Binding of HDL to basolateral membranes of the renal cortex. Evidence for two components in the HDL-membrane association

Abstract

The binding of porcine 125I-HDL to purified basolateral membrane fractions isolated from pig kidney cortex displays two categories of sites, one with high affinity (( K d = (3.0 ± 0.7) · 10 −9M ) and low capacity ( B max = 52 ± 32 ng/mg proteins) another with low affinity ( K d = (5.3 ± 0.7) · 10 −8M ) but a higher capacity ( B max = 795 ± 115 ng/mg proteins). Binding was competitively inhibited to the same extent by unlabeled HDL from swine, human or rat, demonstrating an absence of species specificity. Porcine LDL partially competed for binding even in the presence of 30 mM EDTA which prevents apo B/E specific binding. Membrane proteins solubilized with CHAPS were analyzed by electrophoresis followed by ligand blotting using porcine 125I-HDL and 125I-apoAI-HDL to show that HDL bound to two proteins of respective molecular masses 120 ± 2 and 95 ± 9 kDa. 125I-apoAI associated mostly with the 95 kDa protein. A 100-fold excess of unlabeled HDL greatly decreased binding to the 95 kDa protein but less to the 120 kDa protein. We conclude that part of HDL binding occurs through the lipid moiety, while another is the result of a specific interaction between apoAI and a membrane protein of 95 kDa.