Strain- and sex-related differences of acetohexamide reductase activities in liver microsomes and cytosol of rats: the Wistar-Imamichi strain lacks the male-specific microsomal enzyme activity

Differential pharmacokinetics of acetohexamide in male Wistar-Imamichi and Sprague-Dawley rats: role of microsomal carbonyl reductase

Acetohexamide (AH) is reduced to its alcohol metabolite by carbonyl reductase. We have previously shown that carbonyl reductase present in the liver microsomes of rats is a male-specific and androgen-dependent enzyme. In the present study, the role of microsomal carbonyl reductase in the pharmacokinetics of AH was examined in male Wistar-Imamichi (WI) and Sprague-Dawley (SD) rats after its intravenous administration. AH was eliminated more slowly from plasma in the WI strain, which lacks most of the microsomal carbonyl reductase, than in the SD strain. Furthermore, several pharmacokinetic parameters were derived from the data for the plasma concentrations of AH. The plasma clearance (CL(p)) of AH (72.8+/-11.2 ml/h/kg) in male WI rats was significantly smaller than that (105.5+/-11.1 ml/h/kg) in male SD rats. Testectomy caused a marked decrease, from 105.5+/-11.1 to 44.3+/-11.8 ml/h/kg, in the CL(p) of AH in male SD rats. These results indicate that microsomal carbonyl reductase plays a critical role in the differential pharmacokinetics of AH in male WI and SD rats.

[Strain-, sex- and species-related differences of acetohexamide reductase and 20 beta-hydroxysteroid dehydrogenase activities in liver microsomes of experimental animals]

We examined physiological and genetic factors affecting acetohexamide reductase (AHR) and 20 beta-hydroxysteroid dehydrogenase (20 beta-HSD) activities in liver microsomes of experimental animals. Pronounced strain-related differences were found in both activities of AHR and 20 beta-HSD present in liver microsomes of male rats. Among rat strains tested in this study, even though a Wistar-Imamichi (WIM) rat strain was taken to lack AHR activity, it exhibited a significant 20 beta-HSD activity. These findings appeared to be in conflict with our conclusion reported so far, which AHR and 20 beta-HSD present in liver microsomes of male rats are identical enzymes. Thus the reason for this discrepancy was discussed. Furthermore, AHR and 20 beta-HSD activities were little or not observed in liver microsomes of female rats or male experimental animals other than the rat, indicating the existence of sex- and species-related differences in these two enzyme activities.

Inter-species comparison of microsomal reductive transformation of biologically active benfluron N-oxide

Benfluron N-oxide is an anti-neoplastic active metabolite of benfluron (B) /1/. It is generated by flavine-monooxygenase-catalysed reactions /2/ and immediately undergoes subsequent metabolic transformations, the most important of which are reductive reactions /3/. The products of reductive pathways catalysed by two different microsomal enzymatic systems are the tertiary amine benfluron (i.e. the original parent compound) and/or 7-dihydrobenfluron N-oxide. Our studies on the reductive transformation of B N-oxide in rat, mouse, guinea-pig, rabbit, mini-pig and human microsomes have revealed significant species differences both in the yields of respective reduced metabolites and in the conditions essential for the activity of the reductases involved. While B, the original tertiary amine, is the main product of aerobic incubation of B N-oxide with NADPH in rat, mouse and mini-pig, significantly higher activities of the enzymes catalysing the formation of 7-dihydro-B N-oxide have been detected in rabbit and human microsomes. In rat, mouse and mini-pig, NADPH rather than NADH is the preferred coenzyme for B formation, and NADPH is also the preferred coenzyme for the formation of 7-dihydro-B N-oxide in most of the species used. The yield of tertiary amine B is higher in anaerobic rather than aerobic conditions in most experimental species studied. Aerobic or anaerobic incubating conditions have an insignificant effect on the formation of 7-dihydro-B N-oxide. Based on the inhibitory effect of CO on the reductive transformation of B N-oxide, cytochromes P450 can be assumed to participate in the formation of B both in rat and mini-pig, and, in mini-pig only, also in the formation of 7-dihydro-B N-oxide. Inter-species comparison of the properties of the reductases participating in the transformation of B N-oxide shows that the rabbit is a suitable model to study reductive transformation of B N-oxide in man.

Inheritance of acetohexamide reductase activities in liver microsomes and cytosol of rats

The inheritance patterns of acetohexamide reductase activities in liver microsomes and cytosol of rats were determined by using the inbred Wistar-Imamichi and Fischer-344 strains as a model of low and high metabolizers, respectively. A simple Mendelian genetic analysis for the frequency distribution of acetohexamide reductase activity in liver microsomes of male rats led us to conclude that the phenotype is genetically regulated by an autosomal co-dominant fashion. Female rats, unlike male rats, did not exhibit microsomal enzyme activity in parental, first filial (F1) and second filial (F2) generations, indicating that the inheritance of the microsomal enzyme activity is sex-limited. On the other hand, the frequency distribution of acetohexamide reductase activities in liver cytosol of male and female rats was unimodal in all generations and there was no significant difference among these cytosolic enzyme activities.

Adult-male-specific S-warfarin (11S-OH) and progesterone (20 beta-OH) keto-reductases in rat hepatic microsomes are not identical

Adult-male-specific reductase activities in rat hepatic microsomes use NADPH to reduce S-warfarin and progesterone to their 11S-OH and 20 beta-OH products, respectively (Apanovitch et al. (1992) Biochem. Biophys. Res. Commun. 184, 338-346). When microsomes were treated with increasing concentrations of detergent, S-warfarin (11S-OH) reductase (SW(11S)R) activity was subject to monophasic activation by Triton X-100, monophasic inhibition by sodium cholate, and, activation followed by inhibition with either CHAPS or dodecyl-beta-D-maltoside. A non-dialyzable, heat-sensitive factor in rat and rabbit sera activates microsomal SW(11S)R activity six- to eight-fold. Similar detergent inhibitions but no detergent or serum activations were observed for progesterone (20 beta-OH) reductase (P(20 beta)R) activity. A significant amount of SW(11S)R activity was lost during purification regardless of whether the detergent used for solubilization was activating or inhibiting. Octyl-Sepharose, hydroxyapatite, DEAE-cellulose and carboxymethyl matrices were used to partially purify SW(11S)R. P(20 beta)R activity co-purified with SW(11S)R and the most purified fraction contained two major and several minor polypeptides. Partially purified SW(11S)R is activated by detergents, serum, and salt. These and previous results indicate that SW(11S)R and P(20 beta)R are not identical even though they are both adult male-specific, integral membrane proteins apparently having their active sites exposed on the cytoplasmic surface of the endoplasmic reticulum.