SEASONAL-CHANGES IN ENZYMES OF LIPOGENESIS AND TRIACYLGLYCEROL SYNTHESIS IN THE GOLDEN-MANTLED GROUND-SQUIRREL (SPERMOPHILUS-LATERALIS)

In order to determine whether critical enzyme activities of glycerolip id synthesis change seasonally in the golden-mantled ground squirrel ( Spermophilus lateralis), we collected summer and winter samples of liv er, brown adipose tissue (BAT), and white adipose tissue (WAT). Compar ed with fatty acid synthase activity during hibernation, summer activi ties were 2.5- to 8-fold higher in adipose tissue and liver. Diacylgly cerol acyltransferase (DGAT) activity was 2.6-fold higher in WAT durin g the summer, consistent with increased seasonal triacylglycerol stora ge, but the activity did not change in liver or BAT, suggesting that i n these tissues, triacylglycerol synthesis is equally active in summer and winter. Lack of change in acyl-CoA synthetase in liver and BAT ma y reflect high synthetic rates for acyl CoAs that are destined in the summer for glycerolipid synthesis and in the winter for beta-oxidation . Monoacylglycerol acyltransferase (MGAT) activity increased significa ntly in both liver and WAT during the summer but decreased in BAT. Alt hough the changes were consistent with active year round triacylglycer ol synthesis, the higher summer MGAT activity observed in the squirrel liver and WAT suggest that MGATs function may not be limited to conse rving essential fatty acids during physiological states of lipolysis. Seasonal changes observed in the ground squirrel were similar to those previously reported in the yellow bellied marmot (Marmota flaviventri s), confirming that important adjustments occur in energy metabolism n ecessitated by long seasonal hibernation. (C) 1997 Elsevier Science In c.