Aromatic metabolism in plants III. Quinate dehydrogenase from mung bean cell suspension cultures

Quinate dehydrogenase (Quinate: NAD⁺ oxidoreductase, EC 1.1.1.24) was extracted from liquid suspension cultures of mung bean (Phaseolus aureus Roxb.).

The cells were disrupted by ultrasonic energy, and the enzyme purified by precipitation with ammonium sulfate and elution from columns of Sephadex G-50 and hydroxylapatite. The enzyme was unstable but the activity could be maintained for several weeks after purification of hydroxylapatite when stored at pH 7.5 and at −20°.

The optimum pH for activity was 9.6. The enzyme was specific for NAD⁺. Addition of phenylpyruvate, phenylalanine, cinnamate, or shikimate had no effect on its activity. The enzyme was inhibited by sulfhydryl inhibitors, borate, molybdate, and dehydroquinate. 4-Hydroxybenzoic acid and 3-hydroxybenzoic acid were competitive inhibitors.

The cells also contained 5-dehydroquinate dehydratase (EC 4.2.1.10) and shikimate dehydrogenase (EC 1.1.1.25) and thus contained all the enzymes necessary for the interconversion of quinate and shikimate. Quinate dehydrogenase resembled shikimate dehydrogenase in its pH optimum, inhibition by borate and by the substituted benzoic acids.