SEASONAL-CHANGES IN CRITICAL ENZYMES OF LIPOGENESIS AND TRIACYLGLYCEROL SYNTHESIS IN THE MARMOT (MARMOTA-FLAVIVENTRIS)

Fatty acid metabolism and triacylglycerol synthesis are critical proce sses for the survival of hibernating mammals that undergo a prolonged fasting period. Fatty acid synthase, fatty-acid-CoA ligase, diacylglyc erol acyltransferase, and monoacylglycerol acyltransferase activities were measured in liver and in white and brown adipose tissue, in order to determine whether enzymes of lipogenesis and triacylglycerol synth esis vary seasonally during hibernation in the yellow-bellied marmot ( Marmota flaviventris). Compared with mid-winter hibernation, fatty aci d synthase activity was higher in all three tissues during early sprin g when marmots emerged from hibernation and in mid-summer when they we re feeding, consistent with the synthesis of fatty acids from the carb ohydrate-rich summer diet. Fatty-acid-CoA ligase and diacylglycerol ac yltransferase activities were highest in summer in white adipose tissu e when triacylglycerol synthesis would be expected to be high; diacylg lycerol acyltransferase activity was also high in brown adipose tissue during spring and summer. In liver, however, diacylglycerol acyltrans ferase specific activity was highest during hibernation, suggesting th at triacylglycerol synthesis may be prominent in liver in winter. Mono acylglycerol acyltransferase activity, which may aid in the retention of essential fatty-acids, was 80-fold higher in liver than in white or brown adipose tissue, but did not vary seasonally. Its dependence on palmitoyl-CoA suggests that a divalent cation might play a role in enz yme activation. The high hepatic diacylglycerol acyltransferase activi ty during hibernation suggests that the metabolism of very low density lipoprotein may be important in the movement of adipose fatty acids t o brown adipose tissue and muscle during the rewarming that occurs per iodically during hibernation. These studies suggest that enzymes of li pid metabolism vary seasonally in the marmot, consistent with requirem ents of this hibernator for triacylglycerol synthesis and metabolism.