Oxalacetic acid, transamination reactions

The procedure reported in Scheme 13.11 describes deracemization of an amino acid involving oxidation with an L-specific enzyme and transamination with a D-amino transferase using D-aspartate 10, which is generated from L-aspartate 11 by aspartate racemase, as the amino donor. The oxidative enzyme is defined as an L-amino acid deaminase, a flavoprotein from Proteus myxofadens [34]. The transamination reaction is shifted towards the product since the oxalacetate 12 formed decarboxylates spontaneously to give pyruvate and carbon dioxide. 

The basic construction of the mathematical model using simplified metabolic networks to describe the reactions of the citric acid cycle and associated transamination reactions between pyruvate and alanine, oxalacetate and aspartate and a-ketoglutarate and glutamate, and the use of the FACSIMILE program (Chance et al., 1977) to solve the rather large number of simultaneous differential equations generated by the model was the same as previously described (Chance etal., 1983). For the present experiments the model was expanded to include an input flux at the level of succinate to represent propionate metabolism to succinyl-CoA, and a dilution of the aspartate pool to represent net proteolysis. These input fluxes required an output flux of carbon from the citric acid cycle in order to maintain a steady state carbon balance, for which the conversion of malate to pyruvate via malic enzyme was chosen. The model calculates the unknown flux parameters to provide a minimum least squares fit of the C fractional enrichments of specific carbon atoms of metabolic intermediates as measured by C NMR spectroscopy. 

Sulfinylpyruvic acid accumulates as a result of the transaminating activity. It is an analog of oxalacetic acid, and like this compound, it is decomposed to pyruvate and SOs under the influence of Mn ". The reaction is analogous to the j3-decarboxylation of oxalacetate by Mn++. Mn++ also catalyzes the oxidation of SOj to S04 . As a consequence, reactions 5 and 6 of Fig. 2 are assumed to be nonenzymatic. 

After weighing the different posibilities to counter the lack of equal labeling in carbons 3 and 6 of lysine, Strassman and Weinhouse 180) propose as the more likely synthetic mechanism, a condensation of an acetyl methyl carbon with the carbonyl carbon of a-ketoglutarate, similar to the condensation of acetyl CoA and oxalacetate to yield citrate. This reaction would form homocitric acid. Upon oxidation and decarboxylation of the latter there would be obtained a-ketoadipic acid. Transamination of a-ketoadipic acid produces a-aminoadipic acid, which can be converted to lysine by reduction to the corresponding semialdehyde, followed by transamination. 

Deamination of amino acids in animal tissue is generally effected by transamination with an a-keto-acid. In the majority of cases, this is 2-oxoglutarate formed by the citric acid cycle. Aspartate aminotransferase and alanine aminotransferase are examples of this kind of reaction. In Figure 2.7, transamination involving these enzymes is depicted as it is known to occur in mammalian liver. Note that the scheme shown here requires participation of oxalacetate and pyruvate and thus is intimately connected with metabolic pathways considered earlier. Serine and glycine are readily interconvertible in animal tissue by the enzyme serine hydroxymethyltransferase. It is worth noting also that decarboxylation of serine to ethanolamine as mentioned above can be followed by A -methylation to yield choline. Choline is both an essential component of many... 

Assay of Transamination. Since the sum of keto acids and amino acids does not change in a transamination, specific reactions are required to assay the reaction products. Some of the methods used are oxidation of a-ketoglutarate to succinate and determination of succinate with succinic dehydrogenase decarboxylation of oxalacetate with aniline citrate decarboxylation with specific amino acid decarboxylases separation of products on paper chromatograms and spectrophotometric determination of those keto acids that exhibit specific absorption. 

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