Dihydroxy acid dehydrase

Conversion of the acetohydroxy acids to the a,/3-dihydroxyacid precursors of valine and isoleucine is catalyzed by acetohydroxy acid isomeroreductase. The a,/3-dihydroxy acids are both converted to the a-keto acid precursors of valine and isoleucine by a dihydroxy acid dehydrase. Finally, the two amino acids are formed in trans-... 

Myers, J.W. (1961) Dihydroxy acid dehydrase an enzyme involved in the biosynthesis of isoleucine and valine, /, Biol Chem., 236, 1414-1418. 

The reason for a double requirement of valine and isoleucine in certain mutant strains of Neurospora and E. colt is that a single enzyme catalyzes reactions common to both syntheses. Enz3mes for which there is such evidence are dihydroxy acid dehydrase (160) and a transaminase that converts a-ketovaline and a-ketoisoleucine to valine and isoleucine (143). There is a distinct probability that other enz3mes further back in the bios mthetic sequence also may function in common. 

The dehydrase enzyme, named dihydroxy acid dehydrase was found to have a pH optimum at 8-9. It is inhibited by fluoride and the equilibrium of the reaction appears to be far toward keto acid formation. 

The amount of this metabolite, retention time 31 min (compound 3, fig.lA) accounted for approximately 40% of total recovered radioactivity at pH 7.5. When incubation medium pH was 6.3 the amount of compound 2, retention time 27 min. (fig. IB) was the same as the yield of the main product (about 10%). The change of direction of the substrate metabolism corresponds to different formation of a- and y-ketols under weak acidic and neutral pH of incubation medium. The similar stimulation of y-ketol formation under weak acidic pH values have been observed in the studying of metabolism of a-linolenic acid hydroperoxides by hydroperoxide dehydrases from different plant species [3]. The UV spectrum of metabolite 2 exhibits a typical oxoene chromophore, at 227 nm. The data obtained allow us to suggest that compound 2 is y-ketol of 9-hydroxy derivative, 9,12-dihydroxy-15-oxo-10( ),13( )-octadecadienoic acid. 

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