Role of lipid transfer particle in transformation of lipophorin in insect oocytes.
Oocyte development in Manduca sexta involves the deposition of large amounts of lipoprotein-derived lipid. One source of this lipid is hemolymph high-density lipophorin-adult (HDLp-A) which, upon entry into the oocyte, is transformed into a lipid and apolipophorin III deficient product particle, egg very-high- density lipophorin (VHDLp-E; density = 1.24 g/ml; Kawooya et al. (1988) J. Biol. Chem. 263, 8740-8747). An in vitro model of this transformation has been established using human low-density lipoprotein (LDL) as acceptor of HDLp-A associated lipid in a reaction catalyzed by isolated M. sexta hemolymph lipid transfer particle (LTP). Facilitated vectorial net transfer of lipid from HDLp-A to LDL resulted in formation of a very-high-density lipophorin (VHDLp) product with a density and apolipoprotein content similar to that of VHDLp-E. Lipid was found to comprise 25% of the VHDLp particle mass, whereas over 50% of HDLp-A mass is lipid. Based on these observations it was hypothesized that a lipid transfer factor may be present in M. sexta oocytes and function in the transformation of HDLp-A to VHDLp-E in vivo. Transfer activity was present in the buffer soluble fraction of oocyte homogenates and purification of the active material revealed a catalyst with electrophoretic and immunological properties identical to hemolymph LTP. Incubation of 125I-HDLp-A with an M. sexta oocyte homogenate resulted in transformation of the radiolabeled lipoprotein to a density corresponding to that of VHDLp-E. When the incubation media was preincubated with anti-LTP IgG this conversion was inhibited to a large extent. Inhibition was relieved, however, by addition of exogenous LTP. The results provide the first demonstration of M. sexta LTP in a tissue other than hemolymph and support the concept that LTP-catalyzed lipid transfer plays an integral role in the conversion of HDLp-A to VHDLp-E in vivo.
Liu
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Department of Biochemistry, University of Alberta, Edmonton, Canada.
Ryan
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D000818:Animals; D002352:Carrier Proteins / Q000302:isolation & purification / Q000378:metabolism* / Q000502:physiology*; D002384:Catalysis; D004591:Electrophoresis, Polyacrylamide Gel; D006458:Hemolymph / Q000276:immunology / Q000378:metabolism; D006801:Humans; D005779:Immunodiffusion; D007074:Immunoglobulin G / Q000276:immunology; D008074:Lipoproteins; D008075:Lipoproteins, HDL / Q000302:isolation & purification / Q000378:metabolism; D009036:Moths; D010063:Ovum / Q000378:metabolism*
D002352:Carrier Proteins; D007074:Immunoglobulin G; D008074:Lipoproteins; D008075:Lipoproteins, HDL; C032830:lipophorin