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J. Biochem, 1997, Vol. 121, No. 4 705-710
© 1997 Japanese Biochemical Society

research-article

Characterization of Acetohexamide Reductases Purified from Rabbit Liver, Kidney, and Heart: Structural Requirements for Substrates and Inhibitors

Yorishige Imamura*,1, Toshihisa Koga*, Toyohiko Migita*, Akio Ryu*, Masaki Otagiri*, Masako Nozawa{dagger} and Hiroyuki Akita{dagger}

*Faculty of Pharmaceutical Sciences, Kumamoto University 5-1 Oe-honmachi, Kumamoto 862
{dagger}School of Pharmaceutical Sciences, Toho University 2-2-1 Miyama, Funabashi, Chiba 274

1To whom correspondence should be addressed.

The structural requirements of acetohexamide reductases purified from rabbit liver, kidney,and heart for substrates and inhibitors were examined. Acetohexamide, an oral antidiabetic drug with a ketone group, and analogs of it with various alkyl groups instead of the cyclohexyl group were used as substrates for these three enzymes. The results obtained as to substrate specificity suggested that the nature of the substrate-binding region of the heart enzyme is markedlydifferent from those of the substrate-binding regions of the liver and kidney enzymes. Tolbutamide, which has no ketone group within its chemical structure, strongly inhibited the heart enzyme, whereas it had little ability to inhibit the liver or kidney enzyme. The inhibition of the heart enzyme by tolbutamide was competitive with respect to acetohexamide and uncompetitive with respect to NADPH. Furthermore, tolbutamide analogs with n-pentyl and n-hexyl groups instead of the n-butyl group exhibited very pronounced inhibition of only the heart enzyme. Therefore, it is reasonable to postulate that the heart enzyme, unlike the liver and kidney ones, has a cleft of a strongly hydrophobic nature near its substrate-binding region, andthat this hydrophobic cleft plays a critical role in the interaction of the heart enzyme with the cyclohexyl group of acetohexamide.


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