Stereochemical and positional specificity of the lipase/acyltransferase produced by Aeromonas hydrophila

Aeromonas species secrete a glycerophospholipid-cholesterol acyltransferase (GCAT) which shares many properties with mammalian plasma lecithin-cholesterol acetyltransferase (LCAT). We have studied the stereochemical and positional specificity of GCAT against a variety of lipid substrates using NMR spectroscopy as well as other assay methods. The results show that both the primary and secondary acyl ester bonds of L-phosphatidylcholine can be hydrolyzed but only the sn-2 fatty acid can be transferred to cholesterol. The enzyme has an absolute requirement for the L configuration at the sn-2 position of phosphatidylcholine. The secondary ester bond of D-phosphatidylcholine cannot be hydrolyzed, and this lipid is not a substrate for acyl transfer. In contrast to the phospholipases, but similar to LCAT, the enzyme does not interact stereochemically with the phosphorus of phosphatidylcholine. In fact, the phosphorus is not required for enzyme activity, as GCAT will also hydrolyze monolayers of diglyceride, although at much lower rates.