Partial purification and characterization of β-hydroxybutyric acid dehydrogenase of a methylotrophic bacterium, Pseudomonas 135

An intracellular enzyme, d(—)-β-hydroxybutyric acid dehydrogenase involved in an intracellular poly-d(—)-β-hydroxybutyric acid degredation was isolated from a facultative methylotrophic bacterium, Pseudomonas 135, grown on methanol as a sole carbon and energy source. This enzyme was partially purified to 11.6-fold by ammonium sulphate fractionation and a dye-affinity chromatography. The enzyme catalysed simultaneously the oxidation of d(—)-β-hydroxybutyric acid (Dβ-HB) and the reduction of acetoacetate. The optimum pH was 8.5 for the oxidation reaction and 5.5–6.0 for the reduction reaction, and the enzyme was stable for 2 weeks at — 20° C. The Km values for oxidation and reduction reactions were determined as 1.84 mm for Dβ-HB, 0.244 mm for NAD+, 0.319 mm for acetoacetate and 0.032 mm for NADH, respectively. It was also found that d-lactate and NADH significantly inhibited the oxidation reaction by competitive inhibition, and acetoacetate by non-competitive inhibition, respectively. The inhibition constants were determined as 1.49 mm for d-lactate, 0.196 mm for NADH and 1.82 mm for acetoacetate, respectively. According to an experiment with resting cells, it seemed that the enzyme was constitutive.