A-Keto acid dehydrogenation complexes

Searls and Sanadi (1960a) determined the reduction potential of the pig heart dihydrolipoic dehydrogenase from the extent of its reduction at different DPNH DPN ratios. The value is between —0.332 and —0.320 volt at pH 7.0 and 25°C. Thus the reduction potential of the flavoprotein is close to that of the DPNH-DPN system, —0.320 volt at pH 7.0 and 25°C (Burton and Wilson, 1953), and the Lip(SH)2-LipS2 system, —0.325 volt at pH 7.0 and 25°C (see Section II, A). This is consistent with the ready reversibility of reaction (26) as well as the high initial reaction rates in both directions. It would appear that DPN is the physiological electron acceptor for the a-keto acid dehydrogenation complexes in vivo and that the DPNH formed is reoxidized by way of the electron transport chain. However, the... 

Although there is at present no convincing evidence of a natural function of lipoic acid other than that of a prosthetic group in a-keto acid dehydrogenation complexes, it is conceivable that lipoic acid may function in other enzyme systems as an acyl or electron carrier. 

In a similar way, other a-keto acids, e.g., a-ketoglutarate (in the TCA cycle see below) and branched-chain cc-keto acids derived by transamination from the branched-chain amino acids valine, leucine, and isoleucine (Chapter 17), undergo decarboxylation and dehydrogenation catalyzed by enzyme complexes. These enzyme complexes differ in specificity of Ei and E2, but all contain the same E3 (the dihydrolipoyl dehydrogenase). 

Some studies suggest that the principal pathway of glutamate utilization in liver mitochondria is by transamination (81). GDH decreases the distribution coefficient of glutamate-oxaloacetate aminotransferase on Sephadex G-20Q, possibly by forming a complex with that enzyme (82). In addition, in the presence of NADPH and NHC, GDH appears to catalyze the conversion of the pyridoxal phosphate form of the aminotransferase to the pyridoxamine form, which catalyzes the formation of a-amino acids from a-keto acids (83). This reaction is interesting in view of the inhibition of GDH by pyridoxal phosphate (54) (See Section V,A). If the complex exists in mitochondria, it may provide an efficient mode of dehydrogenation of amino acids that are not normally good substrates of GDH (82). 

The decarboxylation reaction, Eq. (7), is visualized as a cleavage of the a-keto acid to yield CO2 and an enzyme-bound aldehyde-thiamine pyrophosphate (RCHO—TPP) compound, i.e., active aldehyde. There is now unequivocal evidence for this reaction since a pyruvic carboxylase (El) has been shown to be an essential component of the E. coli pyruvate dehydrogenation complex (Koike and Reed, 1961 Gounaris and Hager, 1961) and the nature of the aldehyde-TPP compound has been elucidated (Breslow, 1958 Breslow and McNelis, 1959 Krampitz et al., 1961 Holzer and Beaucamp, 1961 Carlson and Brown, 1961). 

Support for the proposal that lipoic acid is bound in the pyruvate dehydrogenation complex in covalent linkage through its carboxyl group was furnished by studies with a hydrolytic enzyme, lipoyl-X hydrolase, obtained from extracts of S. faecalis (Reed et al., 1958b). Incubation of the E. coli pyruvate and a-ketoglutarate dehydrogenation complexes with lipoyl-X hydrolase released approximately 96% of the bound lipoic acid (Koike and Reed, 1960) and resulted in a loss of the DPN-linked a-keto... 

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