Insect hemolymph lipophorin: a mechanism of lipid transport in insects

Lipophorin, formerly called the "diacylglycerol-carrying lipoprotein," exists in the hemolymph of many insects including locust, cockroach, and silkworm. A rapid and efficient method has been developed for the purification of lipophorin, which includes a specific precipitation under low ionic concentration and DEAE-cellulose column chromatography. The final preparation of lipophorin is highly homogeneous, as judged by gel electrophoresis, electronmicroscopy, and immunodiffusion. Molecules of lipophorin from the above three insects are almost globular in shape with a diameter of 13-16 nm. Molecular weights are 600,000-700,000, and the lipid content totals 40-50%. The lipids are comprised of diacylglycerol, cholesterol, and phospholipid. Of particular interest is that the locust and cockroach lipophorin contains large amounts of hydrocarbons in addition to the above lipids. Apoprotein of lipophorin consists of two-non-identical subunits, heavy chain (M.W. 250,000) and light chain (M.W. 85,000); carbohydrate (mainly mannose) is covalently associated only with the heavy chain. Tests of the physiological function of insect lipophorin indicate that it has multiple roles as a true carrier and a reusable shuttle in transporting diacylglycerol, cholesterol, and hydrocarbon from sites of storage, absorption, and synthesis to sites where these lipids are utilized as metabolic fuel, precursors of triacylglycerol and phospholipid synthesis, or structural components of cell membrane and cuticle. In view of this functional multiplicity as a true carrier and reusable shuttle, therefore, the insect lipophorin is unique lipoprotein and essentially different from mammalian plasma lipoproteins.

Ecdysteroids are bound to vitellin in newly laid eggs of Locusta

The follicle cells of vitellogenic ovaries of Locusta migratoria have been reported to synthesize impressive amounts of ecdysteroids which accumulate inside the oöcytes where they persist during egg-laying; these ecdysteroids are conjugated to more than 95%, and it is believed that their hydrolysis in the egg is the source of the peaks of free ecdysone observed in early embryonic development. The present paper shows that, in the eggs, the ecdysteroid conjugates are bound to a 520 000-dalton macromolecule which shares several characteristics with the major yolk protein vitellin and is precipitated by an anti-vitellin antibody. The physiological relevance of the binding of ovarian ecdysteroid conjugates is discussed in respect to the transfer of maternal ecdysteroids to the embryo.

In vitro incorporation of L-canavanine into vitellogenin of the fat body of the migratory locust Locusta migratoria migratorioides

L-Canavanine competes with L-arginine for incorporation into vitellogenin secreted in vitro by the fat body of the female locust Locusta migratoria migratorioides. Incorporation of L-[guanidinooxy-14C]canavanine into vitellogenin has been established unequivocally by combined arginase and urease hydrolyses of the acid hydrolysate of antibody-precipitated canavanyl vitellogenin. Continued exposure of the fat body to canavanine decreases in vitro protein secretion but the proportion of canavanyl vitellogenin to native vitellogenin increases. Canavanine-mediated inhibition of fat body protein secretion is dependent on both the canavanine concentration and the arginine retention by the fat body. Canavanine replaces about 10% of the arginyl residues of canavanyl vitellogenin. The electrophoretic mobility of canavanyl vitellogenin is greater than that of native vitellogenin but the ability of this aberrant protein to react with vitellogenin antibody is unimpaired.

Production of lipophorin in the fat body of adult Locusta migratoria: comparison with vitellogenin

The lipid-transport lipoprotein, lipophorin (Lp), an abundant plasma protein of insects, is synthesized in the fat body of Locusta migratoria as a polypeptide of relative mass (Mr) = 85 000. No posttranslational modification could be detected. Precipitation of this protein leads to formation of a product of Mr about 240 000, which fails to dissociate under treatment with sodium dodecyl sulfate and beta-mercaptoethanol. In adult female locusts, synthesis and secretion of protein by the fat body increases three- to four-fold from emergence until day 7 of adult life, Lp representing 15-30% of the total. In the subsequent vitellogenic stage (days 7-14 or later), a further impressive increase in protein output up to five times the previtellogenic rate is observed, followed by a decrease to the previtellogenic level at the end of the cycle. This fluctuation is mainly due to production of the yolk precursor protein, vitellogenin (Vg), which makes up some 60% of the secreted protein at the peak of the cycle. Production of Lp does not follow these dramatic changes. Inactivation of the corpora allata, the source of juvenile hormone (JH), by treatment with precocene, prevents the synthesis of Vg in females, and diminishes the production of other proteins, including Lp, in both sexes. Normal secretion patterns can be restored by application of the JH analog, methoprene. It is concluded that, whereas the synthesis of Vg is dependent upon and intensely stimulated by JH, the synthesis of Lp is stimulated only to a minor extent, in common with other proteins.

Biosynthesis of 3-hydroxy-3-methylglutaryl-CoA, 3-hydroxy-3-ethylglutaryl-CoA, mevalonate and homomevalonate by insect corpus allatum and mammalian hepatic tissues

Both radioactively labeled 3-hydroxy-3-methyglutarylcoenzyme A and its homolog 3-hydroxy-3-ethylglutarylcoenzyme A are produced by a cytosolic fraction obtained from corpora allata-corpora cardiaca complexes of the moth Manduca sexta, incubated with [1-14C]acetylcoenzyme A plus unlabeled propionylcoenzyme A. A particulate fraction isolated from the same tissue was able to reduce [Me-3H]hydroxymethylglutaryl-CoA and [3-14C]hydroxyethylglutaryl-CoA to mevalonate and homomevalonate, respectively, when NADPH was used as the electron donor. These reactions are in keeping with the proposed homoisoprenoid biosynthetic pathway for certain of the insect juvenile hormones. Measurement of products produced in picomole levels was accomplished by their conversion to ultraviolet absorbing derivatives which were subsequently analyzed by thin-layer chromatography and liquid chromatography followed by collection and liquid scintillation counting of appropriate fractions. Similar assays conducted with cell-free preparations from corpora allata of the grasshopper Schistocerca nitens, the mealworm beetle Tenebrio molitor, and from rat liver, species in which homoisoprenoid compounds have not been detected, also resulted in formation of the normal and homologous products (although no hydroxymethylglutaryl-CoA/hydroxyethylglutaryl-CoA reductase activity could be demonstrated in T. molitor). The latter results indicate a rather loose substrate specificity of the enzymes leading to mevalonate formation.

The in vivo effects of 3-deoxy-3-fluoro-D-glucose metabolism on respiration in Locusta migratoria

The basis of the toxicity of 3-deoxy-fluoro-D-glucose (3FG) in adult Locusta migratoria is examined in vivo by a radiorespirometric analysis of 14CO2 from the locust after injections of 3FG prior to injections of D-[1-14C]glucose, D-[6-14C]glucose, or [1-14C]acetate. The results indicate that 3FG metabolism irreversibly inhibits glycolysis and not the hexose monophosphate pathway or the tricarboxylic acid cycle. It is also established that during metabolism of 3FG fluoride ion is released. Evidence for the metabolism of 3FG in the whole insect as far as triosephosphate isomerase is based on 3H2O release after injections of D-[3-3H]3FG. Further support for the metabolism of 3FG to fluorinated sugar phosphates is provided by chromatographic and 19F MNR analysis of 3FG poisoned locust tissue extracts. Based on these results a biochemical mode of toxicity of 3FG in locusts is discussed.

Biosynthesis of N-acetyldopamine and N-acetyloctopamine by Schistocerca gregaria nervous tissue

N-Acetyltyramine, N-acetyldopamine and N-acetyloctopamine were the major products when either L-[3H]tyrosine or [3H]tyramine were incubated with thoracic ganglia of the desert locust, Schistocerca gregaria. No label was incorporated into L-DOPA under these conditions, although 2-3% of the radioactivity could be recovered in dopamine and octopamine. Addition of the aromatic amino acid decarboxylase inhibitor, 3-hydroxybenzylhydrazine (NSD 1015), prevented the formation of N-acetylcompounds from L-[3H]tyrosine, without resulting in an accumulation of label in L-DOPA. In contrast, incubation of samples of haemolymph with L-[3H]tyrosine resulted in the recovery of 7% of label in L-DOPA, which was increased to 17% in the presence of NSD 1015. These results provide evidence that the initial step in the synthesis of dopamine and octopamine by S. gregaria nervous tissue is the conversion of L-tyrosine to tyramine, which is subsequently metabolised to N-acetyltyramine, N-acetyldopamine or N-acetyloctopamine.

Formation of the A/B cis ring junction of ecdysteroids in the locust, Schistocerca gregaria

1. The mechanism of formation of the A/B cis ring junction of ecdysteroids in the locust Schistocerca gregaria, was investigated by incorporation of [4-14C,3 alpha-3H], [4-14C,4 alpha-3H] and [4-14C,4 beta-3H]cholesterol into 20-hydroxyecdysone in fifth-instar larvae and into ecdysteroid conjugates in ovaries of maturing adult females. 2. In both systems there was retention of the 4 alpha-3H atom in the ecdysteroid and elimination of the 3 alpha- and 4 beta-3H atoms. 3. The 3H retained in the ecdysone formed from [4 alpha-3H]cholesterol in the ovarian system was probably located at C-4. The results are interpreted by postulating the involvement of a 3-oxo-delta 4 intermediate in ecdysteroid biosynthesis in insects.

The preparation and characterization of locust vitellogenin messenger RNA and the synthesis of its complementary DNA

Poly(A)+ (polyadenylated) RNA was isolated from vitellogenic female-locus fat-body by LiCl/urea extraction and poly(U)-Sepharose 4B affinity chromatography. Agarose-gel electrophoresis of this poly(A)+ RNA under denaturing conditions shows the presence of a high-molecular-weight species (greater than 31 S, 7100 nucleotides) as the major species, which is absent from the RNA prepared from male-locust fat-body. Inclusion of this poly(A)+ RNA in a mRNA-dependent reticulocyte-lysate system directs the synthesis of polypeptides that could be immunoprecipitated with monospecific antibodies against locust egg vitellin. DNA complementary (cDNA) to the poly(A)+ RNA was synthesized, and back-hybridization of the cDNA to its template reveals a major abundant species comprising about 45% of the total poly(A)+ RNA hybridizing with R0t 1/2 of 2 x 10(-2) mol . litre-1 . s. Abundant cDNA isolated from the total cDNA hybridizes to poly(A)+ RNA with a R0t 1/2 of 9 x 10(-3) mol . litre-1 . s. There are 9.1 x 10(3) copies of vitellogenin mRNA per cell of vitellogenic female-locust fat-body, comprising 55% of the poly(A)+ RNA and equivalent to 0.7% of total cellular RNA.

Chiral 1,2-diacylglycerols in the haemolymph of the locust, Locusta migratoria

A 1H-NMR method using chiral shift reagents was applied in the stereochemical analysis of the haemolymph 1,2-diacylglycerols of Locusta migratoria. Conversion of the 1,2-diacylglycerols into 1,2-diacetyl-3-tritylglycerols allowed the accurate determination of the enantiomeric purity, whereas direct trimethylsilylation of the unmodified or hydrogenated haemolymph 1,2-diacylglycerols proved to be less suitable because of signal broadening. In the haemolymph of Locusta, sn-1,2-diacylglycerols with a remarkably high optical purity were found to be present. In the resting locust, at least 96% of the haemolymph 1,2-diacylglycerols have the sn-1,2-configuration, in locusts in which the haemolymph diacylglycerol concentration was elevated by fat body triacylglycerol mobilization induced by flight activity or injection of adipokinetic hormone, over 97% of the 1,2-diacylglycerols is the sn-1,2-enantiomer. The few percent sn-2,3-enantiomer may not have been present initially. Positional distribution of the fatty acids in the fat body triacylglycerols and in the haemolymph sn-1,2-diacylglycerols obtained from locusts after a 2 h flight revealed nearly identical occupation of the sn-2-positions in both acylglycerols. The distribution patterns in the sn-1-position of the 1,2-diacylglycerols and the combined sn-1 and sn-3 positions of the triacylglycerols are compatible with the possible existence of a stereospecific sn-3-triacylglycerol lipase.

Locust collagen: morphological and biochemical characterization

Segment-long-spacing crystallites and reconstituted fibrils have been made from collagen extracted from the ejaculatory duct of the adult male locust, Locusta migratoria. These show the same banding pattern after positive staining as segment-long-spacing crystallites and fibrils made from mammalian type I collagen. Native fibrils show the same periodic pattern as type I fibrils, but it is not so distinct. Biochemical analysis of pepsin-digested locust collagen shows that there are two collagenous components. The alpha chains of the major component are similar to mammalian alpha 1 (I) chains, except that the number of hydroxylysine residues is elevated and the CNBr peptides differ. There are no alpha 2 chains; hence this locust collagen molecule is an alpha 1 trimer. The second component, which is present in only minute quantities, may be a type IV basement membrane collagen. It is concluded that the fibrous collagen molecule of the locust is very similar to that of mammalian type I trimers and to those of other invertebrates which have been examined.

Metabolic fate of [14C]dieldrin in Schistocerca gregaria with particular reference to the nervous system

1. In the metabolism of [14C]dieldrin in Schistocerca gregaria, four metabolites of dieldrin were detected. Three of these have been identified as cis-aldrindiol, trans-aldrindiol and seco-aldrin dicarboxylic acid. 2. Apart from the formation of cis and trans-aldrindiol, another metabolite, M1, was detected in the nervous system. 3. The uptake of the dose of dieldrin was maximal in nerve cords followed by thoracic muscles and gut. Degradation of dieldrin was, however, not detected in thoracic muscles.

Glutamate uptake after stimulation-induced depletion of vesicle numbers in neuromuscular junctions of Locusta migratoria L

The distribution of radioactivity at branches and terminals of the fast axon in extensor tibiae muscle incubated in the radiolabelled putative neurotransmitter L-glutamate was determined by electron microscopic autoradiography. Quantitative analysis of the distribution of silver grains at the axon branches and terminals in preparations stimulated at a low frequency shows that most of the radioactivity is present in the glial cells. In preparations stimulated to the point of fatigue substantial radioactivity is present in both the glial cells and the axoplasm of the terminals. It is suggested that the uptake of L-glutamate into the axoplasm of the terminals is correlated with the depletion and recovery of vesicle numbers after stimulation.

Characteristics of the chitin-synthesizing system of insect tissue

Results obtained from isotopic dilution experiments are consistent with the operation of the chitin pathway as it has been established in fungal preparations. The last enzyme in this pathway, namely chitin synthase, seems to be accessible to substrate from the cell exterior, indicating its presence in the plasma membrane. It can, however, only be saturated (in a manner that partially excludes reaction with a competing substrate) from the cell side of the membrane.

Octopamine levels during the moult cycle and adult development in the migratory locust, Locusta migratoria

Octopamine content of the head of the locust Locusta migratoria has been determined during the last larval stage, moulting and adult life of 3 groups of insects: female and male gregarious, solitary and CO2 solitarized. An important difference was found between these 3 groups. Octopamine contents increased in the middle of the larval life and during the adult life. The moulting time is characterized by a sharp decrease of the octopamine content which becomes identical in the 3 groups of insects. The relation between octopamine content, hormone cycles and motility is discussed.

[Enzymatic N-acetylation of tryptamine by brain homogenates of Locusta migratoria before and after intoxication by chlordimeform or lindane (author's transl)]

Brain homogenates of Locusta migratoria are found to possess enzyme capable of catalyzing the N-acetylation of tryptamine. A main product of the enzymatic reaction is isolated and identified as N-acetyltryptamine by chromatography analyse. Both insecticides, chlordimeform and lindane, inhibit enzyme activity. A direct correlation between the degree of intoxication and acetylation of tryptamine is described.

[Synthesis of juvenile hormones in vitro by the corpora allata of 5th stage larva of Locusta migratoria migratorioides (R and F) (Insecta, Orthopteroida)]

Corpora allata of Locusta migratoria 5th stage larvae synthesize J.H.1, J.H.2 and J.H.3 in vitro. The C.A. of insects of different ages exbit different rates of J.H. synthesis. J.H.1 and J.H.2 synthesis is less than 1 ng/48 h/gland. During the same time the J.H.3 production may be as much as 25.6 ng/gland. J.H. synthetic activity is the same between right and left C.A. The release of J.H. from the C.A. occurs immediately following synthesis. These results are compared with in vivo haemolymphatic J.H. levels.