Hepatic monoacylglycerol acyltransferase. Characterization of an activity associated with the suckling period in rats.

Hepatic monoacylglycerol acyltransferase activity is 700-fold higher during the suckling period than in the adult rat. Specific activity in total particulate preparations rose from 9.4 nmol/min/mg before birth to a peak of 78 nmol/min/mg on the 6th to 8th postnatal days. Monoacylglycerol acyltransferase activity fell sharply after day 8 and was 1.6 and 0.1 nmol/min/mg on day 28 and in adult rats, respectively. The activity had a pH optimum at 8.0 and was activated by albumin and by phospholipids. With [3H]palmitoyl-CoA and sn-2-monooleoylglycerol, more than 96% of the products were di- and triacylglycerols. More than 92% of the diacylglycerol product was the 1,2 isomer. The activity was stable at 43 degrees C for 50 min. Thermal inactivation showed t 1/2 values of 8 min and 4.5 min at 53.5 and 55 degrees C, respectively. In suckling rats, monoacylglycerol acyltransferase activities in liver and intestinal mucosa were 150- to 800-fold higher than in other tissues. Monoacylglycerol acyltransferase activity was 12.5-fold greater with palmitoyl-CoA than with octanoyl-CoA. Acetyl-CoA was not a substrate. The sn-2-monoacylglycerols were strongly preferred over the sn-1 isomers. No direct relationship was noted between 2-monoacylglycerol chain length and apparent Km value. The presence of high levels of monoacylglycerol acyltransferase activity in suckling rat liver suggests that the monoacylglycerol pathway functions as a major route of hepatic glycerolipid synthesis during the suckling period but not in adult animals.