Biosynthesis of isoleucine and valine in Mycobacterium tuberculosis H₃₇ R_v

Abstract

The enzymes involved in the biosynthesis of isoleucine and valine have been shown to be present in cell-free extracts of Mycobacterium tuberculosis H₃₇R_v. In addition to the known enzymes of the pathway, cell-free extracts of this organism contain a new enzyme. When cell-free extracts were incubated with acetolactate and L-ascorbic acid, without reduced nicotinamide adenine dinucleotide phosphate, the isomer of acetolactate, viz., α-keto-β-hydroxyisovalerate, was found to accumulate and was identified by different methods. The reaction is enzymic, and L-ascorbic acid cannot be replaced by other reducing agents such as hydroquinone, 2,6-dichlorophenol indophenol, or glutathione; by derivatives of L-ascorbic acid such as dehydroascorbic acid or dimethyl ascorbic acid; or by cobamide coenzyme. Since the extracts also isomerize α-acetohydroxybutyrate to α-keto-β-hydroxy-β-methylvalerate, the enzyme catalyzing the reaction has been termed "acetohydroxy acid isomerase". This is the first time that the presence of acetohydroxy acid isomerase has been reported in any biological system and that a specific metabolic role has been assigned for L-ascorbic acid. The extract also possesses reductase activity to convert α-keto-β-hydroxyisovalerate to α,β-dihydroxyisovalerate in the presence of reduced nicotinamide adenine dinucleotide phosphate.