Lipid Substrates for LCAT Activity in Liver Disease

The activity of the plasma lecithin:cholesterol acyltransferase (LCAT) was studied in control subjects and in two groups of patients with liver diseases (cholestasis or cirrhosis). Polyunsaturated phosphatidylcholine (PU-PC), as a special substrate, was administered to all subjects by single intravenous infusion of 3g. Before and after infusion the transesterification rate was assayed by three different methods: 1) assay according to Glomset & Wright (7) using plasma against a standard substrate, 2) assay of pooled plasma of normal subjects against substrate prepared from the plasma of the patients, and 3) assay according to Stokke & Norum (24). In normal subjects, no statistically significant differences of the enzyme activity levels were observed before PU-PC infusion. There was a tendency to lower LCAT activity in patients with liver disease compared with normal subjects before PU-PC infusion. After PU-PC infusion a significant increase of LCAT activity was observed with methods 2) and 3) in patients with cholestasis (32, 8/47.2 and 30.4/51.1μM/l/h) and with method 3 in patients with cirrhosis (20.3/34.6μM/l/h). Gaschromatographic analysis of HDL-phosphatidylcholine and HDL-cholesterol revealed a low content of polyunsaturated fatty acids in patients with liver disease as compared to normals. Treatment by PU-PC infusion yielded an increased content particularly of linoleic acid in these high density lipoprotein lipids. Since the LC AT-mediated reaction takes place on or within HDL and since the activation of LCAT was most effective with PU-PC, the transesterification of cholesterol appears to depend on HDL enriched in PU-PC as the preferred substrate for LCAT.