Phosphoenolpyruvate carboxykinase and the synthesis of glyceride-glycerol from pyruvate in adipose tissue.

In the absence of glucose, significant amounts of labeled pyruvate were converted to glyceride-glycerol by rat epididyma1 fat pads in uifro. Using specifically labeled pyruvate-“C we found twice as much C-2 incorporated into glycerideglycerol as C-l. Although this ratio is not affected by changes in dietary conditions, the amount of pyruvate lncorporated into glyceride-glycerol is decreased j-fold ln fasted rats as compared to fasted-refed animals. The addition of glucose to the incubation medium depressed the appearance of label in glycerol and stimulated the formation of fatty acids from pyruvate. Under all dietary conditions, over 90% of pyruvate C-l and 43 to 49% of pyruvate C-2 incorporated into glycerol were in the a! carbon atoms. These results indicate the occurrence of the dicarboxylic acid shuffle in adipose tissue and are supported by the following tidings: phosphoenolpyruvate-carboxykinase activity is present in soluble and mitochondrial fractions of adipose tissue; its activity increases with fasting when compared to the fasted-refed animal; and it is sufficiently high to account for the observed rates of conversion of pyruvate to glycerol. This pathway, under conditions of glucose deprivation, may supply a-glycerophosphate for the re-esterlfication of fatty acids.