Brown adipose tissue triacylglycerol synthesis in rats adapted to a high-protein, carbohydrate-free diet

Adaptation of rats to a high-protein, carbohydrate-free (HP) diet induced a marked reduction of brown adipose tissue (BAT) fatty acid (FA) synthesis from both (H2O)-H-3 and [C-14]glucose in vivo, with pronounced decreases in the activities of four enzymes associated with lipogenesis: glucose-6-phosphate dehydrogenase, melic enzyme, citrate lyase, and acetyl-CoA carboxylase. In both HP-adapted and control rats, in vivo incorporation of (H2O)-H-3 and [C-14]glucose into BAT glyceride-glycerol was much higher than into FA. It could be estimated that most of the glycerol synthetized was used to esterify preformed FA. Glycerol synthesis from nonglucose sources (glyceroneogenesis) was increased in BAT from HP rats, as evidenced by an increased capacity of tissue fragments to incorporate [1-C-14]pyruvate into glycerol and by a fourfold increase in the activity of phosphoenolpyruvate carboxykinase activity, a key glyceroneogenic enzyme. The data suggest that high rates of glyceroneogenesis and of esterification of preformed FA in BAT from HP-adapted rats are essential for preservation of tissue lipid stores, necessary for heat generation when BAT is recruited in nonshivering thermogenesis.