Characterization of dihydrodiol dehydrdrogenase in human liver and lung

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Abstract
Dihydrodiol dehydrogenase (DD; EC 1.3.1.20) will oxidize non-K-region trans-dihydrodiols of polycyclic aromatic hydrocarbons (PAHs), a reaction that can suppress the formation of PAH anti-diol epoxides or ultimate carcinogens. Using benzenedihydrodiol [(±)-trans-1,2-dihydroxy-3,5-cyclo-hexadiene] as a model substrate for trans-dihydrodiol metabolites of PAHs, 23 human liver and eight human lung samples were examined for enzyme activity. un human liver, enzyme activity could be measured spectrophotometrically and specific activities ranged from 0.16 to 6.1 ninol benzenedihydrodiol oxidized/mm/mg protein. Western blot analysis of human liver cytosol using rabbit anti-rat DD serum detected two bands of mol. wts 34 000 and 27 000. The former mol. wt is identical to that observed for the homogeneous rat liver enzyme. Gel-filtration experiments indicate that human liver DD activity elutes as a single peak and co-elutes with the purified rat liver enzyme, suggesting that the lower mol. wt species may be an artefact of degradation. Preparations of the human liver enzyme required NADP+ for activity and were, in general, insensitive to inhibition by dicoutnarol, indomethacin and 6-medroxyprogesterone acetate. These properties distinguish the enzyme from alcohol dehydrogenase, quinone reductase and rat liver DID. Rn human lung, DD activity was barely detectable using a sensitive radiochemical assay in which the oxidation of benzenedihydrodiol to catechol is linked to catechol-O-methyl transferase using [3H]S-adenosyl methionine as methyl donor. Specific activities were ∼1000th of that observed for human liver and ranged from 1 to 4 pmol benzenedihydrodiol oxidized/min/mg protein. Western blot analysis of lung cytosol detected three bands of mol. wts 34 000, 31 000 and 28 000. The relatively high levels of DD in human liver suggest that this enzyme may play an important role in PAH detoxication in this organ, while the low levels of DD in lung may contribute to the susceptibility of this tissue to PAH-induced carcinogenesis.

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