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Microsomal marker enzymes of Manduca sexta (L) midgut.

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Archives of Insect Biochemistry and Physiology 1:311-321 (1984)
Microsomal Marker Enzymes of
Manduca sexta (1) Midgut
Gunter F. Weirich and Jean R. Adams
Insect Physiology Luboratory (G.F.W.) and Insect Pathology Laboratory (J.R.A.),Agricultural
Research Sewice, USDA, Beltsville, Maryland
The subcellular distribution of four enzymes (glucose-6-phosphatase,
phosphodiesterase I, NADPH-cytochrome c reductase, and p-nitroanisole 0demethylase) in the rnidgut of "wandering" fifth-instar larvae of the tobacco
hornworm, Manduca sexta (L), was determined and the composition of
mitochondria1 and microsomal pellets was examined by electron microscopy.
Most of the glucose-6-phosphatase activity and one-third of the
phosphodiesterase 1 activity were found in the high-speed supernatant.
NADPH-cytochrome c reductase activity was marginal and 0-demethylase
activity was undetectable in the supernatant. The highest specific activities
for phosphodiesterase I , NADPH-cytochrome c reductase, and p-nitroanisole
0-demethylase were measured in microsomes, but the relative specific activity
of phosphodiesterase I was only half that obtained with the latter two
enzymes. In all subcellular preparations the relative specific activities of
NADPH-cytochrome c reductase and p-nitroanisole 0-demethylase were
closely correlated. It i s concluded that glucose-6-phosphatase and phosphodiesterase I are not microsomal marker enzymes i n the midgut, but the
activities of NADPH-cytochrome c reductase and p-nitroanisole 0dernethylase are quantitative measures of microsomal content.
Key words: NADPH-cytochrome c reductase, pnitroanisole O-demethylase, Manduca
sexta, midgut, microsomal marker enzymes
INTRODUCTION
In enzyme distribution studies on insects, marker enzymes that have been
established for mammalian tissues are commonly used for the characterization of subcellular fractions. The concept of marker enzymes implies that
specific enzymes are predominantly localized in a single cellular component
[1,2]. Systematic studies to show that mammalian marker enzymes satisfy
this definition when used for the characterization of insect tissue fractions
are lacking.
Address reprint requests to Gunter F. Weirich, Insect Physiology Laboratory, Building 467,
BARC-East, Beltsville, MD 20705.
Received October 7,1983; accepted April 13,1984.
0 1984 Alan R.
Liss, Inc.
312
Weirich and Adams
The aim of the present investigation was to identlfy microsomal marker
enzymes in an insect tissue. We report the subcellular distribution of four
enzymes in the midgut of fifth-instar larvae of the tobacco hornworm, Manduca sexta. Two of these enzymes, glucose-6-phosphatase and phosphodiesterase I, have previously been identified as microsomal enzymes in rat liver
[1,3]. The other two enzymes, NADPH-cytochrome c reductase and p-nitroanisole Odemethylase,* have been found in microsomes of vertebrates
[4-71 as well as insects [5,8-121. Their exclusive microsomal localization,
however, has not been established in insects, and in some vertebrate tissues,
NADPH-cytochrome c reductase has been found in cellular components
other than microsomes [13-151. We report that only two of the enzymes-are
NADPH-cytochrome c reductase and p-nitroanisole 0-demethylase-suitable as microsomal markers in the midgut of M sexta. The subcellular
distributions of glucose-6-phosphatase and phosphodiesterase I, on the other
hand, are inconsistent with a predominantly microsomal localization. These
findings suggest that enzyme distribution patterns of mammalian tissues
should not be applied to insect tissues without verification.
MATERIALS AND METHODS**
Chemicals
NADP+, sodium salt; NADPH, tetrasodium salt, type I; glucose 6-phosphate, monosodium salt; glucose-6-phosphate dehydrogenase, type XV (from
baker’s yeast); cytochrome c, type VI; thymidine 5’-monophospho-p-nitrophenyl ester, sodium salt, grade I; and bovine serum albumin,? fraction V,
were obtained from Sigma Chemical Co (St Louis, MO). p-Nitroanisole was
purchased from Aldrich Chemical Co (Milwaukee, WI), and recrystallized
from hexane. All other chemicals were of the highest purity commercially
available, and solvents were redistilled in glass.
Homogenization and Fractionation
Tobacco hornworms were reared on artificial diet [16]. Fifth-instar larvae
were taken on the first day of ”wandering” [17,18]and the midguts dissected
as described previously [19]. All of the following operations were carried out
at 5°C. The carefully cleaned and rinsed guts (inside and outside) were
minced with scissors and homogenized in 6 ml of buffer A (10 mh4 TRISHC1, pH 7.4, 300 mM sucrose, 1 mM EDTA) per gram gut tissue in a Ten
‘The term p-nitroanisole 0-demethylase is used in this paper for the total 0-demethylation
activity measured with p-nitroanisole as substrate and does not imply the involvement of
only one cytochrome P-450monooxygenase.
**
Mention of a company name or proprietary product does not constitute an endorsement
by the U.S. Department of Agriculture.
+Abbreviations:bovine serum albumin = BSA; y intercept of linear regression line = b; slope
of linear regression line = m; correlation coefficient = r; time integral of the squared angular
velocity, W = 981 x 60 (g min)/raV.
Microsomal Marker Enzymes
313
Broeck homogenizer (average clearance 125 pm) cooled in ice. Homogenization was initially performed by hand until the tissue was broken up and
suspended in the buffer and subsequently with a motor-driven pestle (two
strokes at approximately 650 rpm). Homogenates were fractionated as shown
in Figure 1. The duration of the initial centrifugations was reduced to a
minimum by increasing the speed [20], and some centrifugations consisted
only of acceleration and deceleration with no plateau phase in between. Each
centrifugation is described by the averaged g force and the time integral of
the squared angular velocity W (given in brackets; Fig. 1)[2]. Centrifugations
were carried out in a Beckman model L Ultracentrifuge with a Rotor Type 30
(Beckman Instruments, Inc, Palo Alto, CA) at 5°C. The W values were
calculated by integration, based on the linear acceleration and deceleration
curves obtained and the plateau phases, where applicable.
An intermediate fraction (containing residual mitochondria as well as
microsomes) was removed from the postmitochondrial supernatant prior to
the high-speed sedimentation of the bulk of the microsomes. Mitochondria1
and microsomal pellets were washed by gentle mixing with the indicated
volumes of buffer A in glass-Teflon homogenizers (100-200 pm clearance).
The supernatants from washings of mitochondria were not subjected to
further fractionation, and the supernatant from the washing of the microsomes was not combined with the original postmicrosomal supernatant.
HOMOGENATE
I
accelerated to 3,0009
12.4 x lO'rad'sec-']
rehomogenized in
3 vols buffer A
I
centrifuged as above
CRUDE.NUCLEAR
FRACTION
Repeated
one or two
more times
I
Supernatant
I
accelerated to 20,OOOg
14.0 x 108rad'sec-']
Pellet
resuspended in
3 vols buffer A
Supernatant
accelerated to 13,0009
[2.0 x 108rad'sec-']
centrifuged at 28.OOOg
19.0x 1OBrad2sec-']
I
MITOCHONDRIA
Supernatant
1
INTERMEDIATE
FRACTION
Supernatant
I
centrifuged at 8O.OOOg
13.6 x 10i0rad2sec-']
I
POSTMICROSOMAL
Pellet
resuspended In
3 vols byffer A
SUPERNATANT
centrifuged as above
A
MICROSOMES
Supernatant
Fig. 1. Flow diagram for fractionating M sexta midgut homogenates.
314
Weirich and Adams
For use in enzyme assays and protein determinations [21], all particulate
fractions were suspended in buffer A by gentle mixing in glass-Teflon
homogenizers.
Enzyme Assays
Glucose-6-phosphatase (D-glucose-6-phosphate phosphohydrolase; EC
3.1.3.9) activity was determined according to Baginski et a1 [22] and phosphodiesterase I (oligonucleate 5'-nucleotidohydrolase; EC 3.1.4.1.) activity according to Touster et a1 [3]. Initial tests showed that the phosphodiesterase I
activities could be approximately doubled by the addition of 25 mM MgC12.
Therefore, all subsequent assays were performed with 15 mM MgC12. Glucose-6-phosphatase and phosphodiesterase activities were determined immediately after preparation of the subcellular fractions. For the glucose-6phosphatase assays, 0.2-2.0 mg protein were incubated in a total volume of
0.8 ml; for the phosphodiesterase assays, 2.0-7.0 mg protein were used in
1.0 ml of incubation mixture. Both assays were conducted at 30°C. For most
fractions, the reaction was linear for the first 60 min of incubation.
The NADPH-cytochrome c reductase (NADPH:ferricytochrome oxidoreductase; EC 1.6.2.4) assay was adapted from Sottocasa et a1 [23]. In addition
to the subcellular preparation (0.1-1.0 mg protein), the assay mixtures contained 50 pM cytochrome c, 0.33 mM KCN, 48 mM potassium phosphate,
pH 7.8, 0.33 mM TRIS, and 1.0 mM EDTA. The total volume was 3.0 ml. The
reactions were started by the addition of NADPH (final concentration 0.1
mM). The reduction of cytochrome c at 30°C was monitored by recording the
increase in absorbancy at 550 nm with the isosbestic point for reduced and
oxidized cytochrome c at 541 nm as reference. All measurements were made
with an Aminco DW-2a TJViVIS Spectrophotometer (American Instrument
Co. Urbana, IL). Initial rates were determined and corrected for the absorbancy change recorded prior to the addition of NADPH less than 10% of the
total). Enzyme activities were calculated using 18.5 mM- cm-' as absorption
coefficient for reduced cytochrome c [24], which was verified for our experimental conditions. The rates were proportional to the amounts of mitochondrial or microsomal protein added to the incubation mixtures, and in mixed
incubations the activities of mitochondria and microsomes were additive.
p-Nitroanisole 0-demethylase (p-nitroanisole monooxygenase (0-demethylating); not yet registered) activity was determined at 30°C by a modification
of the method of Hansen and Hodgson [8].In addition to the buffer components, the reaction mixtures contained an NADPH-generating system, consisting of 0.5 mM NADP+, 2.5 mM glucose 6-phosphate, and 0.25 units of
glucose-6-phosphate dehydrogenase per ml, and the subcellular fraction
(0.3-5.0 mg protein). Total volume was 3.0 ml. After 5 min of preincubation
at 30T, the reactions were initiated by the addition of 20 pl of a 75 mM pnitroanisole solution in methanol (0.5 mM in the incubation mixture). The
reactions were terminated by addition of 1ml 1.0 N HCl and the reaction
products were transferred to 0.5 N NaOH via chloroform partitions. Absorbancies were measured at 400 nm (dual wavelength mode with 500 nm as
reference), and p-nitrophenol standards, carried through the same partition
procedure, were used to determine the product concentrations.
i
Microsomal Marker Enzymes
315
Preliminary assays were conducted in three different buffers: 33 mM
potassium phosphate, 3 mM TRIS, pH 7.8,1 mM EDTA, and 300 mM sucrose
(buffer B); 37 mM TRIS-HCI, pH 7.9, 0.3 mM EDTA, 5 mM MgC12, 3.3 mg
BSA/ml, and either 100 mM sucrose (buffer C) or 300 mM sucrose (buffer D).
Mitochondria1 as well as microsomal activities were lowest when assayed in
isotonic phosphate-sucrose buffer (B), and highest when assayed in isotonic
TRIS-sucrose buffer (D). Buffer D was used for all subsequent assays.
The demethylation reaction was linear for 15 min, and the rate decreased
slightly between 15 and 30 min of incubation. The amount of product was
proportional to the amount of protein in both 15- and 30-min incubations of
reaction mixtures containing up to 10 mg mitochondrial protein or 4 mg
microsomal protein. In mixed incubations of mitochondria and microsomes,
the yield was additive up to 5 mg mitochondrial protein plus 1.5 mg microsoma1 protein. In all assays of mitochondria or microsomes the protein
content was within these limits.
The NADPH-cytochrome c reductase proved to be a very stable enzyme.
Its activity did not change for 48 h after isolation of the subcellular fractions,
provided they were kept on ice. The 0-demethylase activity of microsomal
suspensions was stable for 24 h, but the activity of mitochondrial suspensions
decreased substantially. Therefore, the O-demethylase activity was routinely
determined in fresh subcellular preparations, and the NADPH-cytochrome c
reductase activity in preparations stored overnight on ice.
Electron Microscopy
Aliquots of mitochondrial and microsomal suspensions were centrifuged
in small tubes to form very small pellets (200-300 pg protein). The pellets
were fixed in 3% glutaraldehyde in 10 mh4 HEPES buffer (containing 300
mM sucrose and 1 mh4 EDTA), pH 7.40, for 16 h at 4°C. The pellets were
rinsed in HEPES buffer three times and postfixed in 1%osmium tetroxide in
HEPES buffer for 1h at 4°C. After two rinses in HEPES buffer, pellets were
dehydrated in an ethanol series and then infiltrated and embedded in DER
324 [25] (DER 324 is the replacement for DER 334, which is no longer
available). Sections were obtained with an LKB Ultratome 111, stained with
alcoholic uranyl acetate and lead citrate [26], and examined in a Philips 400 T
electron microscope.
RESULTS
Glucose-6-Phosphataseand Phosphodiesterase I
In contrast to the situation in rat liver [1,3], microsomes from M sextu
midgut did not exhibit the highest glucose-6-phosphatase activity (Fig. 2). In
fact, the microsomes had the lowest activity. Although some enzyme activity
was associated with the particulate fractions, the highest activity (both specific and total) was found in the supernatant.
The specific activity of phosphodiesterase I was highest in the microsomes;
the intermediate fraction had somewhat lower activity. The crude nuclear
fraction, mitochondria, and postmicrosomal supernatant had similar, low
316
Weirich and Adams
Nucl
0
25
Mit I M
50
75
100
0
0
25
Mit I M
50
slo
75
Parcent 01 Total Prolsh
25
50
75
100
p-Nitroaniade O-demethy(.m
NADFltcytochome c reductare
Nucl
Mil I M
Nucl
SUP
Mil I M
Nucl
100
0
25
50
SUD
75
100
Percent 01 Total Proteh
Fig. 2. Distribution of glucose+-phosphatase, phosphodiesterase I, NADPH-cytochrome c
reductase, and p-nitroanisole 0-demethylase i n midgut fractions of M sexta. Enzyme activities
are shown on the ordinate as relative specific activities ((total activity in fraction)/(total activity
in all fractions))/((total protein in fraction)/(total protein in all fractions)), a measure for the
degree of enrichment in each fraction [2]. The percentage of the total protein recovered in
each fraction is shown on the abscissa, and the areas represent the total enzyme activities for
each fraction. The fractions are (from left to right) crude nuclear (Nucl), mitochondria (Mit),
intermediate (I), microsomes (M), and postmicrosomal supernatant (Sup). The sums of the
83.9, 36.15 f 5.44, 710.5 f 15.1, and 1.34 f 0.20 nmole/
activities in all fractions were 293.3
midmidgut (means SD of 2 experiments) for glucose-6-phosphatase, phosphodiesterase I,
NADPH-cytochrome c reductase, and p-nitroanisole 0-demethylase, respectively. Data for
glucose-6-phosphatase were corrected for a slow release of inorganic phosphate occurring in
incubations without added Glc-6-P.
specific activities. Thus, in the M sexta midgut, phosphodiesterase I exists in
particle-bound as well as in soluble form.
NADPH-Cytochrome c Reductase and p-Nitroanisole 0-Demethylase
The specific activity of NADPH-cytochrome c reductase was very high in
the microsomes, low in the crude nuclear fraction and in the mitochondria,
intermediate in the intermediate fraction, and marginal in the supernatant
(Fig. 2). p-Nitroanisole 0-demethylase was similarly distributed in the subcellular fractions, although it was not detectable in the supernatant.
Microsomal Marker Enzymes
317
For both enzymes, the total activity in the nuclear fraction was slightly
higher than that in the microsomes. Apparently, a large proportion of the
endoplasmic reticulum cosedimented with the nuclei and was retained by
this fraction. Sedimentation of microsomal enzyme activities at unusually
low g forces has been reported for other insect tissues [27l, and a similar
tendency for cosedimentation has been observed in rat liver fractionations
[28]. This problem may have been aggravated in our preparations by the
muscle component of the midgut. Moreover, the homogenization of the
midguts was very gentle and the isolation procedure was designed for
optimum purity, not maximum yield of microsomes. While it cannot be ruled
out that other fractions contain some NADPH-cytochrome c reductase andl
or 0-demethylase of their own, the pattern of specific activities suggested
that in the larval midgut of M sextu, NADPH-cytochrome c reductase and 0demethylase are enzymes primarily associated with the endoplasmic reticulum and that the activities found in other subcellular fractions are mostly due
to carryover or entrapment of microsomes.
This conclusion was supported by electron-microscopic examinations of
mitochondrial and microsomal pellets. As shown in Figure 3, the microsomal
pellets consisted mostly of smooth membrane vesicles and contained a few
fragments of microvilli. The mitochondrial pellets contained predominantly
mitochondria of various sizes and degrees of structural integrity, low concentrations of autophagic vacuoles, multilamellar bodies or lamellar lysosomal
bodies, and a few multivesicular bodies, secretory granules of the Golgi, and
smooth membrane vesicles. The biochemical data (6% microsomes) were in
good agreement with the ultrastructural observations.
To determine whether both of these enzymes are equally reliable as microsomal markers, we analyzed the results of three experiments. In these three
experiments, the specific activities of NADPH-cytochrome c reductase in the
microsomal fraction ranged from 139 to 177 nmollminlmg protein and those
of 0-demethylase from 255 to 379 pmollminlmg. The variations in the two
specific activities were not correlated, but for each experiment the distributions of NADPH-cytochrome c reductase and 0-demethylase among the
subcellular fractions were identical. The specific activities declined by identical proportions with each washing of the mitochondrial pellets, and the
enzyme activities in the supernatants were equal to the amounts removed
from the mitochondria. Relative specific activities for mitochondrial pellets of
varying purity ( l x , 2 x , 3x washed) were highly correlated (Fig. 4).Linear
regression analysis yielded a correlation coefficient (r) of 0.994, a slope (m) of
1.01, and a y intercept (b) of -1.41. The results of a similar analysis of all
other fractions and of whole homogenates were r = 0.983, m = 1.03, and b
= 1.06. Thus, both enzyme activities seem to be equally reliable indicators of
microsomal content.
DISCUSSION
Two of the four microsomal marker enzymes commonly used for mammalian tissues, glucose-6-phosphatase and phosphodiesterase I, were not
predominantly associated with any one subcellular fraction of M sexta mid-
318
Weirich and Adams
Fig. 3. Electron micrographs of sections of mitochondria1 (A) and microsomal (B) pellets of
M sexta midgut prepared as described in Materials and Methods. NADPH-cytochrome c
reductase activity indicated a 6% microsome content in the mitochondria1 preparation.
Microsomal Marker Enzymes
319
.Ot
- 0'
10
20
~ O d T O l T 8 C r e d r c t a s e
Fig. 4. Correlation between relative specific activities of p-nitroanisole 0-demethylase and
NADPH-cytochrome c reductase in mitochondria1 preparations from M sexta midgut. The
activities are shown as percentages of the relative specific activity found in rnicrosornes.
gut. The bulk of the glucose-6-phosphatase activity was found in the postmicrosomal supernatant. Storey and Bailey [29] reported the solubilization of
glucose-6-phosphatase of cockroach fat body microsomes in 100 mM TRISHC1 buffer, pH 8.0, containing 15 mM mercaptoethanol. It is uncertain
whether solubilization was responsible for the glucose-6-phosphatase distribution we observed in the midgut fractions. However, if this enzyme is easily
extracted from insect microsomes, it is not a suitable marker enzyme.
The highest relative specific activity of phosphodiesterase I was found in
the microsomes, but it was only half that observed for NADPH-cytochrome
c reductase or p-nitroanisole 0-demethylase. Furthermore, the substantial
soluble component precludes the use of phosphodiesterase I as a microsomal
marker enzyme.
The other two enzymes, NADPH-cytochrome c reductase and p-nitroanisole 0-demethylase, were most abundant in microsomes (Fig. 2). In fact,
their relative specific activities, 6.2 and 6.5, respectively, were higher than
those previously reported for either mammalian [3,4]or insect tissues [12].
The NADPH-cytochrome c reductase activities were several hundred times
as high as the p-nitroanisole 0-demethylase activities. Thus, NADPH-cytochrome c reductase can be measured more easily and more economically
than 0-demethylase. The greater stability also makes the reductase the enzyme of choice.
The p-nitroanisole 0-demethylase activities reported here are similar to
those reported by Tate et a1 [30] for the same tissue and species, but the
NADPH-cytochrome c reductase activities we report are substantially higher.
The high relative specific activities of NADPH-cytochrome c reductase and
pnitroanisole 0-demethylase in microsomes, the close correlation of their
activities in subcellular fractions, and the good agreement between biochemical and electron-microscopic observations support the conclusion that these
two enzymes are reliable microsomal marker enzymes in the midgut of M
sextu and do not occur at substantial levels in other subcellular components.
320
Weirich and Adam
ACKNOWLEDGMENTS
We thaAk Kathryn J. Green and Daniel J. Scholfield for their competent
technical assistance, Dr Jan P. Kochansky for purifying p-nitroanisole, and
Drs Jeffrey R. Aldrich, Mark F. Feldlaufer, and Raziel S. Hakim for valuable
suggestions during the preparation of the manuscript.
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