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Paths of Carbon in Gluconeogenesis and Lipogenesis

II. CONVERSION OF PRECURSORS TO PHOSPHOENOLPYRUVATE IN LIVER CYTOSOL
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      Previous studies have indicated that, in rat liver, gluconeogenesis from pyruvate and lactate involves carboxylation of pyruvate in mitochondria but the oxalacetate formed does not diffuse to the cytosol. Instead it is reduced to malate or transaminated to aspartate. The present study indicates that aspartate and malate may be converted to phosphoenolpyruvate by enzymes in rat liver cytosol almost as rapidly as is oxalacetate. The rates of conversion are sufficient to account for the formation of carbohydrate in livers of normal rats and are greatly elevated in preparations from livers of diabetic rats. The synthesis of phosphoenolpyruvate from pyruvate via the malic enzyme or pyruvate carboxylase in the cytosol was found to be insignificant. Quantitative measurements of enzymes in the cytosol indicate that fumarase is elevated in diabetes and fasting but is not altered by adrenalectomy or administration of hydrocortisone. Malate dehydrogenase is elevated in diabetes and after hydrocortisone. Both glutamate-oxalacetate and glutamate-pyruvate transaminase are elevated in diabetes, in fasting, and after hydrocortisone. In agreement with other workers, citrate cleavage enzyme was depressed in diabetes and fasting, and elevated after insulin administration and refeeding of fasted animals. The enzyme adaptations and the rates of metabolite conversion in rat liver cytosol are thus in keeping with the proposed pathway of carbon in gluconeogenesis.

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