Subject oxaloacetate

The formation of oxaloacetic acid by dehydrogenation implies that this acid may be dissimilated by two mechanisms. It is known (62), (114) that oxaloacetic acid is subject to decarboxylation under acid conditions, and that higher pH is favorable to its stability. Thus, alkaline media enable the add to remain unchanged long enough to be split, yielding acetate and oxalate, while acidic media cause decarboxylation. 

Further developments are shown in Figure 4. On the basis that glucosamine reacted with pyruvic acid in the presence of alkali to yield pyrrole-2-carboxylic acid, in 1% yield, Gottschalk (21) proposed that sialic acid was formed by an aldol condensation reaction between N-ace-tylglucosamine and pyruvic acid. Kuhn and Brossmer (15) and Zilliken and Glick (22) showed that the reverse reaction also took place under alkaline conditions. Cornforth, Firth, and Gottschalk (23) synthesized crystalline N-acetylneuraminic acid (NANA) from N-acetylglucosamine and oxaloacetic acid (pH 11, 20°C). Under conditions less subject to misinterpretation, Heimer and Meyer (24) found that Vibrio cholerae enzymes cleaved NANA into an N-acetylhexosamine and pyruvic acid. 

The acetyl-CoA transfers its acetyl group to oxaloace-tate, thereby generating citrate. In a cyclic series of reactions, the citrate is subjected to two successive decarboxylations and four oxidative events, leaving a four-carbon compound malate from which the starting oxaloacetate is regenerated. 

The glutamate-oxaloacetate aminotransferase stimulation test involves the reconstitution of PLP with the apoenzyme. The enzyme activity in broken red blood cells is measured with and without PLP added. Addition of PLP would be expected to result in little or no stimulation of enzyme activity if the subject had been consuming a Bg-adequate diet, whereas an increase in enzyme activity with the addition of pure PLP to the enzyme assay mixtures would indicate that the subject had been consuming a Bg-deficient diet. Consumption of a Bg-deficient diet allows continued synthesis of the apoenzyme in the cell, but not conversion of the apoenzyme to the holoenzyme. A marked increase occurring with the addition of PLP could indicate that the subject had been consuming a Bg-deficient diet or that absorption of dietary vitamin Bg was impaired or defective in some way. 

Other biotin-dependent enzymes include propionyl-CoA carboxylase and pyruvate carboxylase (Chapter 15). The latter, like acetyl-CoA carboxylase, is subject to allosteric regulation. Pyruvate carboxylase, a mitochondrial enzyme, is activated by acetyl-CoA and converts pyruvate to oxaloacetate which, in turn, is converted to glucose via... 

Although the citric acid cycle is generally regarded as a pathway for the oxidation of four- and five-carbon compounds arising from amino acids, such as fumarate, oxaloacetate, OC-ketoglutarate and succinate (see Figure 5.20), it does not, alone, permit complete oxidation of these compounds. Four-carbon intermediates are not overall consumed in the cycle, as oxaloacetate is reformed. Addition of four- and five-carbon intermediates will increase the rate of cycle activity (subject to control by the requirement for ATP) only until the pool of intermediates is saturated. 

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