A kinetic study of glycerophosphate acyltransferase of rat adipocytes in relation to its control by noradrenaline.

Glycerophosphate acyltransferase present in an extract of rat adipocytes is strongly inhibited by excess palmitoyl-CoA. This inhibition is released by serum albumin but an excess of serum albumin is inhibitory, particularly at low palmitoyl-CoA concentrations. An optimal activity is reached when the ratio palmitoyl-CoA/albumin is in the range of 3-6. In the absence of albumin, oleic acid inhibits the activity at all palmitoyl-CoA concentrations. This inhibition is released by albumin and, inversely, oleic acid releases the inhibition by high concentrations of albumin. Another effect of fatty acids is to favour the inactivation of the glycerophosphate acyltransferase in extracts of adipocytes kept at 0 degree C. This inactivation is time-dependent and cannot be reversed by the addition of albumin to the assay mixture. Treatment of adipocytes with noradrenaline had no effect on the activity of the enzyme as long as the cells had been separated from fatty acids and albumin. With extracts of unwashed cells, the effect of noradrenaline on both the activity and stability of glycerophosphate acyltransferase could be explained by the presence of fatty acids in the extract.