Synthesis of citrate from phosphoenolpyruvate and acetylcarnitine by mitochondria from rabbit enterocytes: implications for lipogenesis.

Enterocytes from fasted rabbits make glucose from exogenous fructose and dihydroxyacetone at rates of 180 and 91 nmol/min/10(8) cells but do not make glucose from glycerol, aspartate, malate, lactate, alpha-ketoglutarate, glutamate or glutamine. Total activities of phosphoenolpyruvate carboxykinase, fructose 1,6-bisphosphatase and glucose 6-phosphatase in isolated enterocytes are 0.44, 0.60 and 1.90 mumol/min/10(8) cells, and > or = 95% of carboxykinase activity is intramitochondrial. Enterocytes contain marginal glycerol kinase (0.05 mumol/ min/10(8) cells) and essentially no pyruvate carboxylase activities. Enterocyte mitochondria synthesize citrate from exogenous phosphoenolpyruvate and acetylcarnitine at a rate of 2.40 nmol/min/mg protein. Citrate formation is highly dependent on exogenous HCO3 and inhibited strongly by 3-mercaptopicolinate and 1,2,3-benzenetricarboxylate. Citrate synthesis is stimulated consistently by GDP and significantly so by GTP. Citrate production is unaffected by ADP or ATP. Enterocytes from fasted-refed rabbits contain activities of 0.05, 0.12, 0.39 and 0.56 mumol/min/mg cytosolic protein of ATP:citrate lyase, NADP:malate dehydrogenase, glucose 6-phosphate dehydrogenase and NADP:isocitrate dehydrogenase. Activities of NADP:malate dehydrogenase, glucose 6-phosphate dehydrogenase and NADP:isocitrate dehydrogenase are significantly higher in enterocytes from fasted-refed rabbits than those from fasted rabbits. Mitochondrial phosphoenolpyruvate carboxykinase in enterocytes in vivo could convert glycolysis-derived phosphoenolpyruvate to oxaloacetate that, with acetyl CoA, could form citrate for export to support cytosolic lipogenesis as an activator of acetyl CoA carboxylase, a source of carbon via ATP:citrate lyase and of NADPH via NADP:malate dehydrogenase or NADP:isocitrate dehydrogenase.