Substrate Specificity of Rat Plasma Lecithin-cholesterol Acyltransferase towards a Molecular Species of Phosphatidylcholine

The substrate specificity and the affinity of rat purified lecithin-cholesterol acyltransferase towards the molecular species of phosphatidylcholine (PC) were studied in comparison with the human enzyme. The substrate vesicles were prepared with 40% of a test PC, 60% of egg yolk PC, and tritium cholesterol. Both human and rat enzymes showed similar high reactivity to the substrates containing three major PCs (16: 0-18: 1 PC, 16: 0-18: 2 PC, and 18: 0-18: 1 PC) of egg yolk compared with the vesicles of egg yolk PC alone. In the case of 18: 0-20: 4 PC, the rat enzyme had the highest activity among all the test PCs, but the human enzyme only had a moderate activity. Even when the substrate consisted of 18: 0-20: 4 PC alone, the rat enzyme had a high activity, but the activity of the human enzyme was not detected. Symmetrical diacyl-PCs (18: 2-18: 2 PC, 18: 1-18: 1 PC, 18: 0-18: 0 PC) were not a preferable substrate for either enzyme. The transfer of both the human and rat enzymes from the vesicles containing 18: 0-20: 4 PC to the egg yolk PC vesicles was on a higher level than that from the vesicles containing 18: 2-18: 2 PC. This suggests that the activity of the LCA T can be easily influenced by the kinds of PC molecular species and its relative content in the substrate and that the substrate may provide the extent of the enzyme transfer between the substrate particles.