Human lactate dehydrogenase and malate dehydrogenase possess the catalytic properties to produce aromatic -hydroxy acid

Lactate dehydrogenase and malate dehydrogenase are the two main alpha-hydroxy acid dehydrogenases in the human body. We investigated the catalytic properties of human lactate dehydrogenase LDHC, LDHL6A and malate dehydrogenase MDH1 on aromatic alpha-keto acids phenylpyruvic acid, p-hydroxyphenylpyruvic acid and 3,4-dihydroxyphenylpyruvic acid. The optimum temperatures for LDHC, LDHL6A, and MDH1 are 37 degree celsius, 35 degree celsius, and 45 degree celsius, respectively; and the optimum pH is 6.5, 6.5, and 5.5, respectively. The K-m of LDHC for phenylpyruvic acid and 3,4-dihydroxyphenylpyruvic acid were 0.90 mM and 0.92 mM, respectively. LDHL6A has a high affinity for phenylpyruvic acid and 3,4-dihydroxyphenylpyruvic acid with K-m of 0.77 mM and 0.80 mM, respectively; MDH1 has an extremely high affinity (K-m = 0.46 mM) and catalytic efficiency (k(cat)/K-m = 23.87 s(-1)mM(-1)) for p-hydroxyphenylpyruvic acid. It also has a high affinity for 3,4-dihydroxyphenylpyruvic acid with a K-m of 0.90 mM, but with a low affinity for phenylpyruvic acid (K-m = 3.76 mM). The catalytic properties of human LDHC, LDHL6A, and MDH1 for the abovementioned aromatic alpha-keto acids may be one of the sources of L-phenyllactic acid, L-p-hydroxyphenyllactic acid, and L-3,4-dihydroxyphenyllactic acid in humans.