Structural studies on lipophorin, an insect lipoprotein.

An insect high density lipoprotein, lipophorin, can be rapidly isolated from larval Manduca sexta (tobacco hornworm) hemolymph by single vertical spin density gradient ultracentrifugation. The two apolipoproteins (Mr = 245,000 and 78,000; designated apoLp-I and apoLp-II, respectively) were readily dissociated and separated in 6 M guanidine HCl by gel permeation chromatography. ApoLp-I and apoLp-II showed no immunological cross-reactivity on electrophoretic blots of sodium dodecyl sulfate-polyacrylamide gels. ApoLp-I and apoLp-II from lipophorin of adult M. sexta behaved identically to their larval counterparts. Amino acid compositions of larval apoLp-I and apoLp-II were similar except with respect to tryptophan and cysteine; apoLp-I contained 32 residues/mol of tryptophan (1.5 mol%) and 22 residues/mol (1.1 mol%) of cysteine; apoLp-II contained 2 residues/mol of tryptophan (0.2 mol%) and 14 residues/mol of cysteine (2.1 mol%). In double immunodiffusion tests, antiserum against apoLp-I or whole lipophorin strongly precipitated lipophorin, while antiserum against apoLp-II caused only minor precipitation. This indicates relatively greater exposure of apoLp-I to the aqueous environment.