Purification and characterization of hydroxypyruvate reductase from a serine‐producing methylotroph, Hyphomicrobium methylovorum GM2

Hydroxypyruvate reductase of a serine‐producing methylotroph, Hyphomicrobium methylovorum GM2, was purified to complete homogeneity, crystallized and characterized, the first time for an enzyme from a methylotroph. The enzyme was found to be a dimer composed of identical subunits (38 kDa), the molecular mass of the enzyme being about 70 kDa. The enzyme was stable against heating at 25°C for 10 min at pH values between 5 and 9. Optimal activity was observed at pH 6.8 and around 45°C. The enzyme catalyzed the reduction of hydroxypyruvate with the oxidation of only NADH. Other than hydroxypyruvate, only glyoxylate served as a substrate. The K_m values were found to be 0.175 mM for hydroxypyruvate and 10.8 mM for glyoxylate. Taking advantage of the high substrate specificity of this enzyme, a means of enzymatic determination of hydroxypyruvate was established.