Catalytic Properties for Naphthoquinones and Partial Primary Structure of Rabbit Heart Acetohexamide Reductase.

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The catalytic properties of rabbit heart acetohexamide reductase (RHAR) for naphthoquinones were examined. RHAR efficiently reduced 1, 4-naphthoquinone and juglone (5-hydroxy-1, 4-naphthoquinone), whereas it had little or no ability to reduce menadione (2-methyl-1, 4-naphthoquinone) or plumbagin (5-hydroxy-2-methyl-1, 4-naphthoquinone). The structural requirements for these four naphthoquinones and one acetohexamide analog, and the kinetic mechanism for the inhibition of acetohexamide reduction by juglone led us to conclude that the 2-methyl group of menadione and plumbagin prevents access of the substrates to the catalytic site of RHAR. Five of six peptides derived from RHAR showed 30-42% residue identities with regions in the amino acid sequence of mouse lung carbonyl reductase (MLCR) belonging to the short-chain dehydrogenase/reductase (SDR) family. The catalytically important residues (Arg-39, Ser-136, Tyr-149 and Lys-153) of MLCR were found in the peptide sequences of RHAR, despite the low residue identities between the two enzymes. RHAR is probably bast classified as a member of the SDR family similar to MLCR.

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