The effect of cadmium ions and cadmium-metallothionein on the activities of phospholipid-synthesizing enzymes of rat liver microsomes in vitro

Abstract

The effects of cadmium ions or cadmium-metallothionein on the activities of acyl-CoA:1acyl- sn-glycerol 3-phosphoric acid or 1-acyl- sn-glycero 3-phosphocholine acyltransferase of rat liver microsomes have been studied, in vitro. Cadmium ions were found to cause a noncompetitive type inhibition of these two acyltransferases. The K i values were calculated, and found to be smallest (1.7 × 10 −5 m) for palmitoyl-CoA and greatest (1.0 × 10 −4 m) for linoleoyl-CoA, among the several fatty acyl-CoA's tested on the 1-acyl- sn-glycerol 3-phosphoric acid acyltransferases. With the 1-acyl- sn-glycero 3-phosphocholine acyltransferase, the K i values were found to be smallest for the plamitoyl-CoA acyltransferase (3.8 × 10 −5 m) and largest for thearachidonoyl-CoA acyltransferase (1.1 × 10 −4 m). In contrast, mouse liver cadmium-metallothionein, including 4 mol of cadmium and 2 mol of zinc in one molecule of metallothionein, was not found to be inhibitory or rather stimulative on the above two acyltransferases at the same concentration of cadmium tested in the cadmium ion inhibitor experiments. The above results demonstrate that there is a strong and irreversible inhibition by cadmium ions on acyl-CoA acyltransferases, but that when cadmium acts on the enzyme in the form of a cadmium-metallothionein complex, the inhibition effect does not occur. These findings may reflect differing degrees of toxicity of these two types of cadmium compounds in mammalian tissues.