Characterisation of heterologous and homologous low-density lipoprotein binding to apolipoprotein B,E receptors on porcine adrenal cortex membranes: enhanced binding of trypsin-modified human low-density lipoprotein

The characteristics of the binding of homologous and heterologous (human) LDL to membrane preparations from porcine adrenal cortex have been determined. The membranes displayed a single class of high-affinity, saturable binding site for both 125I-labelled porcine and human LDL, which was dependent on divalent cations, in addition to a low-affinity, non-saturable component(s). Porcine LDL displaced both 125I-labelled porcine and 125I-labelled human LDLs from the high-affinity binding site more effectively than human LDL, reflecting the lower Kd, (13.2 micrograms/ml) for porcine than human (Kd 19.2 micrograms/ml) LDL. These values are comparable to those obtained for half-maximal binding of human and bovine LDLs in a bovine adrenocortical membrane system (Kovanen, P.T., Basu, S.K., Goldstein, J.L. and Brown, M.S. (1979) Endocrinology 104, 610-616). Tryptic modification of porcine LDL (T-LDL) diminished its ability to compete with 125I-labelled native LDL for the high-affinity binding site; in contrast, 125I-labelled porcine T-LDL showed an elevated receptor affinity (Kd 9.7 micrograms/ml) and was more efficiently displaced by its unlabelled counterpart than by native porcine LDL. Tryptic treatment of human LDL similarly increased its binding affinity (Kd 8.3 micrograms/ml), although in this case, the unlabelled T-LDL displaced not only 125I-labelled human T-LDL but also 125I-labelled human LDL from the high-affinity site more effectively than native LDL. We conclude that (i) porcine adrenocortical membranes possess binding sites specific for LDL and resembling the apolipoprotein B,E receptors already demonstrated in murine, bovine and human adrenal cortex; (ii) tryptic modification of porcine LDL may remove or destroy segments of apolipoprotein B100 which contribute to receptor recognition sites on the surface of the particle; (iii) trypsinised porcine LDL may interact with the membrane binding site by a mechanism differing from that by which native LDL binds, and (iv) trypsinisation of human LDL may cleave or remove species-specific segments of the B100 protein at or close to the receptor recognition site(s) on the particle, thus decreasing structural differences between porcine and human LDL, and thereby enhancing its binding affinity for the porcine receptor.