Requirement of low density lipoproteins for the lysolecithin acyl transferase activity in human plasma: assay of enzyme activity in abetalipoproteinemic patients.

Previous studies from this laboratory have shown that the lecithin:cholesterol acyl transferase (LCAT) of normal human plasma can convert lysolecithin to lecithin in the presence of low density lipoproteins (LDL). In order to study the importance of LDL for the lysolecithin acyl transferase (LAT) activity, the LCAT and LAT activities were assayed in two patients with abetalipoproteinemia. The plasma from both patients had only 5%-6% of the LAT activity present in normal plasma. This activity was stimulated up to 22-fold by the addition of normal LDL but not very low density lipoproteins (VLDL) or high density lipoproteins. The LAT activity of normal plasma was only stimulated by two-threefold by LDL. The LCAT activity in both patients was reduced to 42%-46% of the control values. This activity was stimulated up to fourfold by the addition of LDL as well as VLDL which is comparable to the activation obtained with control plasma. These results therefore suggest that LDL is physiological activator of LAT reaction in normal human plasma.