Evidence for an Enol Intermediate in the Enzymatic Conversion of α,β-Dihydroxyisovalerate to α-Ketoisovalerate

α,β-Dihydroxyisovalerate-α-3H was converted to α-ketoisovalerate by a partially purified α,β-dihydroxyacid dehydrase from Salmonella typhimurium. More than 98% of the radioactivity was found in the water. When unlabeled substrate was incubated with the enzyme in 3H2O, there was significant incorporation of protons from the medium. These results suggest the formation of an enol intermediate in the conversion of α,β-dihydroxyisovalerate to α-ketoisovalerate catalyzed by α,β-dihydroxyacid dehydrase.