Abstract
Activities of TPN-dependent enzymes that oxidize glucose 6-phosphate (glucose-6-phosphate and gluconic acid 6-phosphate dehydrogenases), isocitrate (isocitrate dehydrogenase) and malate (malic enzyme) were determined in particle-free supernatant fractions prepared from normal and diabetic rat liver homogenates and from lactating and nonlactating rat mammary-gland homogenates. These activities were compared with actual oxidations of their respective substrate under conditions where TPN availability was limited. 1.1. The decrease in activities of glucose 6-phosphate-oxidizing enzymes in diabetic rat livers was not associated with a reduction in the rate of oxidation of glucose 6-phosphate. Thus, reoxidation of TPNH limits the oxidation of glucose 6-phosphate2.2. The inhibitory effect of citrate on oxidation of glucose 6-phosphate and malate in livers of normal and diabetic rats is principally due to high activities of isocitrate dehydrogenase. The inhibitory effect of glucose 6-phosphate on oxidation of citrate and malate by lactating gland fractions is due mainly to high activities of glucose 6-phosphate-oxidizing enzymes.3.3. Malic enzyme activity in diabetic liver was one-fourth that of normal liver.4.4. Weaning decreased activities of glucose 6-phosphate-oxidizing and malic enzymes in mammary gland preparations.5.5. Further evidence is presented for conversion of DPN to TPN in the presence of ATP in rat liver and lactating mammary gland fractions.
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