Hydroxyhydroquinone reductase, the initial enzyme involved in the degradation of hydroxyhydroquinone (1,2,4-trihydroxybenzene) by Desulfovibrio inopinatus

The recently isolated sulfate reducer Desulfovibrio inopinatus oxidizes hyd roxyhydroquinone (1,2,4-trihydroxybenzene; HHQ) to 2 mol acetate and 2 mol CO2 (mol substrate)(-1), with stoichiometric reduction of sulfate to sulfid e. None of the key enzymes of fermentative HHQ degradation, i.e. HHQ-1,2,3, 5-tetrahydroxybenzene transhydroxylase or phloroglucinol reductase, were de tected in cell-free extracts of D. inopinatus, indicating that this bacteri um uses a different pathway for anaerobic HI-IQ degradation. HHQ was reduce d with NADH in cell-free extracts to a nonaromatic compound, which was iden tified as dihydrohydroxyhydroquinone by its retention time in HPLC separati on and by HPLC-mass spectrometry. The compound was identical with the produ ct of chemical reduction of HHQ with sodium borohydride. Dihydrohydroxyhydr oquinone was converted stoichiometrically to acetate and to an unknown copr oduct. HHQ reduction was an enzymatic activity which was present in the cel l-free extract at 0.25-0.30 U (mg protein)(-1), with a pH optimum at 7.5. T he enzyme was sensitive to sodium chloride, potassium chloride, EDTA, and o -phenanthroline, and exhibited little sensitivity towards sulfhydryl group reagents, such as copper chloride or p-chloromercuribenzoate.