Kinetics of lecithin-cholesterol acyltransferase reaction with discoidal complexes of apolipoprotein...

Discoidal substrates for purified human lecithin-cholesterol acyltransferase were prepared with human apolipoprotein A-I, cholesterol, and egg phosphatidylcholine (PC) or dipalmitoyl PC, including dihexadecyl PC in various proportions as an enzymatically inert dilutor of the interfacial PC substrate. All the complexes, prepared by the sodium cholate dialysis method, were found to be very similar in size, lipid/apolipoprotein stoichiometry, and apolipoprotein spectral properties to the small discoidal complexes without any dihexadecyl PC, described previously (Jonas, A., and Matz, C.E. (1982) Biochemistry 21, 6867-6872; Jonas, A., and McHugh, H. T. (1984) Biochim. Biophys. Acta 794, 361-372). The kinetic results presented in the form of double reciprocal plots of initial velocity against bulk PC or interfacial PC concentration were linear according to the Verger et al. kinetic model (Verger, R., Mieras, M. C. E., and de Haas, G. H. (1973) J. Biol. Chem. 248, 4023-4034) for an initial enzyme binding via an interfacial recognition site followed by interfacial substrate binding and catalysis, in the presence of a competitive interfacial inhibitor. The results indicate, furthermore, that the affinity of the active site for the substrate and inhibitor is quite similar.