Studies on the nature and reactions of protein-bound lipoic acid.

Studies on the activation of an apopyruvate dehydrogenation system obtained from extracts of lipoic acid-deficient Streptococcus jaecalis cells indicated that in its functional form lipoic acid is bound to protein in covalent linkage through its carboxyl group, i.e. as “lipoyl enzyme” (1). Further support for this proposal is furnished by the present finding that a partially purified enzyme, “lipoyl-X hydrolase,” obtained from S. juecalis extracts, liberated lipoic acid from the protein-bound form present in the Escherichia coli (Crookes strain) pyruvate dehydrogenation system, thereby inactivating the latter system. Reactivation required the same components and conditions as were found necessary to activate the S. jaecalis apopyruvate dehydrogenation system. A preliminary communication of this work has been reported elsewhere (2). The availability of a method of releasing lipoic acid from the proteinbound form and of reactivating the apoenzyme has enabled us to study the mechanism of certain enzymatic reactions which have been carried out with free lipoic acid or structurally related compounds. Gunsalus and his collaborators reported (3-6) that E. coli Fraction A, in the presence of CoA’ and phosphotransacetylase, catalyzed Reactions 1 and 2, and that Fraction B, in the presence of DPN and lactic dehydrogenase, catalyzed Reaction 3.