Binding affinity and reactivity of lecithin cholesterol acyltransferase with native lipoproteins

The first step in the reaction of lecithin cholesterol acyltransferase (LCA T) with lipoproteins is the interfacial binding of the enzyme to the lipid surfaces. In this study the equilibrium dissociation constants (K(d)s) for the interaction of pure human plasma LCAT with LDL, HDL2, HDL3, and a recon stituted discoidal HDL (rHDL) were determined by the activity-inhibition me thod. In addition, enzyme kinetics were measured with each of the lipoprote in substrates. Based on phospholipid concentrations, the K-d values (0.9 x 10(-5) to 4.6 x 10(-5) M) increased in the order rHDL = HDL3 less than or e qual to HDL2 < LDL while the relative reactivities (app V-max/ app K-m) wit h LCAT were 100, 16, 1, 6%, respectively, for the different lipoproteins. T hese quantitative measures were used to predict the distribution of LCAT in plasma and to explain cholesterol esterification when HDL are absent or in effective as substrates. (C) 1999 Academic Press.