Purification and properties of a very high density lipoprotein from the hemolymph of the honeybee Apis mellifera

A larval-specific very high density lipoprotein (VHDL) has been isolated from the hemolymph of the honeybee Apis mellifera. VHDL was isolated by a combination of density gradient ultracentrifugation and gel filtration. The purified protein is a dimer of Mr 160,000 apoproteins as shown by chemical cross-linking with dimethyl suberimidate. N-Terminal sequence analysis indicates that the two polypeptide chains are identical. The holoprotein contains 10% lipid by weight and 2.6% covalently bound carbohydrate. A native Mr 330,000 species was obtained by gel permeation chromatography. Antiserum directed against VHDL was used to show that VHDL is distinct from other hemolymph proteins and appears to constitute a novel lipoprotein of unknown function. However, the lipoprotein is present in high amounts in hemolymph only at the end of larval life, suggesting a potential role in lipid transport and/or storage protein metabolism during metamorphosis.

The role of apolipophorin III in in vivo lipoprotein interconversions in adult Manduca sexta

Sustained flight in the moth, Manduca sexta, necessitates lipid mobilization and transport to flight muscle, a process mediated by the adipokinetic hormone. An adult specific high density lipophorin (lipoprotein, HDLp-A, Mr = 7.68 X 10(5)) accepts diacylglycerol from the fat body, increasing in size and decreasing in density, to give a low density lipophorin (lipoprotein, LDLp, Mr = 1.56 X 10(6)). During this process, several molecules of the small apolipoprotein, apolipophorin III (apoLp-III), are added to the two molecules originally present in HDLp-A. A study of the time course of adipokinetic hormone-induced loading of diacylglycerol onto HDLp-A, using the analytical ultracentrifuge and gel filtration, suggests that a lipoprotein of density intermediate between HDLp-A and LDLp was formed transiently. Analysis of lipoproteins separated by density gradient ultracentrifugation in the course of the loading process indicates that apoLp-III is added more rapidly than diacylglycerol and that it changes its conformation on the surface as more diacylglycerol is added. Taken together with the known properties of apoLp-III, a prolate ellipsoid with an axial ratio of 3, we suggest that initially apoLp-III adds to the expanded hydrophobic surface of the lipoprotein with its short axis parallel to the surface and that apoLp-III subsequently unfolds to cover a greater area of hydrophobic surface. Exchange experiments with labeled apoLp-III showed that the two apoLp-III molecules in HDLp-A do not exchange with free apoLp-III, even when the lipoprotein passed through a loading and unloading cycle, suggesting a structural role for apoLp-III in HDLp-A.

Purification of very high density lipoproteins by differential density gradient ultracentrifugation

Differential density gradient ultracentrifugation procedures, utilizing a vertical rotor, were developed for the preparative purification of very high density lipoproteins (VHDL, density greater than 1.21 g/ml). The VHDLs of several insect species were purified as follows. An initial density gradient ultracentrifugation step removed lipoproteins of lower density from the VHDL-fraction, which partially separated from the nonlipoproteins present in the infranatant. A complete separation was achieved by a second centrifugation step employing a modified gradient system. The use of a vertical rotor and specially designed discontinuous gradients allows a relatively fast, efficient, and economical isolation of the class of very high density lipoproteins. Similar gradient systems should be useful for the detection and purification of VHDLs from other sources.

Lipid transfer protein from Manduca sexta hemolymph

A hemolymph lipid transfer protein (LTP) was isolated from the tobacco hornworm, Manduca sexta. LTP catalyzes net lipid transfer between isolated hemolymph lipoproteins in vitro. An isolation procedure employing density gradient ultracentrifugation and gel permeation chromatography produced a purified protein. LTP is a very high density lipoprotein with a particle Mr greater than 500,000. Sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed that LTP is comprised of two apoproteins: apoLTP-I (Mr approximately 320,000) and apoLTP-II (Mr approximately 85,000). LTP may have a physiological role in altering the lipid content and composition of the major hemolymph lipoprotein, lipophorin.

Lipoprotein interconversions in an insect, Manduca sexta. Evidence for a lipid transfer factor in the hemolymph

Hemolymph lipoproteins (lipophorins) of adult Manduca sexta are disinct from larval forms in density, lipid content, composition, and the presence of a third, low molecular weight apoprotein. Generally, only one lipoprotein species exists in M. sexta hemolymph during any given life stage. Progression through the life cycle results in alterations of existing lipoproteins to produce new forms, without new protein synthesis. The observed alterations in lipoprotein density could result from facilitated lipid transfer in insect hemolymph. An in vitro assay of facilitated lipid transfer was developed which employs a high density lipophorin from the wandering larva (density = 1.18 g/ml) as acceptor and adult low density lipophorin (density = 1.03 g/ml) as donor. Adult lipophorin-deficient hemolymph was shown to catalyze a time-dependent equilibration of the starting lipoproteins to produce a new intermediate lipophorin, Lp-I. Hydrodynamic experiments on the donor, acceptor, and product lipoproteins excluded fusion as the mechanism whereby Lp-I is produced. Thus, it is concluded that Lp-I results from facilitated net lipid transfer from low to high density lipoprotein. Furthermore, experiments conducted with radioiodinated donor and radioiodinated acceptor lipoproteins demonstrated that apoprotein exchange does not occur during the lipid transfer reaction. When donor lipoprotein was labeled in the lipid moiety with carbon-14, evidence of diacylglycerol and phospholipid exchange was obtained. Partial characterization of the lipid transfer factor revealed a relationship between incubation time, donor concentration, acceptor concentration, lipophorin-deficient hemolymph concentration, and transfer activity, as measured by Lp-I production. It is concluded that lipophorin-deficient hemolymph contains one or more factor(s) that catalyze net lipid transfer as well as diacylglycerol and phospholipid exchange between lipophorins to produce a single form at equilibrium.

Changes in lipoprotein composition during larval-pupal metamorphosis of an insect, Manduca sexta

During the transition from the last feeding larval stage to the pupal stage of the tobacco hornworm, Manduca sexta, significant changes occur in the properties of lipophorin, the major hemolymph lipoprotein. Within the first 24 h after cessation of feeding, the larval lipophorin (HDLp-L) is first converted to a higher density form (HDLp-W2) and then HDLp-W2 is converted to a lower density form (HDLp-W1). HDLp-W1 remains in the hemolymph until pupation, when another form, HDLp-P, with a density between HDLp-W1 and HDLp-L, is present. Although all the lipophorins contain identical apoproteins, they differ in lipid content and composition; the differences in density being primarily related to diacylglycerol content. The conversion of HDLp-L to HDLp-W1 is accompanied by a loss of hydrocarbon and uptake of carotenes. These latter changes in lipophorin composition reflect alterations in cuticular lipid composition. HDLp-L was radiolabeled in the apoproteins by injecting animals with 3H-amino acids early in the last larval stage. Subsequently HDLp-L was isolated at the end of the larval stage, HDLp-W2 and HDLp-W1 were isolated during the wandering stage, and HDLp-P was isolated after pupation. The specific activity of the apoproteins in the four lipophorins was not significantly different, suggesting that the observed alterations in lipophorin properties do not require synthesis of new apoproteins but result from retailoring the lipid composition of preexisting molecules. Examination of the hemolymph of individual animals during these transitions showed that only one species of lipoprotein was present, never a mixture of two or more species. These observations suggest that the lipoprotein conversions are precisely timed and that lipoprotein metabolism during larval development and pupation cannot be considered a static process. The unique finding of these studies was that synthesis of lipophorin apoproteins proceeds actively during the first part of the fifth instar but then ceases and does not recommence during the wandering or early pupal stages.

Arylphorin from Manduca sexta: carbohydrate structure and immunological studies

The major hemolymph protein in the last larval stage of Manduca sexta is a hexameric glycoprotein, arylphorin (Mr = 450,000). Sodium dodecyl sulfate polyacrylamide gel electrophoresis of purified arylphorin reveals the presence of two subunits, A1 and A2. Both subunits are glycosylated and have apparent Mr = 77,000 and 72,000, respectively. Pronase digestion of arylphorin yielded a single major glycopeptide. 250 MHz NMR spectroscopy of arylphorin glycopeptide revealed a Man9GlcNAc2 oligosaccharide structure similar to that observed in mammalian glycoproteins. Endoglycosidase-H treatment of arylphorin was employed to remove covalently linked carbohydrate residues. The carbohydrate removal lowered the apparent Mr of subunits A1 and A2 to 72,000 and 69,000, respectively, indicating that the difference in arylphorin subunit size is not due to levels of glycosylation. Immunoblotting experiments with anti-arylphorin antiserum and Bombyx mori storage proteins indicated cross reactivity with the corresponding arylphorin of this insect. Preparation of subunit A2 monospecific antibodies, followed by immunoblotting of arylphorin showed a close immunological relationship between subunits A1 and A2.

Lipophorin of the larval honeybee, Apis mellifera L

Most insects have a major lipoprotein species in the blood (hemolymph) that serves to transport fat from the midgut to the storage depots in fat body cells and from the fat body to peripheral tissues. The generic name lipophorin is used for this lipoprotein. In larvae of the honeybee, Apis mellifera, a lipophorin has been found with properties that correlate well with those of the only other lipophorin reported for an immature insect, that of the tobacco hornworm, Manduca sexta. The honeybee lipophorin (Mr = 530,000) has a density of 1.13 g/ml, contains approximately 41% lipid and 59% protein, and contains two apoproteins, apoLp-I, Mr = 250,000 and apoLp-II, Mr = 80,000, both of which are glycosylated. The lipids consist predominantly of polar lipids, of which phospholipids and diacylglycerols represent 60% of the total. When the intact lipophorin is treated with trypsin, apoLp-I is rapidly proteolyzed, while apoLp-II is resistant, indicating a difference in exposure of the two apoproteins to the aqueous environment. Honeybee apoLp-II cross-reacts with antibodies to M. sexta apoLp-II, but not to anti-M. sexta apoLp-I. No cross-reactivity of honeybee apoLp-I to anti-M. sexta apoLp-I was observed.

Purification and properties of a predominantly female-specific protein from the hemolymph of the larva of the tobacco hornworm, Manduca sexta

A major serum protein was isolated from the hemolymph of larvae of the female tobacco hornworm, Manduca sexta, just prior to metamorphosis. After 3 or 4 days, this predominantly female-specific protein is rapidly cleared from the hemolymph and taken up and stored by the fat body. This larval serum protein was purified by density gradient ultracentrifugation, gel permeation, and ion-exchange chromatography. The purified protein exhibits a single band on native gel electrophoresis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Chemical cross-linking with dimethylsuberimidate indicates a hexameric subunit arrangement for the native protein. The amino acid composition, relatively rich in methionine but poor in cysteine, was used to calculate a v = 0.75 cm3/g. Analytical ultracentrifugation experiments yielded S020,w = 16.9 S and D020,w = 3.23 X 10(-7) cm2/s. From these values Mr = 510,000, f/f0 = 1.22, and Stokes radius = 66.3 A were calculated. Immunoblotting experiments with anti-larval serum protein serum indicate a cross-reactivity with storage protein-1 of Bombyx mori. The amino acid composition and immunological data suggest that larval serum protein may be an example of a class of insect storage proteins distinct from the arylphorins, which are characterized by high content of aromatic amino acids.

Insect apolipophorin III. Purification and properties

The hemolymph of adult Manduca sexta (tobacco hornworm) contains a 17,000-dalton protein that can associate reversibly with the insect lipoprotein lipophorin. The protein is abundant in the hemolymph of the adult, but is found in larval hemolymph in only small amounts, and does not associate with larval lipophorin. On the basis of its association with adult lipophorin, we have designated the protein apolipophorin III. Apolipophorin III was dissociated from adult lipophorin by guanidinium chloride treatment and isolated by gel permeation and ion exchange chromatography. The unassociated apolipophorin III was also purified from lipophorin-free hemolymph by gel permeation, ion exchange, and lectin chromatography. Both preparations have identical isoelectric points and amino acid composition as well as the following properties. Apolipophorin III is a non-glycosylated polypeptide lacking cysteine and tryptophan. The 17,000-dalton polypeptide dimerizes in solution to a protein of Mr = 34,000.

The amount and distribution of water, dry matter, and sugars in the digestive tract of rats fed xanthan gum

Diets containing a nutritionally adequate, high-maltose nutrient mixture and either 4% xanthan gum or 4% cellulose were fed ad libitum to rats. The feeding of this gum increased the combined weight of the small intestine and its contents by 110%. This effect was partially due to an enlarged intestinal cell mass and to extra dry matter in the contents but chiefly to a 400% increase in intraluminal water. Xanthan feeding enhanced greatly the persistence of sugars beyond the proximal quarter of the small intestine and increased their total recovery in the first three quarters of that organ by 150%. The xanthan-induced increase in intraluminal water in the small intestine was partially due to a slowed absorption of osmotically active substances from the gut.

Gastric emptying of glucose and nutrient energy in meal-fed rats

Male rats (designated "meal-feeders") were trained for at least 21 days to eat voraciously for 2 hour a day a high-glucose, nutritionally adequate diet, while "nibblers" were fed the same diet ad libitum. In experiment 1, a second control group received the same amount of diet as eaten by meal-feeders but in four meals per day. Final test meals comprised either 10g per (kilogram body weight)3/4 or all that the animal would eat in a 2-hour period. With both sizes of test meal, gastric emptying of fat and/or glucose was greater in meal-feeders than in either control groups. In experiment 2, gastric emptying of glucose and fat was measured in meal-feeders and nibblers during three intervals of the dark period and during the 12-hour light period. An unusually rapid gastric emptying of glucose and of metabolic energy was observed in meal-feeders but was confined to the early period after the start of the daily meal. Glucose was emptied preferentially to fat under all test conditions. This tendency was prominent, however, only when a small meal was fed or after stomach contents had declined.