Glyceride-Glycerol Synthesis from Pyruvate

Abstract

The dicarboxylic acid shuttle in adipose tissue and its role in the conversion of pyruvate to glyceride-glycerol has been studied in rat epididymal fat pads in vitro.Phosphoenolpyruvate carboxykinase and pyruvate carboxylase have also been measured in rat liver and the adaptive response of these enzymes to alterations in diet has been compared to alterations of these enzymes in adipose tissue from the same animals.Fasting for 24 hours caused a 5-fold increase in the activity of P-enolpyruvate carboxykinase in adipose tissue and a 3-fold increase in the activity of the enzyme in liver.Fasting for periods longer then 24 hours reduced the increase in P-enolpyruvate carboxykinase in adipose tissue.Actinomycin D blocked this increase in activity in adipose tissue but had no effect in liver.Refeeding the animals for 24 hours after fasting for 24 hours reduced the activity of Penolpyruvate carboxykinase to levels below those of normal fed rats.The stimulatory effect of fasting on P-enolpyruvate carboxykinase activity in adipose tissue but not liver was markedly age-dependent, being reduced in older rats.Pyruvate carboxylase, unlike P-enolpyruvate carboxykinase, was found in both the mitochondria and cytosol of liver and adipose tissue.Fasting, and fasting followed by refeeding, had no effect on pyruvate carboxylase activity in adipose tissue but fasting caused a P-fold increase in the activity of the enzyme in rat liver cytosol without affecting the mitochondrial enzyme.The conversion of pyruvate-3-l% (25 mu) to glyceride-glycerol by rat epididymal fat pads in vitro was increased by fasting.Actinomycin D blocked this increase, and refeeding the animals after fasting reduced the incorporation to levels below those found in adipose tissue from normal fed rats.At a pyruvate concentration of 25 mu, fatty acid synthesis was not significantly different in adipose tissue from normal and 24-hour-fasted rats.At lower pyruvate concentrations (0.25 mu) relatively more