The incorporation of 3-phenoxybenzoic acid and other xenobiotic acids into xenobiotic lipids by enzymes of the monoacylglycerol pathway in microsomes from adult and neonatal tissues

Abstract

The incorporation of 3-phenoxybenzoic acid (3PBA) into xenobiotic lipids by enzymes of the monoacylglycerol (MG) pathway was measured using microsomes prepared from rat liver as an enzyme source. The mean activities of the three enzymes involved were: acyl-CoA synthetase, 1.1 nmol/min/mg protein; MG acyltransferase, 75 pmol/min/mg protein; and diacylglycerol acyltransferase, 11.4 pmol/min/mg protein. MG and DG acyltransferases also showed activity with benzoyl-CoA or 1-naphthylacetyl-CoA as acyl donor but none with clofibryl-CoA or 2,4-dichlorophenoxyacetyl-CoA. MG acyltransferase activity, using 3PBA-CoA, was higher in microsomes from rat intestinal mucosa and pig liver, and lower in rat adipose tissue, rat liver and mouse liver. This ranking of activities corresponds to published activities using natural substrates. There was a large increase in MG acyltransferase, using either 3PBA-CoA or palmitoyl-CoA as substrate, in microsomes from the livers of rats 16–18 days old. Lysophosphatidic acid (lyso-PA) and lysophosphatidylethanolamine (lyso-PE), but not other phospholipids or detergents, stimulated MG acyltransferase activity more than two-fold. Lyso-PA (5 μM) increased the V max but had little effect on the K m for 2-hexadecylglycerol, whereas 100 μM lyso-PE decreased the K m and had a smaller effect on the V max . These results illustrate that the incorporation of xenobiotic acids into diacyl- and triacylglycerol by enzymes of the MG pathway may be a more general phenomenon than was previously suspected and that it may be subject to a variety of developmental and physiological controls.