Pathway of carbon flow during fatty acid synthesis from lactate and pyruvate in rat adipose tissue

Abstract

The metabolism of pyruvate and lactate by rat adipose tissue was studied. Pyruvate and lactate conversion to fatty acids is strongly concentration-dependent. Lactate can be used to an appreciable extent only by adipose tissue from fasted–refed rats. A number of compounds, including glucose, pyruvate, aspartate, propionate, and butyrate, stimulated lactate conversion to fatty acids. Based on studies of incorporation of lactate-2-3H and lactate-2-14C into fatty acids it was suggested that the transhydrogenation sequence of the “citrate–malate cycle”1 was not providing all of the NADPH required for fatty acid synthesis from lactate. An alternative pathway for NADPH formation involving the conversion of isocitrate to α-ketoglutarate via cytosolic isocitrate dehydrogenase was proposed. Indirect support for this proposal was provided by the rapid labeling of glutamate from lactate-2-14C by adipose tissue incubated in vitro, as well as the demonstration that glutamate can be readily metabolized by adipose tissue via reactions localized largely in the cytosol. Furthermore, isolated adipose tissue mitochondria convert α-ketoglutarate to malate, or in the presence of added pyruvate, to citrate. Glutamate itself can not be metabolized by these mitochondria, a finding in keeping with the demonstration of negligible levels of NAD-glutamate dehydrogenase activity in adipose tissue mitochondria. Pyruvate stimulated α-ketoglutarate and malate conversion to citrate and reduced their oxidation to CO2.It is proposed that under conditions of excess generation of NADH malate may act as a shuttle carrying reducing equivalents across the mitochondrial membrane. Malate at low concentrations increased pyruvate conversion to citrate and markedly decreased the formation of CO2 by isolated adipose tissue mitochondria. Malate also stimulated citrate and isocitrate metabolism by these mitochondria, an effect that could be blocked by 2-n-butylmalonate. This potentially important role of malate in the regulation of carbon flow during lipogenesis is underlined by the observation that 2-n-butylmalonate inhibited fatty acid synthesis from pyruvate, but not from glucose and acetate, and decreased the stimulatory effect of pyruvate on acetate conversion to fatty acids.

Keywords

• fasting–refeeding
• glutamate
• alanine aminotransferase
• asparate aminotransferase
• NADP-isocitrate dehydrogenase
• mitochondrial metabolism