SYNTHESIS OF CITRATE FROM PHOSPHOENOLPYRUVATE AND ACETYLCARNITINE BY MITOCHONDRIA FROM RABBIT ENTEROCYTES - IMPLICATIONS FOR LIPOGENESIS

Enterocytes from fasted rabbits make glucose from exogenous fructose a nd dihydroxyacetone at rates of 180 and 91 nmol/min/10(8) cells but do not make glucose from glycerol, aspartate, malate, lactate, alpha-ket oglutarate, glutamate or glutamine. Total activities of phosphoenolpyr uvate carboxykinase, fructose 1,6-bis-phosphatase and glucose 6-phosph atase in isolated enterocytes are 0.44, 0.60 and 1.90 mu mol/min/10(8) cells, and greater than or equal to 95% of carboxykinase activity is intramitochondrial. Enterocytes contain marginal glycerol kinase (0.05 mu mol/min/10(8) cells) and essentially no pyruvate carboxylase activ ities. Enterocyte mitochondria synthesize citrate from exogenous phosp hoenolpyruvate and acetylcarnitine at a rate of 2.40 nmol/min/mg prote in. Citrate formation is highly dependent on exogenous HCO3- and inhib ited 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. Enterocyte s from fasted-refed rabbits contain activities of 0.05, 0.12, 0.39 and 0.56 mu mol/min/mg cytosolic protein of ATP:citrate lyase, NADP:malat e dehydrogenase, glucose 6-phosphate dehydrogenase and NADP:isocitrate dehydrogenase. Activities of NADP:malate dehydrogenase, glucose 6-pho sphate dehydrogenase and NADP:isocitrate dehydrogenase are significant ly higher in enterocytes from fasted-refed rabbits than those from fas ted rabbits. Mitochondrial phosphoenolpyruvate carboxykinase in entero cytes in vivo could convert glycolysis-derived phosphoenolpyruvate to oxaloacetate that, with acetyl CoA, could farm citrate for export to s upport cytosolic lipogenesis as an activator of acetyl CoA carboxylase , a source of carbon via ATP:citrate lyase and of NADPH via NADP:malat e dehydrogenase or NADP:isocitrate dehydrogenase. (C) 1997 Elsevier Sc ience Inc.