Uptake of the major hemolymph lipoprotein and its transformation in the insect egg.

Abstract

The egg of Manduca sexta contains a very high density lipophorin (VHDLp-E; Mr approximately equal to 4.14 x 10(5), rho = 1.238 g/ml) that is derived from the high density lipophorin (HDLp-A; Mr approximately equal to 7.63 x 10(5), rho = 1.076 g/ml) of the hemolymph. The selective uptake of HDLp-A into the egg and its subsequent conversion to VHDLp-E was studied both in vivo and in vitro. Upon entering the egg, an estimated 530 mol of lipid were stripped from each mol of HDLp-A, and 68% of the diacylglycerol fraction was converted to triacylglycerol. In addition, the two molecules of the low molecular weight apolipoprotein, apolipophorin-III, of HDLp-A were dissociated from the lipophorin particle. The VHDLp-E thus formed consisted of 80% protein and 20% lipid, 75% of which was phospholipid. HDLp-A labeled in vivo with [35S]methionine in its apoprotein moiety was injected into females at the onset of egg development, and its incorporation in a series of follicles at different stages of growth was measured. There was increased accumulation of [35S]HDLp-A in the follicles as they matured. The apoproteins of [35S]HDLp-A were not hydrolyzed when the particle was internalized by the follicle. In the accompanying paper we have presented the evidence that the apoproteins of HDLp-A are retained in the follicles (Kawooya, J.K., and Law, J.H. (1988) J. Biol. Chem. 263, 8748-8753).