Bufuralol

Abstract: The anti-arrhythmic quinidine has been reported to be a competitive inhibitor of the catalytic activities of human liver P-450DB, including sparteine delta 2-oxidation and bufuralol 1'-hydroxylation, and we confirmed the observation that submicromolar concentrations are strongly inhibitory. Human liver microsomes oxidize quinidine to the 3-hydroxy (Km 4 microM) and N-oxide (Km 33 microM) products, consonant with in vivo observations. Both bufuralol and sparteine inhibited microsomal quinidine 3-hydroxylation. Liver microsomes prepared from DA strain rats showed a relative deficiency in quinidine 3-hydroxylase activity in females compared to males. These observations might suggest that quinidine oxidation is catalyzed by the same P-450 forms that oxidize debrisoquine, bufuralol, and sparteine; i.e., rat P-450UT-H and P-450DB. However, neither of these two purified enzymes catalyzed quinidine 3-hydroxylation, and anti-P-450UT-H, which strongly inhibits human liver microsomal bufuralol 1'-hydroxylation, did not substantially inhibit quinidine 3-hydroxylation or N-oxygenation. P-450MP, the human S-mephenytoin 4-hydroxylase, also does not appear to oxidize quinidine but P-450NF, the human nifedipine oxidase, does. Anti-P-450NF inhibited greater than 95% of the 3-hydroxylation and greater than 85% of the N-oxygenation of quinidine in several microsomal samples. Quinidine inhibited microsomal nifedipine oxidation and, in a series of human liver samples, rates of nifedipine oxidation were correlated with rates of quinidine oxidation. Thus, quinidine oxidation appears to be catalyzed primarily by P-450NF and not by P-450DB. Quinidine binds 2 orders of magnitude more tightly to P-450DB, which does not oxidize it, than to P-450NF, the major enzyme involved in its oxidation. The substrate specificity of human P-450NF is discussed further in terms of its regioselective oxidations of complex molecules including quinidine, aldrin, benzphetamine, cortisol, testosterone and androstenedione, estradiol, and several 2,6-dimethyl-1,4-dihydropyridines.

Abstract: Human cytochrome P450 (P450) 1B1 (CYP1B1) has recently been shown to be an important enzyme in the activation of diverse procarcinogens such as arylarenes, nitroarenes, and arylamines to reactive metabolites that cause DNA damage in the cells. However, it is not known whether this P450 enzyme also plays roles in the oxidation of certain drugs or model substrates commonly used in P450 assays. We examined the substrate oxidation activities of recombinant human CYP1B1 in yeast microsomes and compared these activities with those catalyzed by reconstituted systems containing recombinant CYP1A1 and CYP1A2 which were isolated from membranes of Escherichia coli in which respective cDNAs have been expressed. Catalytic activities towards some of the model substrates of other human P450 enzymes including CYP2A6, 2C9, 2C19, 2D6, 2E1, and 3A4 were also determined and compared. CYP1B1 catalyzed benzo[a]pyrene 3-hydroxylation at rates lower than those of CYP1A1 but higher than those of CYP1A2. The activity towards 7-ethoxyresorufin O-deethylation catalyzed by CYP1B1 was about one-tenth of that of CYP1A1, but the Km values were lower for CYP1B1 than those for CYP1A1 and CYP1A2. CYP1B1 was also able to catalyze the oxidation of theophylline and caffeine, two prototypic substrates for CYP1A2. CYP1B1 did not oxidize other typical P450 substrates such as coumarin, tolbutamide, S-mephenytoin, chlorzoxazone, nifedipine, and testosterone, while low rates of oxidation of bufuralol and 7-ethoxycoumarin were found for CYP1B1. These results indicate that CYP1B1 has catalytic activities overlapping CYP1A1 and CYP1A2 with respect to the oxidation of drugs and model P450 substrates, although the relative catalytic roles in these three P450 enzymes differ depending upon the substrates examined. A distinct marker activity of CYP1B1 has not been identified.