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Human liver 3α-hydroxysteroid/dihydrodiol dehydrogenase (DD) is involved in the metabolism of steroid hormones and polycyclic aromatic hydrocarbons, and is also responsible for the reduction of ketone-containing drugs. To account for the interindividual difference in the activity, we isolated and characterized clones for the human liver enzymes. The sequence of the cDNA clone coding for the variant differed from that coding for the wild-type DD by two nucleotides (substitutions of C with G at positions 434 and 931) which caused two amino acid replacements, Ser145 to Cys (S145C) and Leu311 to Val (L311V). The heterologous expression of the variant mRNA was confirmed in four of 31 liver samples from Japanese by an allele-specific polymerase chain reaction. The effects of the mutations on the catalytic properties were examined with the recombinant enzymes expressed in Escherichia coli. The introduction of S145C/L311V double mutations resulted in three- to five-fold decreased activities for xenobiotic and steroidal substrates, whereas no significant change was observed by an introduction of the S145C mutation alone. The results substantiate the existence of polymorphic forms for human liver DD, and also suggest the importance of the residue at position 311 for substrate binding to the enzyme.