NOVEL EVIDENCE FOR AN ECTO-PHOSPHOLIPID METHYLTRANSFERASE IN ISOLATEDRAT HEPATOCYTES

Phospholipids of isolated rat hepatocytes were labelled by preincubati on with either 2 mu M [methyl-C-14] S-adenosylmethionine (AdoMet) or 2 mu M [methyl-C-14]methionine. Subsequent addition of phospholipase C to the suspension removed 95 % of the radioactivity from phospholipids methylated by [methyl-C-14]AdoMet within a few minutes, but was witho ut effect on phospholipids methylated by [methyl-C-14]methionine radio activity from the latter could, nevertheless, be removed by phospholip ase C after permeabilization of the cells with digitonin. The results clearly show that the methyl group of exogenous AdoMet, contrary to th at of methionine, is transferred on to phospholipids located on the ex ternal face of the plasma membrane. Accordingly, pretreatment of isola ted hepatocytes with trypsin prevented the methylation of phospholipid s from exogenous AdoMet by 60-80 %, whereas it was almost without effe ct when exogenous methionine was the methyl donor. Our data corroborat e previous work [Bontemps and Van den Berghe (1997) Biochem. J. 327, 3 83-389], which indicated that AdoMet methylates hepatocyte phospholipi ds without penetrating the cells.