Direct oxidation of glucose-6-phosphate, 6-phosphogluconate and pentose-5-phosphates by enzymes of animal origin

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Abstract
A direct pathway for the oxidation of D-glucose-6-phosphate, 6-phospho-D-gluconate and D-ribose-5-phosphate was demonstrated in animal tissues and shown to resemble closely that previously described for yeast. Hexose monophosphate and 6-phosphoglucon-ate are actively oxidized by extracts of liver (horse, rat and rabbit), kidney (rat and rabbit), brain (rat and rabbit) and rat liver carcinoma. Skeletal muscle (rat and rabbit) and Rous sarcoma of the fowl are weakly active. Ribose-5-phosphate was oxidized by liver and kidney, and less actively by skeletal muscle. Oxidation of all 3 substrates is coenzyme II-specific and proceeds independently of the glycolytic route, since inorganic phosphate is not essential and high concns. (0.01 [image]) of fluoride and iodoacetamide do not inhibit. Partial separation of the systems from rat and horse liver was effected by fractional ammonium sulfate precipitation. Fraction C (60-70% saturation) oxidizes 6-phosphogluconate specifically. Fraction B (50-60% saturation) oxidizes glucose-6-phosphate and fructose 6-phosphate, but not fructose-1,6-diphosphate. Fractions B+C are generally necessary for D-ribose-5-phosphate oxidation, which is about 5 times as rapid as the oxidation of D-arabinose-5-phosphate and D-xylose-5-phosphate. At least 2 liver factors are necessary for ribose-5-phosphate oxidation. One of these is a coenzyme II-specific dehydrogenase and the other possibly a type of aldolase. The dehydrogenation of ribose-5-phosphate is very slow at 20[degree], but rapid at 37[degree], and thus differs from the rapid dehydrogenation of glucose-6-phosphogluconate at 20[degree]. Cyanide (0.01 [image]) does not inhibit the oxidation of these substrates and may accelerate the early stages of oxidation. The substrate concns. producing 50% of the max. initial velocities are approx. 7.5 x 10-5[image] for hexose monophosphate and 1.5 x 10-5[image] for 6-phosphogluconate dehydrogenases. A mechanism is suggested for the oxidative formation of D-ribose-5-phosphate from D-glucose-6-phosphate.