Modulation of the positional specificity of lecithin-cholesterol acyltransferase by the acyl group composition of its phosphatidylcholine substrate: role of the sn-1-acyl group.

Human lecithin-cholesterol acyltransferase (LCAT), which is normally specific for the sn-2 position of phosphatidylcholine (PC), derives a significant percentage of acyl groups from the sn-1 position, when sn-2 is occupied by 18:0, 20:4, or 22:6. We investigated the relative importance of the two acyl groups of PC in determining the positional specificity by first analyzing the cholesteryl esters formed in the presence of symmetric PCs labeled at sn-2. Both human and rat LCATs transferred exclusively the sn-2-acyl group from all symmetric PCs, including 18:0-18:0, and 20:4-20:4, showing that the presence of these fatty acids at sn-2 does not automatically alter the positional specificity. The role of the sn-1-acyl group was then tested by using PCs containing 20:4 or 18:0 at sn-2 and fatty acids of various chain lengths and unsaturation at sn-1. With 20:4 at sn-2 and saturated fatty acids of various chain lengths at sn-1, human and rat LCATs derived, respectively, 5-72% and 1-20% of the total acyl groups from the sn-1 position. However, the chain length of the sn-1-acyl did not correlate with its utilization by either enzyme. Various unsaturated fatty acids at sn-1 also were transferred by human LCAT at a higher rate (5-75% of total) than they were transferred by rat LCAT (0-21%). With sn-2-18:0 PCs, however, rat LCAT exhibited greater alteration in positional specificity (30-95% from sn-1) than human LCAT (15-83% from sn-1). These results show that while the primary determinant of positional specificity is the sn-2-acyl group of PC, the structure of sn-1-acyl significantly modifies it.