The Effect of Cysteine on L-α-Glycerophosphate and Lactate Dehydrogenase Reactions

Under conditions for the optimum activities of l-α-glycerophosphate and lactate dehydrogenases, cysteine reacts with dihydroxyacetone phosphate and pyruvate. The thiohemiketals presumably formed are not substrates for the enzymes. By competing with the enzymes for the substrate, the reaction of cysteine with dihydroxyacetone phosphate and pyruvate inhibits their enzymatic reductions to α-glycerophosphate and lactate; the reverse reactions are promoted through removal of the products of the enzymatic reactions. The reaction between cysteine and certain carbonyl compounds may be responsible for several unexplained results in enzymatic systems where both α-keto acid and cysteine, or homocysteine, are the substrates.