Regulation of triacylglycerol synthesis in the liver: a decrease in diacylglycerol acyltransferase activity after treatment of isolated rat hepatocytes with glucagon.

Isolated rat hepatocytes were used to investigate the possibility of a short-term effect of glucagon on the synthesis of triacylglycerols in the liver. Incubation of hepatocytes in the presence of glucagon, followed by homogenization in a buffer containing F- (50 mM) and EDTA (2.5 mM), resulted in a 53% decrease in activity of microsomal diacylglycerol acyltransferase (EC 2.3.1.20), the only enzyme that is exclusively involved in the synthesis of triacylglycerols. The activity of cholinephosphotransferase (EC 2.7.8.2), which also uses diacylglycerols as substrate, was not decreased after exposure of the hepatocytes to glucagon. This may imply that triacylglycerol synthesis can be regulated independently of phosphatidylcholine synthesis. The activity of diacylglycerol acyltransferase in microsomes isolated from a homogenate of whole liver could be reduced by preincubating the microsomes with Mg2+ (5 mM), ATP (1 mM) and 105 000 X g supernatant. The enzyme could be reactivated by incubation of the washed microsomes with a 105 000 X g supernatant in the presence of dithiothreitol (5 mM). Fluoride (50 mM) inhibited this reactivation. It is concluded that the activity of diacylglycerol acyltransferase is subject to hormonal short-term control, possibly via a phosphorylation-dephosphorylation mechanism.