Apolipoprotein A-I(R151C)(Paris) is defective in activation of lecithin : cholesterol acyltransferase but not in initial lipid binding, formation of reconstituted lipoproteins, or promotion of cholesterol efflux

ApoA-I(R151)(Paris) is a natural apolipoprotein (apo) A-I variant that is a ssociated with low levels of high-density lipoprotein cholesterol (HDL-chol esterol) and the partial deficiency of lecithin:cholesterol acyltransferase (LCAT) in the plasma of heterozygous carriers. We compared the abilities o f recombinant normal apoA-I and recombinant apoA-I(R151C)(Paris) to clear a n emulsion of dimyristoylphosphatidylcholine (DMPC), to form reconstituted lipoproteins with dipalmitoylphosphatidylcholine (DPPC), to activate LCAT, and to promote efflux of biosynthetic cholesterol from porcine aortic smoot h muscle cells (SMCs) or of exogenous cholesterol from lipid-loaded mouse p eritoneal macrophages. Recombinant apoA-I(R151C)(Paris) occurred in monomer ic and dimeric forms at a ratio of 60:40. Normal apoA-I and apoA-I(R151C)(P aris) cleared DMPC emulsions at equal rates. Both isoforms associated compl etely with DPPC during cholate dialysis. Normal apoA-I formed one single pa rticle with a mean diameter of 9.3 nm, whereas apoA-I(R151)(Paris) gave ris e to three particles with mean diameters of 9.3 nm (containing 74% of apoA- I), 10.6 nm, and 12.1 nm, respectively. Compared to normal apoA-I, apoA-I(R 151C)(Paris) had a reduced LCAT-cofactor activity with a 60% lower V-max/K- m ratio due to a 50% higher affinity constant, K-m. During incubations for 10 min and 360 min, normal apoA-I/DPPC complexes and apoA-I(R151 C)(Paris)/ DPPC complexes were equally efficient in releasing biosynthetic cholesterol from SMCs. In the lipid-free form, apoA-I(R151C)(Paris) induced normal hyd rolysis of cholesteryl esters and normal cholesterol efflux from lipid-load ed mouse-peritoneal macrophages. In conclusion, in addition to its ability to form homo- and heterodimers, apoA-I(R151C)(Paris) is characterized by de fective LCAT-cofactor activity but by normal lipid binding and cholesterol- efflux-promoting abilities.