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Lipophorin in the larval and adult stages of Musca domestica.

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Archives of Insect Biochemistry and Physiology 6:39-48 (1987)
Lipophorin in the Larval and Adult Stages of
Musca dornestica
Antonio G. de Bianchi, Margareth de L. Capurro, and Osvaldo Marinotti
Departamento de Bioquimica, lnstituto de Quimica, Universidadede Sflo Paaulo, Brasil
The major Mosca domestica hemolymph lipoprotein, lipophorin, was purified
from larval and from adult animals. The housefly lipophorin is composed of
two apoproteins, apolipophorin I (M, 253,000) and apolipophorin II (M,
85,000). The lipophorin contains about 3.9% carbohydrates and reacts
positively with concanavalin A. The density of larval lipophorin is equal t o
1.152 glml and of adult lipophorin to 1.106 glml. The amount of lipophorin
per animal increases during the larval stage, is constant during pupal stage,
and suffers a great reduction at the pharate adult stage. The amount of
lipophorin remains stable during the whole first gonotrophic cycle of the
housefly. Lipophorin is not detected in the eggs of this insect.
Key words: housefly, hemolymph proteins, metamorphosis
Lipophorin, the major lipoprotein of insect hemolymph, is involved in
lipid transport between the sites of absorption, storage, and utilization [1,2].
The lipophorin of several insects have been isolated and analyzed in the
recent past [3-61. In general, the lipophorins are composed of two types of
apoprotein, ApoLpI" and ApoLpII, with relative molecular weights of about
250,000 and 78,000, respectively [6]. In some insects, a third apoprotein has
been found in the adult lipophorin. This apoprotein, ApoLpIII, has a molecular weight of about 17,000 [6-81.
*Abbreviations: ApoLp = apolipophorin; PAS = periodate-Schiff reagent; PTC = phenylthiocarbamide; SDS-PAGE = sodium dodecylsulfate polyacrylamidegel electrophoresis; TLCK =
Acknowledgments: This work was supported by grants from the FundaC%ode Amparo B
Pesquisa do Estado de SPo Paulo (FAPESP), Financiadora de Estudos e Projetos (FINEP),
ConvCnio No. 43.84.0725.00. M.deL.C. i s a graduate fellow and O.M. a postdoctoral fellow of
FAPESP. A.G.deB. is a staff member of the Biochemistry Department and a research fellow
from CNPq.
Received August 7, 1986; accepted March 16,1987.
Address reprint requests to A.G. de Bianchi, Departamento de Bioquimica, lnstituto de
Quimica, Universidade de S%oPaulo, C.P. 20780,01498, S%oPaulo, SP, Brasil.
0 1987 Alan R. Liss, Inc.
de Bianchi, de L. Capurro, and Marinotti
The association of ApoLpIII with lipophorin is induced by the presence of
adipokinetic hormone in the hemolymph, increasing the lipophorin lipidcarrying capacity [9,10]. ApoLpIII was found, until now, only in insects of
the orders Lepidoptera, Hemiptera, and Orthoptera [6,7j. In addition to the
ApoLpIII association with adult lipophorin, several stage-specific forms of
this protein were recognized in Munducu sextu. These forms differ in their
densities, and these differences are due to variation in lipid contents and
composition [ll].
In adult Surcophugu bulZutu, the lipophorin is composed of ApoLpI and
ApoLpII and has a density of 1.12g/ml[6]. These are the only available data
about lipophorin of dipteran insects. To obtain further information about
dipteran lipophorins, we isolated and analyzed the Muscu dornestica lipophorin. In this paper, we also describe some aspects of qualitative and
quantitative variations of this protein during the housefly life cycle.
M . dornesticu of a wild strain were reared at 25°C as described by Targa
and Peres [12].
Hemolymph, Fat Body, and Ovary
Larval hemolymph was collected by puncturing the anterior end of the
animals. The hemolymph was pooled in centrifuge tubes containing 0.1 M
NaH2P04-NaOH,pH 7.0, 0.15 M NaC1, 5 mM EDTA, 1mM benzamidine,
0.5 mM PTC, and 1mM TLCK (buffer A). The diluted hemolymph (1:l)was
centrifuged at 3,0008 for 10 min at 4"C, and the supernatant was used for the
experiments. Adult hemolymph was obtained from C02-anesthetized flies.
An abdominal incision was made in each fly with microscissors, and 5 pl of
buffer A was used to wash out the hemolymph from the abdominal cavity.
The diluted hemolymph was pooled in centrifuge tubes and centrifuged
under the conditions described above. The ovaries and fat bodies were
dissected out in 0.15 M NaC1.
Isolation of Lipophorin
Lipophorin was isolated by a modification of the density-gradient ultracentrifugation method described by Shapiro et al. [13]. Hemolymph (2.3 ml)
diluted in buffer A containing 3.7 M KBr was put into ultracentrifuge tubes
and overlayed with 2.3 ml of buffer A. The tubes were centrifuged for 16 h
at 96,0008 and 4°C in a Spinco SW50.1 rotor. The lipophorin, visible as a
yellow band, was removed by inserting a syringe and withdrawing the
contents. The density of lipophorin was determined from the refractive index
of KBr of each sample at 25°C.
Larval lipophorin isolated under these conditions is contaminated with
low-molecular-weight polypeptides (see Results) and needs a glycerol-gradient ultracentrifugation step to separate it from these contaminants. A
sample of partially purified lipophorin was laid on the top of a linear glycerol-
Musca dornestica Lipophorin
gradient (5-30%) and centrifuged for 16 h at 96,OOOg at 4°C in a Spinco
SW50.1 rotor. The lipophorin was collected as described above for the KBr
Polyacrylamide Gel Electrophoresis
SDS-PAGE was carried out according to the procedure of Laemmli [14]. A
5-12% acrylamide gradient gel slab (9 cm) with a 1cm stacking gel was used.
Electrophoresis was performed at a constant voltage of 7.8 Vlcm until the
tracking dye reached the bottom of gels. Native electrophoresis was performed in 4% polyacrylamide gels according to the system described by
Davis 1151.
Gels were stained for protein with Coomassie blue R [16]. The staining for
glycoproteins was done with PAS [17I and for lipoproteins with Sudan black
[18]. Protein standards, for molecular weight determinations by the method
of Lambin et al [19], were purchased from Sigma Chemical Co, (St. Louis,
MO) and Bio-Rad Laboratories (Richmond, CA).
About 0.6 mg of purified larval lipophorin was dissolved in 0.2 ml of 0.1
M NaH2P04-NaOHbuffer, pH 7.0, 0.15 M NaCl and emulsified with 0.5 ml
of Freund’s complete adjuvant. This emulsion was injected subcutaneously,
at multiple sites, into a rabbit. After 7 days, a similar injection was administered but with Freund’s incomplete adjuvant. Seven days after the last
injection, the rabbit was bled and 0.01% (wh) NaN3 was added to the serum
obtained before storage at -20°C.
Double i ~ u n o d i f f u s i o nwas carried out according to the method of
Ouchterlony [20] using 1%noble agar in 0.10 M NaH2P04-NaOHbuffer, pH
7.0, 0.3 M NaC1, 0.5 mM PTC, 5 mM EDTA, and 0.01% (wh) NaN3. Radial
immunodiffusion [21] and rocket immunoelectrophoresis [Z]were used to
estimate the lipophorin amount during the Musca life cycle. For radial immunodiffusion, samples of soluble proteins (see below), were applied to 3mm-diameter wells hollowed out in a 1-mm-thick slab of 1.5%noble agar in
0.1 M NaH2PO4-NaOH buffer, phf 7.0, 0.3 M NaCI, 0.05 m.M PTC, 0.01%
(wfv) NaN3, and 1%(vh) antilipophorin serum. The rocket immunoelectrophoresis was run using 1.0% (wiv) agarose Type I (Sigma Chemical Co.) in
30 mM Tris, 83 mM sodium acetate, pH 8.6, O.OlO/c (wiv) NaN3. The serum
antilipophorin was used at a concentration of 0.5%. The run was at 4.3 Vlcm
for 16 h at room temperature.
Quantificationof Lipophorin During Musca Life Cycle
Animals were homogenized in buffer A (20 mg wet weight per 0.1 ml of
buffer) in a Potter-Elvehjem homogeneizer. The homogenates were centrifuged at 11,OOOg for 10 min at 4°C. The supernatants (soluble proteins) were
used for lipophorin quantification by radial immunodiffusion.
Protein and Carbohydrate Determinations
Protein was determined by the Coomassie blue method [23] using bovine
serum albumin as standard. Carbohydrate was determined by the method of
de Bianchi, de 1. Capurro, and Marinotti
Dubois et al. [24] using glucose as standard. To determine the contents of
carbohydrate of purified lipophorin, protein was estimated by the method of
Ellman [25].
Lipophorin from the hemolymph of adults and larva of M. domestica was
isolated as described in Materials and Methods. The purity of the lipophorin
preparations was assessed by SDS-PAGE (Fig. 1A). After the KBr densitygradient ultracentrifugation, the larval Iipophorin is contaminated with lowmolecular-weight polypeptides (Fig. 1A) that are removed by the glycerolgradient ultracentrifugation step (Fig. 1A). The adult lipophorin obtained is
free of contaminants after the one-step KBr density-gradient ultracentrifugation (Fig. 1A).
The purified larval lipophorin of M. domestica, when analyzed by native
electrophoresis, gives a very well defined band that is stained by PAS and
Sudan black (Fig. 1B). Under these conditions, the lipophorin shows some
aggregation as evidenced by the protein band retained at the top of the
running gel (Fig. 1B). When this retained band is cut out of the polyacrylamide cylinder and analyzed by SDS-PAGE, only lipophorin subunits are
detected (results not shown).
The M. dumestica lipophorin (from adult and larva) is composed of two
types of apoprotein, ApoLpI and ApoLpII, with relative molecular weights
of 253,000 f 6,000 and 85,000 f 3,000, respectively (Fig. 2). The subunits are
dissociated by 0.1% (wh) SDS. The purified lipophorin has about 3.9%
Fig. 1. A SDS-PACE (5-12% acrylamide gradient slab) of Musca domestica lipophorin. Lane
1: Purified larval lipophorin after KBr gradient centrifugation. Lane 2: Purified larval lipophorin after glycerol gradient centrifugation. Lane 3: Purified adult lipophorin after KBr
gradient centrifugation. B: Native electrophoresis of purified larval M. domesfica lipophorin
in 4% polyacrylamide gels. Lane 1: Coomassie blue R staining. Lane 2: PAS staining. Lane 3:
Sudan black staining.
Musca domestica Lipophorin
Log x T
Fig. 2. Determination of the molecular weights of apolipophorin I and apolipophorin I I from
larval M. domestica lipophorin. The proteins were subjected to SDS-PAGE (5-12% acrylamide
gradient slab), and the molecular weights were estimated by the method of Lambin et al. [19].
The arrows indicate the molecular weights of ApoLpl and ApoLpll. The proteins used as
molecular weight markers were 1, thyroglobulin; 2, myosin; 3, P-galactosidase; 4, phosphorilase B; 5, bovine serum albumin; 6, ovalbumin; 7, carbonic anhydrase; and 8, soybean trypsin
inhibitor. The straight line was constructed using the least-squares method. The correlation
coefficient (r) was equal to 0.999.
0.7% of carbohydrates, and, since lipophorin reacts positively with concanavalin A (Fig. 3C), glucose andlor mannose must be associated with this
The densities of lipophorins of the several developmental stages of M .
domeslim are given in Table 1. The adult lipophorin shows a significantly
lower density than the lipophorin at other stages.
The serum antilipophorin is monospecific when tested by double immunodiffusion against larval and adult hemolymph (Fig. 3). In spite of the
difference in density values, the adult and larval lipophorins are immunologically identical (Fig. 3). The double immunodiffusion tests were unable to
demonstrate lipophorin presence in fat bodies, ovaries, and eggs (Fig. 3).
Analysis by rocket immunoelectrophoresis indicates that lipophorin, if present, must represent less than 0.02% of total egg protein (Fig. 4).
de Bianchi, de L, Capurro, and Marinotti
Fig. 3. A Ouchterlony immunodiffusion in 1%noble agar. Central well contained antilipophorin serum and the others contained 1 and 4, purified larval lipophorin; 2, adult hemalymph; and 3, larval hemolymph. B Ouchterlony immunodiffusion in 1%noble agar. Central
well contained a n t i l i ~ ~ h o serum
~ i n and the others contained 1 and 4, purified larval lipophorin; 2, egg soluble proteins: 3, soluble ovarian proteins; 5, soluble larval fat body proteins;
and 6, soluble adult fat body proteins. C Double diffusion of concanavalin A against purified
larval M. domestica lipophorin. Central well contained 20 pg of concanavalin A, and wells 1,
2, 3, 4,5 and 6 contained 20, 10,5,2,1, and 0.5 p g of purified lipophorin, respectively.
TABLE 1. Lipophorin Density Values During
Musca domestica Life Cycle
Density (gimlfa
Developmental stages
Larval (feeding)
Larval (wandering)
1.152 f 0.004
1.145 f 0.003
1.142 f 0.002
1.106 f 0.007
“Values are mean f SD (n = 3).
Lipophorin Quantification
The growth of M. ~ o ~ ebys wet
t ~weight
~ under our rearing conditions is
shown in Figure 5. After the beginning of the adult stage, the female flies
grow larger than males because of ovarian development.
The quantification by radial immunodiffusion of lipophorin in houseflies
during their life cycle is presented in Figure 6. The quantity of lipophorin
increases during the larval stage, remains at a high and relatively stable level
during all the pupal stage, and suffers a great reduction at the pharate adult
stage. The lipophorin level is stable during the entire first gonotrophic cycle
of the housefly. The reduction in lipophorin level at the pharate adult stage
is not due to a reduction in weight of the flies, which is stable in the pupaladult transition (Fig. 5).
The M . domesticrz lipophorin is composed of two types of apolipoproteins,
ApoLpI (N, 253,000) and ApoLPII (M, 85,000). The molecular weights
of the apolipophorins are reported to be about 250,000 for ApoLpI and 78,000
for ApoLpII across a wide spectrum of insect orders [6]. Some insects in the
adult stage show a lipophorin formed by three apoproteins [6]; in others, the
lipophorin is assembled only from two apolipoproteins, as is the case for
housefly lipophorin.
Musca dornestica Lipophorin
ad u lt
Age in days
Fig. 4. Rocket immunoelectrophoresis of lipophorin. Lipophorin was quantitated by rocket
immunoelectrophoresis as described in Materials and Methods. Samples of recently formed
pupa homogenates were applied to wells 1, 2, and 3. The samples contained 0.60, 0.56, and
0.68 pg of lipophorin, respectively. Well 4 contained egg homogenate (94 p g of total soluble
Fig. 5. M. domestica wet weight during development. Means & SD of three determinations
on groups of five to 20 animals. The age in days after egg laying i s indicated as are the larval,
pupal, and adult stages. S4, Sbr and S8 indicate the ovarian stages during the first gonotrophic
cycle according to Adams 1321. After emergence, the values for females ( 0 - - - - 0 )and males
are individualized.
de Bianchi, de 1. Capurro, and Marinotti
Age in d a y s
Fig. 6. Total lipophorin in M. dornestica during development. Each point is the mean k SD
of three determinations on groups of five to 20 animals. Lipophorin was quantitated by radial
immunodiffusion as described in Materials and Methods. The developmental stages are
indicated at the top. S4, S6, S,, and S,, indicate the ovarian stages during the first gonotrophic
and males
cycle according to Adams [32].After emergence, the values for females -I(.
are individualized.
The carbohydrate contents of housefly lipophorin (3.9%) are similar to
those described for Locustu rnigrutoria [2]. Covalently bound mannose-containing oligosaccharide chains are reported to occur in the lipophorin of
several insect orders 161 as demonstrated by positive reaction of lipophorin
with concanavalin A. We also reported the presence of this type of oligosaccharide chain in lipophorin from M . dornesticu larvae.
The densities of housefly lipophorin suggest that, during the feeding,
wandering, and pupal stages, the lipoprotein does not undergo major modifications in its lipid composition. On the other hand, the adult lipophorin
shows a lower density, suggesting a qualitative andlor quantitative modification in the lipids carried by the protein. Modifications in the densities of
Munducu sextu lipophorin during the life cycle of this insect were reported by
Sarvamangala et al. [ll]. The alteration of Munducu sextu lipophorin density
values were correlated with quantitative and qualitative modifications of the
lipid core of the protein [ll].The several forms of Munducu sextu lipophorin
are interconvertible by action of a lipid transfer protein [26,27]. The density
of housefly lipophorin (1.106 glml) is lower than that reported for Surcophugu
bullutu (1.12 glml) [6], the only density value for adult dipteran lipophorin
reported until now.
The quantification of lipophorin during the life cycle of the housefly shows
very interesting features. The results (Fig. 6) suggest that lipophorin is
synthesked in very significant amounts only during the larval stage. During
the pupal stage, the synthesis of lipophorin is probably at a low level,
because the amount of the protein is stable (Fig. 6), and the turnover of
Musca domestica Lipophorin
lipophorin is known to be very slow in other insects [28]. The very wellmarked time of reduction in the lipophorin level suggests a well-tuned
mechanism for protein degradation that is activated in the pharate adult
stage. The level of M. dornesficu storage protein is reduced at precisely the
same time as lipophorin 1291.
The relatively stable level of lipophorin contents during the gonotrophic
cycle indicates that the lipid mobilization for egg formation does not need a
higher level of this protein. A similar result was obtained for Locusfa rnigruforia [30].
The lack of detection of lipophorin in housefly eggs contrasts with the
presence of lipophorin in the eggs of several other insects [31]. Whether
these data indicate a difference in the mechanisms of lipid accumulation in
housefly oocytes in relation to other insects is not yet known.
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