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Quinone and quinone methide as transient intermediates involved in the side chain hydroxylation of N-acyldopamine derivatives by soluble enzymes from Manduca sexta cuticle.

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Archives of Insect Biochemistry and Physiology 16:123-138 (1 991)
Quinone and Quinone Methide as
Transient Intermediates Involved in the
Side Chain Hydroxylation of N-Acyldopamine
Derivatives by Soluble Enzymes from
Manduca sexta Cuticle
Steven J. Saul, Hemalata Dali, and Manickam Sugumaran
Departvierit of Biology, University of Massachusetts at Boston, Harbor Campus, Massachusetts
Proteins solubilized from the pharate cuticle of Manduca sexta were fractionated by ammonium sulfate precipitation and activated by the endogenous
enzymes. The activated fraction readily converted exogenously supplied
N-acetyldopamine (NADA) to N-acetylnorepinephrine (NANE). Either heat treatment (70°C for 10 min) or addition of phenylthiourea (2.5 pM) caused total
inhibition of the side chain hydroxylation. If chemically prepared NADA quinone was supplied instead of NADA to the enzyme solution containing phenylthiourea, it was converted to NANE. Presence of a quinone trap such as
N-acetylcysteine in the NADA-cuticular enzyme reaction not only prevented
the accumulation of NADA quinone, but also abolished NANE production.
In such reaction mixtures, the formation of a new compound characterized
as NADA-quinone-N-acetylcysteine adduct could be readily witnessed. These
studies indicate that NADA quinone is an intermediate during the side chain
hydroxylation of NADA by Manduca cuticular enzyme(s). Since such a conversion calls for the isomerization of NADA quinone to NADA quinone methide
and subsequent hydration of NADA quinone methide, attempts were also made
to trap the latter compound by performing the enzymatic reaction in rnethanol. These attempts resulted in the isolation of p-methoxy NADA (NADA quinone methide methanol adduct) as an additional product. Similarly, when
the N-P-alanyldopamine (NBAD)-Manduca enzyme reaction was carried out
i n the presence of L-kynurenine, two diastereoisomers of NBAD quinone
methide-kynurenine adduct ( = papiliochrome Ila and Ilb) could be isolated.
The above results are i n agreement with our hypothesis that N-acylnorepinephrine formed in Manduca cuticle is biosynthesized by an indirect route
involving intermediary formation of N-acyldopamine quinone and N-acyldopamine quinone methide as established in the case of Sarcophaga bullata and
is not produced by the action of a p-hydroxylase.
Acknowledgments: This work was supported by grants from NIH (R01-AI-14753) and University of Massachusetts, Boston. We thank Mr. Victor Semensi for his help. Our thanks are also
due to Dr. M. Yago for a generous supply of synthetic papiliochrorne Ila and Ilb.
Received June15,1990; accepted September 24,1990.
Address reprint requests to Dr. Manickam Sugumaran, Department of Biology, University of
Massachusetts at Boston, Harbor Campus, Boston, M A 02125.
0 1991 Wiley-Liss, Inc.
124
Saul et al.
Key words: cuticular sclerotization, N-acylnorepinephrine,quinone isornerization, P-hydroxylation, phenoloxidase, quinone isomerase
INTRODUCTION
(3-Hydroxylation of NADA,*a well known insect cuticular sclerotizing precursor, by cuticular enzyme(s) from Manduca sexta was first reported by Peter
[l]nearly a decade ago. Using optical rotatory dispersion studies, he also found
that the NANE formed in the reaction is a racemic mixture. Unlike the mammalian dopamine-P-hydroxylase which performs the stereoselective hydroxylation of dopamine to L-norepinephrine [2], the cuticular enzymes generated
racemic NANE [l],the significance of which was not clear, until we first reported quinone methide production from 4-alkylcatecholby cuticular enzymes from
Sarcophaga bullata [3]. Based on the reaction of different substrates with insoluble cuticular enzyme(s), we proposed that quinone methides are involved as
transient intermediates during the activation of NADA and related compounds
and put forward the quinone methide sclerotization hypothesis [3,4]. According to our proposal, enzymatic activation of 4-alkyl catechols generates
2-hydroxy-p-quinone methides, which are one of the reactive species involved
in sclerotization [3,4]. Since quinone methides undergo rapid Michael-1,6addition reactions with nucleophiles [S], they accounted for the formation of
catechol-cuticle adducts. Since the reaction is nonenzymatic and hence
nonstereoselective, it generated racemic products as found in the case of NANE
[l].Two years after the advocation of quinone methide sclerotization hypothesis, Peter and Vaupel[6] reconfirmed the racemic nature of NANE formed in
Manduca cuticle by an entirely different experiment and proposed three different routes for NANE formation, only one of which included the intermediary
formation of NADA quinone methide. However, we maintained that quinone
methide is the key intermediate in the conversion of NADA to NANE [7-lo].
Using trapping experiments, we also found that NADA quinone is another
intermediate in the enzymatic conversion of NADA to NANE in Manduca and
other insects and invoked the isomerization of NADA quinone to its quinone
methide [11,12]. Although quinone isomerization to quinone methides can
occur nonenzymatically, as in the case of methylene substituted benzoquinones [13-151 and carboxyethyl-o-benzoquinonederivatives [16,17], we did identify an enzyme catalyzing this reaction in insect cuticle [11,12,18] and reported
the purification and characterization of o-quinone isomerase from the hemolymph as well as cuticle of S. bullata [19-221. Thus, the side chain hydroxylation of NADA to NANE observed in insect cuticles is caused by the combined
enzymatic (phenoloxidase and quinone isomerase) and nonenzymatic reactions, rather than the action of a single P-hydroxylase (Fig. 1).
In this paper we examine the mechanism of side chain hydroxylation of
N-acyldopamine derivatives by the soluble enzymes from the pharate pupal
*Abbreviations used: m.pt. = melting point; NAcCys = N-acetylcysteine; NADA = N-acetyldopamine; NANE = N-acetylnorepinephrine; NBAD = N-P-alanyldopamine; NBANE =
N-P-alanylnorepinephrine.
Phenoloxidase/Quinone lsornerase of Manduca Cuticle
125
OH
1
2
3
4
Fig. 1. Alternate routes for the P-hydroxylation of N-acyldopamine derivatives. N-acyldopamine
(1) (R = CH3; NADA; R = CH2CH2NH2;NBAD) can be directly hydroxylated bya P-hydroxylase
type enzyme or converted to N-acylnorepinephrine (2) through the intermediary formation
of quinone (3) and quinone methide (4). A = phenoloxidase; 5 = quinone isomerase;
C = nonenzymatic reaction; D = P-hydroxylase.
cuticle of M. sextn. Our results confirm the contention that in Manduca cuticle
the conversion of N-acyldopamine to N-acylnorepinephrine involves the intermediary formation of N-acyldopamine quinone and N-acyldopamine quinone
methide as established in the case of S. b u h t a and not direct P-hydroxylation.
MATERIALS AND METHODS
Enzyme Preparation
Eggs of M. sexta larvae were kindly donated by Dr. J.S. Buckner of Metabolism and Radiation laboratory, Agricultural Research Service, U. S. Department
of Agriculture, Fargo, ND. Larvae were reared on a synthetic medium [23],
and kept at 25°C during a 16 h light-8 h dark photoperiod. The cuticles were
harvested from pharate pupae by dissection and washed thoroughly with ice
cold water. They were then homogenized in a Waring blender (DynamicsCorp.
America New Hartford, CT) for 30 s and washed with excess water. After
repeating homogenization and washing once more, the recovered cuticle was
suspended in 0.1% sodium borate, pH 8.0, containing 1 p,M phenylthiourea
for 3 h. At the end of this period, the solubilized proteins were collected by
passing the mixture through cheese cloth and the contents centrifuged. The
clean supernatant was subjected to ammonium sulfate fractionation. The
proteins precipitated between 20-40% ammonium sulfate saturation were
collected by centrifugation and individually suspended in 5 ml of water. The
contents were left at 5°C for 1week and used as the enzyme source.
Chemicals
P-Methoxy-NADA was synthesized as follows: NANE (270 mg) was stirred
in 1N methanolic hydrochloric acid (25 ml) at room temperature for 2 h. After
this period, the reaction mixture was neutralized with ammonium hydroxide
and concentrated on a rotary evaporator. The solid material obtained was extract-
126
Saul et al.
ed with ethyl acetate. Distillation of the organic solvent on a rotary evaporator
gave a white hygroscopic solid of m. pt. 53-55°C (160 mg; yield, 56%).
H-nuclear magnetic resonance (NMR) (DMSO-d6)6 = 1.85 (s, 3H, COCH3),
2.85-3.40 (m, 2H, CH2),3.11 (s, 3H, OCH3),3.80-4.25 (m, lH, CH), 6.40-6.90
(m, 3H, ArH), 7.70-8.05 (br s, lH, NH), 8.45-9.30 ppm (br s, 2H, OH); NH
and OH signals were exchanged by D20. NADA-quinone-NAcCys adduct was
synthesized by the procedure published from this laboratory 1241. NADA,
3,Pdihydroxyphenylglycol were procured from Sigma Chemical Co. ,St. Louis,
MO. NANE was prepared by acetylation of norepinephrine [25]. A similar procedure was used to synthesize N-acetylarterenone. 3,4-Dihydroxyphenethyl
alcohol was prepared as outlined in an earlier publication [18].Papiliochrome
IIa and IIb were gifts from Dr. Motoko Yago of Iwate Medical College,
Morioka, Japan.
High Performance Liquid Chromatography
HPLC analysis of catechols was performed using a Beckman (Berkeley, CA)
332 liquid chromatography system equipped with two model 1IOB pumps,
a model 420 controller, a model 160 absorbance detector (280 nm), and model
427 integrator. Chiral separation of P-methoxy NADA was achieved on an
acetylated-P-cyclodextrin column as described in an earlier publication [20].
RESULTS
Figure 2 shows the HPLC analysis of different reaction mixtures containing
NADA and soluble enzyme(s). As soon as NADA and the enzyme fraction
were mixed, the only catecholic compound detected by HPLC analysis of the
reaction mixture is parent NADA (Fig. 2, trace A). However, 5 min incubation
with the Manduca enzyme fraction generated additional products (Fig. 2, trace
B). The peak at 1.99 min was identified as NANE by comparison of its elution
time with that of synthetic NANE. Both this material and NANE co-chromatographed as a single symmetrical peak and exhibited the same ultraviolet
absorbance spectrum (X max = 278 nm), confirming that they are one and the
same compound. The peak at 2.62 min was confirmed as NADA quinone by
comparison of its visible spectrum (data not shown) and elution profile (Fig. 2,
trace C). Heat inactivated enzymes failed to cause these conversions (Fig. 2,
trace D).
From these studies, it can be concluded that NADA is converted by the
Manduca cuticular enzymes to NADA quinone and NANE. NANE can be formed
either by P-hydroxylation of NADA, or by hydration of NADA quinone methide
formed by the isomerization phenoloxidase generated NADA quinone. The following line of evidences indicates that NANE is generated from NADA quinone
and NADA quinone methide and not directly from NADA. When the NADAenzyme reaction is performed in presence of phenylthiourea, a specific inhibitor of phenoloxidase, not only the formation of NADA quinone is totally
inhibited, but also NANE production is abolished (Fig. 2, trace E), indicating
that phenoloxidase is essential for the synthesis of both NADA quinone
and NANE.
In order to check whether NADA quinone is the precursor for NANE, chemically synthesized NADA quinone was supplied to the Manduca enzyme prep-
PhenoloxidaselQuinone Isomerase of Manduca Cuticle
127
E
0
m
N
I-
4
w
0
z
U
m
U
0
v)
m
d
0
2.0
4.0
0
2.0
4.0
ELUTION TIME (mln)
Fig. 2. Oxidation of NADA by soluble enzymes from M . sexta. A reaction mixture (1 ml) containing 0.22 mM NADA in 50 mM sodium phosphate buffer, pH 6.0, was incubated at room
temperature. A 50 pI aliquot of enzyme preparation (14 mg/ml) was added to the reaction mixture and at the indicated times, a 20 pI aliquot was subjected to HPLC analysis on a Beckman
C t 8ultrasphere reversed phase column (5 km, 4.6 x 150 rnm) using isocratic elution with
50 rnM acetic acid containing 0.2 mM sodium octylsulfonate in 20% methanol at a flow rate of
1.0 ml/min. Generation of NADA quinone was achieved by incubating the reaction mixture
(lacking enzyme) with excess silver oxide for 2 min. The contents were filtered and subjected
to HPLC analysis. Inactivation of enzyme was achieved by heating at 70°C for 10 min. A = zero
time reaction; B = 5 min reaction; C = silver oxide generated NADA quinone reaction;
D = 5 min reaction with heat inactivated enzyme; E = same as B, but the reaction mixture
contained 2.5 pM phenylthiourea. The peaks at 1.99,2.62, and 4.0 min were identified as NANE,
NADA quinone, and NADA, respectively.
aration and its fate analyzed by HPLC studies. As shown in Figure 3 (trace A),
when chemically synthesized NADA quinone was immediately subjected to
HPLC analysis, three compounds could be identified. The major peak at 2.62 min
was identified as NADA quinone by spectral analysis and comparison with
mushroom tyrosinase generated NADA quinone. The peak at 4.0 min was due
to unoxidized NADA. The minor peak observed at 1.99 min was identified as
NANE. Five minute incubations of the NADA quinone with Munduca enzyme
preparation caused complete conversion of NADA quinone to NANE (Fig. 3,
trace B), attesting that NADA quinone is the precursor for NANE.
If this is true, trapping of NADA quinone formed in the NADA-Manduca
enzyme reaction should cause total inhibition of NANE formation. Therefore,
we performed the NADA-Munduca enzyme reaction in the presence of NAcCys,
a well established quinone trap. As shown in Figure 4, if NAcCys is included
in the NADA-enzyme reaction, formation of both NADA quinone and NANE
128
Saul et al,
E
0
Q
N
2
w
0
z
4
m
c
0
fn
m
4
1
1
C
ELUTION TIME (rnln)
Fig. 4. Effect of NAcCys o n NADA-enzyme reaction. The reaction conditions are as outlined
for Figure 2, except for the inclusion of NAcCys (2mm) in the experiment. Incubation time was
S rnin. A = Control without NAcCys; B = experiment with NAcCys; C = standard NAcCysNADA quinone adduct. The peaks at 2.0,2.62,4.0,and 9.3 min were identified as NANE, NADA
quinone, NADA, and NAcCys-NADA quinone adduct, respectively.
Phenoloxidase/Quinone lsomerase of Manduca Cuticle
129
1.0
YI
0
2
4
m
E 0.5
0
v)
rn
U
0
220
290
360
WAVELENGTH (nm)
Fig. 5. Ultraviolet absorbance spectrum of NADA-quinone-NAcCys adduct in 0.2 M acetic
acid. A = Synthetic material; 6 = enzymatic product.
were totally abolished (Fig. 4, trace B). But, formation of a new product eluting 9.30 min could be observed. This product exhibited the same retention
time as that of an authentic NADA-quinone-NAcCys adduct (Fig. 4, trace C)
and possessed the same spectra1 characteristics as that of the synthetic NADA
quinone-NAcCys adduct (Fig. 5). Therefore, trapping of NADA quinone by
NAcCys prevented the formation of NANE. From these studies, it was concluded that NADA quinone and not NADA itself is the precursor for NANE
formation in Marzduca. Chemical considerations indicated that NADA quinone
has to be tautomerized to its quinone methide analog before yielding NANE
(Fig. 1). If indeed NADA quinone methide is the precursor for NANE, then
one should be able to trap the transiently formed NADA quinone methide.
Earlier we have demonstrated that NADA quinone methide formed in S. bullata
can be trapped by methanol to yield p-methoxy NADA [20,22]. We have used
the same approach to prove the formation of NADA quinone methide in Munducu
as well. Figure 6 shows the HPLC analysis of the NADA-enzyme reaction mixture conducted in the presence of methanol (traces A, 9). From Figure 6 it is
evident that there is a new product formed (4.76 min peak in traces A, B) which
is dependent on the presence of methanol. This compound could be either a
NADA quinone methanol adduct or a NADA quinone methide methanol adduct.
In our previous work [20], we relied on the ultraviolet absorbance spectra to
distinguish between these two adducts. However, to unambiguously establish the structure of the adduct, we chemically synthesized P-methoxy NADA
by employing the procedure outlined in the Materials and Methods section.
The synthetic compound exhibited the same retention time as that of the biological isolate (Fig, 6, trace C) and co-chromatographed as a single symmetrical peak with the latter, confirming the trapping of NADA quinone methide
as its methanol adduct. The NMR spectrum of synthetic P-methoxy NADA
shown in Figure 7 confirmed its structural assignment. The synthetic (3-methoxy
NADA exhibited the same ultraviolet absorbance spectrum (Fig, 8) as that of
the biological isolate, and could be resolved into the individual stereoisomers
by HPLC chromatography on a chiral column (Fig. 8, inset).
Isomerization of NADA quinone to its quinone methide tautomer could occur
130
Saul et al.
!-
U
c
0
2.0
4.0
ELUTION TIME ( m l d
Fig. 6. Trapping of NADA quinone methide with methanol. A reaction mixture containing
0.22 m M NADA, enzyme protein, 50 m M sodium phosphate buffer, pH 6.0, in 10% methanol
was incubated at room temperature and an aliquot was subjected to HPLC analysis as outlined
for Figure 2. A = 5 rnin reaction; B = 15 min reaction; C = authentic p-methoxy NADA.
either enzymatically or nonenzymatically. Kramer and his associates have
claimed that the reaction occurs spontaneously and nonenzymatically [26,27].
To determine whether or not NANE is formed by an enzymatic route, the
Manduca enzymes were heat inactivated at 70°C for 10 min and substituted
for the native enzyme in NADA. quirtone-enzyme reaction. Such an experiment failed to produce NANE over the nonenzymatic rates reported from this
laboratory [17], confirming the actual presence of a quinone isomerase in
the enzyme preparation. Accordingly, the NANE formation exhibited a pH
optimum characteristic of enzymatic reactions. The coupled phenoloxidase/
Fig. 7. NMR spectrum of p-methoxy NADA in DMSO (d6).
Phenoloxidase/Quinone lsomerase of Manduca Cuticle
285
220
131
350
WAVELENGTH f n m )
Fig. 8. Ultraviolet absorbance spectrum of p-methoxy NADA. A = In 0.2 M acetic acid;
B = In sodium borate, pH 8.5. Inset: HPLC analysis of p-methoxy NADA on acetylated cyclobond
column. Conditions are as outlined in Materials and Methods.
quinone isomerase exhibited a pH optimum of 6.5 (Fig. 9). Murtducu phenoloxidases have been shown to exhibit a pH optimum of 6 [27,28]. Therefore, it
is possible that the pH optimum of quinone isomerase is about 6.5.
Apart from NADA, the combined phenoloxidase/quinone isomerase system
of Manducu also attacked NBAD and 3,4-dihydroxyphenethyl alcohol and generated their side chain hydroxylated products. Figure 10, for instance, shows
the action of Manduca enzymes on 3,4-dihydroxyphenethylalcohol. As is shown,
the enzyme action produced 4-hydroxyethyl-o-benzoquinoneand 3,kdihydroxyphenyl glycol analogous to NADA oxidation reaction.
Figure 11gives the HPLC analysis of two reaction mixtures containing NBAD,
L-kynurenine, and mushroom tyrosinase with and without Manducu soluble
enzymes. The peak at 1.44 min was identified as NBANE in comparison with
the chromatographicbehavior of the authentic compound. The peak at 1.82 min
was identified as NBAD quinone. While NBAD gave rise to the 1.97 min peak,
kynurenine eluted at 2.23 min. In addition to the two unidentified peaks elut-
4.0
5.0
0.0
7.0
8.0
PH
Fig. 9. pH optimum of phenoloxidase/quinone isomerase. NANE formed in a Manduca
phenoloxidase/quinone isomerase was quantified at different pH values and plotted.
132
Saul et al.
E
0
W
N
s
W
0
2
U
m
a
2m
U
1
0
2.0
4.0
ELUTION TIME (mln)
6 .O
Fig. 10. Enzymatic oxidation of 3,4-dihydroxyphenethyl alcohol. A reaction mixture (1 ml) containing 0.35 m M 3,4-dihydroxyphenethyl alcohol in 50 mM sodium phosphate buffer, pH 6.0,
was incubated at room temperature. A 50 pI aliquot of soluble enzyme solution was added to
the reaction mixture and an aliquot (20 kI) was subjected to HPLC analysis as outlined for
Figure 2. A = zero time reaction; B = 5 min reaction. The peaks at 1.88, 2.46, and 3.54 min
were identified as 3,4-dihydroxyphenyl glycol, hydroxyethyl-o-benzoquinone,and 3,4-di hydroxyphenethyl alcohol, respectively.
ing at about 3 min, two peaks eluting at 3.94 and 4.65 min were observed.
The last two peaks did not appear in the chromatograms of the reaction mixture lacking either L-kynurenine or NBAD (data not shown), indicating that
they are some kind of NBAD-kynurenine adducts. Papiliochrome IIa and IIb
are two diastereoisomeric structures arising from the nonstereospecific condensation of L-kynurenine with NBAD quinone methide [29-321. An authentic sample of papiliochrome IIa and IIb supplied by Dr. Yago exhibited the
same retention time as the above two peaks (Fig. 11, dotted chromatograms)
and co-chromatographed with each other indicating that they are indeed
papiliochrome IIa and IIb. Comparison of the two reactions also reveals that
NBANE production as well as papiliochrome formation is a lot higher in the
reaction mixture containing Manduca enzymes than in the control reaction,
attesting to the presence of an enzyme system generating NBAD quinone
methide in the Manduca enzyme preparation.
DISCUSSION
P-Hydroxylation of NADA by cuticular enzyme(s) from M. sexta was reported nearly a decade ago by Peter [l],but the mechanism of the hydroxylation
has not been clarified. Based on Peter’s [l]finding that NANE formed in the
enzymatic hydroxylation of NADA side chain in M. sexta cuticle is a racemic
PhenoloxidaseIQuinone lsomerase of Manduca Cuticle
A
133
?I3
7 Il
V-
E
N I s
' 9 c
?
-
0
m
N
t
* .
y1
0
2
U
m
a
E
U
Fig. 11. Enzymatic oxidation of NBAD in presence of L-kynurenine. A reaction mixture (1 mi)
containing 2 mM NBAD, 2 mM L-kynurenine in 50 mM sodium phosphate buffer, p H 6.0, and
mushroom tyrosinase (40 pg) was incubated at room temperature for 76 min and a20 pl aliquot
was subjected to HPLC analysis on a Beckman Cq8ultrasphere reversed phase column (5 pm,
4.6 x 150 rnrn) using isocratic elution with 100 mM sodium phosphate buffer, pH 3.3 in 10%
acetonitrile at a flow rate of 1 ml/min (trace A). Trace 6 is the chromatogram of reaction as
described above, but also contained Manduca enzyme. The peaks at 1.44, 1.02, 1.97, and
2.23 min were identified as NBANE, NBAD quinone, NBAD, and L-kynurenine, respectively.
The two peaks at 3 min are due to unidentified compounds. The peaks at 3.94 and 4.65 min
are due to papiliochrorne Ilaand Ilb. The chromatograms with broken lines are due to authentic papiliochrome Ilaand Ilb.
mixture and our own experiments on the substrate specificity and product
analysis of sarcophagid cuticular phenoloxidase 131, we proposed that quinone methides are transient intermediates in the reaction and put forward
the quinone methide sclerotizationhypothesis [3,4]. Initially, we failed to observe
the accumulation of NADA quinone in reaction mixtures containing NADA
and cuticular enzymes [3] and believed that NADA oxidation to NADA quinone methide involves a direct two electron oxidation process [3,4,7-91, although
we did acknowledge the feasibility of quinone to quinone methide isomerization route for this conversion.
The possibility of N-acyldopamine quinone isomerizing nonenzyma tically
to N-acyldopamine quinone methide was considered as early as 1958by Witkop
and his associates, who also proposed that the (3-hydroxylation of dopamine
to norepinephrine involves a similar reaction [33-351. However, in a dramatic
turn of events, it was demonstrated that the dopamine-P-hydroxylationinvolves
the stereospecific incorporation of one atom of molecular oxygen into the substrate and not a nonstereoselective and nonenzyinatic hydration of dopamine
quinone methide [2,36].
As a consequence of the well established enzymology of dopamineP-hydroxylasereaction, it was often assumed that N-acyldopamine-P-hydroxylation commonly observed in insect cuticle is also catalyzed by P-hydroxylase.
134
Saul et al.
Thus, Kramer and his associates recently designated the enzyme causing the
conversion of N-acyldopamine to N-acylnorepinephrine as P-hydroxylase [37-391.
However, recent studies from this laboratory [10-12,17-221 conclusively prove
the operation of the original pathway proposed by Witkop and his associates
[33-351, viz., hydration of the N-acyldopamine quinone methide generated
from the isomerization of N-acyldopamine quinones as the route for the side
chain hydroxylation of N-acyldopamine derivatives.
We first reported the presence of an enzyme catalyzing the isomerization of
N-acyldopamine quinones to N-acyldopamine quinone methides in the insoluble pharate cuticle of M . sexta [ll],and subsequently obtained the enzyme
in the soluble form from both the cuticle and hernolymph of S. bullata larvae
[ 19-22]. Apart from trapping the enzymatically generated NADA quinone
methide as its methanol adduct, we also demonstrated that the resultant pmethoxy NADA formed was a racemic mixture and confirmed the nonenzymatic,
and hence nonstereoselective nature of the observed Michae1-lf6-additionreaction [20,22].In addition, we invoked the quinone methides in immune responses in insects [21,40] and established that the biosynthesis of the sclerotizing
precursor, 1,2-dehydro-NADA (discovered by Andersen and Roepstorff [41])
involves direct tautomerization of NADA quinone methide [42-441.
Independent of the discovery of quinone isomerase, we delineated a few
facile nonenzymatic routes for quinone methide formation in the case of
methylene- and carboxy ethyl-substituted benzoquinones [13-171. As a result,
it was essential to compare the rate of enzymatic with nonenzymatic rates of
synthesis of NANE from NADA. In a reaction mixture (1ml) containing 1mM
NADA and 10 pg tyrosinase, NANE production was found to be about 1nmol/
min [17,221. In contrast, inclusion of 1unit of purified quinone isomerase from
S. bullutu in the above reaction dramatically increased the rate of NANE production to about 400 nmoVmin [17,22]. However, the specific activity of quinone isomerase is about 4,000 unitsimg 1221. Therefore, on a per milligram
basis, if the tyrosinase reaction is generating about 100 nmol of NANE/min,
quinone isomerase is able to generate as much as 1.6 mmol of NANE/min.
In spite of our claim that there is a quinone isomerase in Manducu cuticle
[11,12], Kramer and his associates recently doubted the existence of quinone
isomerase in the cuticle of Manduca [26,27]. They asserted that the side chain
hydroxylation could be due to nonenzymatic origin [26] or phenoloxidase catalyzed [27] and also suggested that the side chain hydroxylase activity was
due to P-hydroxylase [37-391. In order to prove the existence of quinone isomerase, clarify the role of P-hydroxylase, and to conclusively establish the intermediary nature of N-acyldopamine quinone and N-acyldopamine quinone
methide, we undertook the present study. The results presented in this paper
with soluble enzyme preparation from Manducu cuticle confirm our contention that NADA conversion to NANE observed in Manduca is caused by the
intermediate formation of NADA quinone and NADA quinone methide [ll,121.
Accordingly, the side chain hydroxylation involved enzymatic isomerization
of phenoloxidase generated NADA quinone to its quinone methide tautomer
and its subsequent nonenzymatic hydration (Figs. 1,12). That the NADA quinone is an obligatory intermediate in this conversion is confirmed by trapping experiments with NAcCys, as well as inhibition by phenylthiourea, while
PhenoloxidasdQuinone lsomerase of Manduca Cuticle
135
OH
HO
2
1
4'
Me 0
OH
HoJ&R
'
3$
b
I , " "
--3
HO
HO
3
4
L-KYNURENINE
HO
6
Fig. 12. Summary of the observed reactions. The side chain of N-acyldopamine derivatives
(1: NADA, R = CH2NHCOCH3;NBAD, R = CH2NHCOCH2CH2NH,)i s not directly hydroxylated by a dopamine-p-hydroxylasetype enzyme (D) in Manduca cuticle but by the combined
action of phenoloxidase (A) and quinone isomerase ( 8 ) through the intermediary formation
of 2: quinone, and 3: quinone methide. Accordingly, the reaction of NADA with cuticular enzymes
generates quinone methide (3),which can be trapped either by water to produce 4: NANE, or
by exogenously added methanol to generate 5: racemic p-methoxy NADA. In the case of NBAD
reaction, the corresponding quinone methide reacts with water to yield NBANE or reacts with
the exogenously added L-kynurenine to give 6 : papiliochrome Ila and Ilb.
the intermediacy of NADA quinone methide has been confirmed by trapping
experiments with methanol (Fig. 12). Experiments with NBAD and kynurenine
also confirmed the formation of quinone methide as papiliochrome IIa and IIb
were recovered as additional products in the reaction mixture (Figs. 1 1 , l Z ) .
Evidence for the existence of quinone isomerase (or a similar enzyme involved
in side chain hydroxylation) in Manduca cuticle comes from the work of Kansas group only. During their studies on NBANE biosynthesis by Manduca cuticle, they observed the formation of about 22 Fmol of NBANE per gram of wet
cuticle from 67 pmol of NBAD in 10 min [45]. Therefore, the rate of NBANE
synthesis is about 2.2 pmol/min/g tissue. If we assume that the wet cuticle
had about 0.01% by weight phenoloxidase, this gives a specific activity of
22 pmol/min/mg of phenoloxidase(s).But in reality, the purified phenoloxidase(s)
from Maizdtaca exhibit a rate of about 300 nmol of side chain hydroxylated product formed/min/mg of protein [27], which is of the same order as that of purified mushroom tyrosinase reaction (about 100 nmol/min/mg protein), but at
least 70 times lower than the rate reported for whole cuticle that calls for enzymes
other than phenoloxidase(s) to be responsible for side chain hydroxylation of
N-acyldopaminederivatives.This enzyme could be either a f3-hydroxylase[37-391
or the quinone isomerase. The mechanistic studies outlined in the present
paper favor the route involving quinone isomerization to quinone methide,
and discount the p-hydroxylase action.
Evidence for the presence of quinone isomerase in the soluble cuticular preparation comes from the following studies. If NADA quinone is chemically synthesized and provided to the enzyme preparation treated with phenylthiourea,
136
Saul et al.
it is still converted to NANE (Fig. 3). This conversion, however, returns to basal level if the heat inactivated enzyme is substituted to the native enzyme.
Our repeated attempts to separate the quinone isomerase from phenoloxidase
ended in vain. This could be due to the formation of a complex between
phenoloxidase and quinone isomerase. Such a complex formation is common
among enzymes catalyzing the consecutive reactions in a metabolic pathway
and the complexes are called metabolons [46]. Metabolon formation is advantageous especially for those enzymes dealing with unstable intermediates. If
phenoloxidase and quinone isomerase exist independently, the phenoloxidase
generated quinone has to be released into solution before it can diffuse to the
active site of quinone isomerase. On the other hand, if phenoloxidase/quinone
isomerase active sites are coupled by complex formation, loss of reactive NADA
quinone intermediate into the solution could be avoided and efficiency can be
achieved by directly transferring the NADA quinone generated at the active
site of phenoloxidase to the active site of quinone isomerase (channeling),which
also ensures raising of the local concentration of the substrate for effective catalysis [46]. We have already reported that in Surcophuga phenoloxidase/quinone
isomerase comigrate on Sepharose6B column chromatography, probably reflecting complex formation. In this organism, we could separate these two enzyme
activities independent of each other. However, we have so far been unsuccessful in resolving these two activities from Munducu cuticle. We are continuing our studies with the Munducu cuticle to obtain more information on the
nature of quinone isomerase in this organism.
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methide, chains, intermediate, quinone, involved, derivatives, acyldopamine, side, transiente, enzymes, hydroxylation, soluble, sexta, manduca, cuticle
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