Characterization of acetohexamide reductases purified from rabbit liver, kidney, and heart: structural requirements for substrates and inhibitors.
(9/10)
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 markedly different 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, and that this hydrophobic cleft plays a critical role in the interaction of the heart enzyme with the cyclohexyl group of acetohexamide. (+info)
20beta-hydroxysteroid dehydrogenase catalyzes ketone-reduction of acetohexamide, an oral antidiabetic drug, in liver microsomes of adult male rats.
(10/10)
We examined the catalytic properties and physiological function of an enzyme responsible for the ketone-reduction of acetohexamide, an oral antidiabetic drug, in liver microsomes of adult male rats. Progesterone, 17alpha-hydroxyprogesterone, cortisone and cortisol, which have a ketone group at 20-position of C21-steroids, were potent inhibitors for ketone-reduction of acetohexamide in liver microsomes of adult male rats. Progesterone was also found to inhibit competitively the ketone-reduction of acetohexamide, suggesting that the ketone-reduction of acetohexamide and progesterone is catalyzed by the same enzyme. When progesterone was used as a substrate, 20beta-hydroxysteroid dehydrogenase present in liver microsomes of adult rats, such as acetohexamide reductase, exhibited a male-specific and androgen-dependent activity. Furthermore, a significant correlation was observed between the activities of 20beta-hydroxysteroid dehydrogenase and acetohexamide reductase in liver microsomes of individual male rats at various ages. Based on all results, we conclude that 20beta-hydroxysteroid dehydrogenase catalyzes the ketone-reduction of acetohexamide in liver microsomes of adult male rats. (+info)