Rat liver cytoplasmic dihydrodiol dehydrogenase. Purification to apparent homogeneity and properties

A method is described for the purification of an enzyme, which catalyzes the conversion of benzene dihydrodiol to catechol, from rat liver cytoplasmic fraction to apparent homogeneity. The purification involved (NH4)2SO4 fractionation, DEAE-cellulose chromatography, interfacial salting-in and gel filtration through Sephadex G-100 superfine. The end product, which was purified over 500-fold with a yield of about 14% when compared to rat liver 100,000 X g supernatant, was judged to be homogeneous by several criteria, including sodium dodecyl sulfate-polyacrylamide gel electrophoresis, analytical ultracentrifugation, gel filtration, and immunoprecipitation. Physical studies suggested that the protein was a monomer with a molecular weight of 35,000 and one NADPH binding site per molecule. Amino acid analysis showed that the enzyme had a relatively high content of acidic and neutral amino acids in agreement with its isoelectric point which was at pH 6.2 Apparent Km values for benzene dihydrodiol and NADP+ were found to be 2.2 mM and 7.7 microM, respectively. The apparent Vmax value for the conversion of benzene dihydrodiol to catechol using NADP+ as cofactor was calculated to be 6.67 mumol/mg of enzyme/min. Substrate specificity studies showed that, in addition to benzene dihydrodiol, the dehydrogenase could oxidize acenaphthenol and the 3 alpha-hydroxy group of steroids. No activity was observable with a large number of other hydroxylated steroids possessing hydroxy groups at positions 3 beta, 11 beta, 17 alpha, 17 beta, 20 alpha, 20 beta, 21, and 22 of the steroid skeleton. Furthermore, only steroids which contained a 3-keto group and no double bond at the delta 4 position were reduced. This, and the fact that a range of nonsteroidal vicinal diols did not serve as substrates, indicates a relatively narrow substrate specificity. When benzene dihydrodiol was used as substrate, NADP+ was the preferred coenzyme but NAD+ was also accepted, whereas with the hydroxylated steroids the difference between the specific activities with NAD+ and NADP+ or with NADH and NADPH was less striking. The role of the enzyme in the metabolism of carcinogenic polycyclic hydrocarbons is discussed.