Lipoic acid arsenite, reaction with

Pyruvate produced by the glycolytic pathway may be transported into the mitochondria (via an antiport with OH"), where it is converted to acetyl-CoA by the action of the enzyme complex pyruvate dehydrogenase. The pertinent enzyme activities are pyruvate dehydrogenase (PD), lipoic acid acetyltransferase, and dihydrolipoic acid dehydrogenase. In addition, several cofactors are utilized thiamine pyrophosphate (TPP), lipoic acid, NAD+, Co A, and FAD. Only Co A and NAD+ are used in stoichiometric amounts, whereas the others are required in catalytic amounts. Arsenite and Hg2+ are inhibitors of this system. The overall reaction sequence may be represented by Figure 18.5. The NADH generated may enter the oxidative phosphorylation pathway to generate three ATP molecules per NADH molecule reduced. The reaction is practically irreversible its AGq = -9.4 kcal/mol. 

Musilek and SevCik (1958a and b) reported that 4 X 10 m arsenite inhibited erythromycin biosynthesis by 87% with a simultaneous accumulation of pyruvate and acetaldehyde in the medium, but the growth of S. erythreus was not affected. The inhibitory effect of arsenite was reversed to some extent by acetate, propionate and formate (MusIlek and SevCIk, 1959 SevCIk and MusIlek, 196I). Since arsenite may inhibit a series of reactions catalyzed by lipoic acid (decarboxylation of other 2-oxoacids than pyruvic acid), these results do not prove conclusively the role of Cj- or Cg-units in the biosynthesis of erythromycin. 

The acyl-transfer reaction, Eq. (9), is visualized as an acyl exchange between protein-boimd S-acyl(iihydrolipoic acid and CoA to give protein-bound dihydrolipoic acid and acyl CoA. Evidence for this reaction is based on model reactions carried out with substrate amounts of dihydrolipoic acid and on the arsenite inhibition studies of Sanadi et al. (1959b) discussed in Section III, A, 2. An enzymatic component, lipoic reductase-transacetylase, which apparently catalyzes reactions (8) and (9), has been isolated from the E. colt pyruvate dehydrogenation complex (Koike and Reed, 1961). It is uncertain whether this component consists of one or two enzymes. 

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