In-vitro metabolic interaction of bunitrolol enantiomers in rabbit liver microsomes.
We have examined the 4-hydroxylation of bunitrolol in rabbit and rat liver microsomes. Significant species differences (rabbit < rat of both sexes) and sex (male > female of both species) were observed in the formation of 4-hydroxybunitrolol from racemic bunitrolol (10 microM). The 4-hydroxylation of bunitrolol racemate and enantiomers showed biphasic kinetics, a low-Km system and a high-Km system, in liver microsomes from rabbits of both sexes. There were significant differences in Km and Vmax values [(+) > (-)] for 4-hydroxylations of (+)-bunitrolol and (-)-bunitrolol in the low-K(m) system. Furthermore, the rate of clearance (Vmax/Km) was 20- to 200-fold for the low-Km system compared with the high-Km system, indicating that enzymes in the low-Km system play a major part in the rabbit liver microsomal bunitrolol metabolism. Inhibition studies using cytochrome P450 inhibitors such as quinidine, quinine, and alpha-naphthoflavone or polyclonal antibodies raised against rat P450-2D and -1A enzymes did not make clear which P450 enzymes are involved in bunitrolol 4-hydroxylation in rabbit liver microsomes. The 4-hydroxylase activity of (+)-bunitrolol was slightly higher than that of (-)-bunitrolol in separated incubations containing male rabbit liver microsomes and an enantiomer concentration of 10 microM. However, the 4-hydroxylation of (+)-bunitrolol (10 microM) was markedly suppressed in the presence of its antipode (10 microM), whereas (-)-bunitrolol 4-hydroxylation was not affected by the presence of its antipode, resulting in a change of the stereoselectivity from (+) > (-) for enantiomer to (+) < (-) for racemate. The difference in the Michaelis constants in the low-Km system, where the Km value of (-)-bunitrolol is one-eighth that of (+)-bunitrolol, is thought to cause the change in the stereoselectivity in rabbit liver microsome-mediated bunitrolol 4-hydroxylation.
Narimatsu
S
S
Laboratory of Biopharmaceutics, Faculty of Pharmaceutical Sciences, Chiba University, Japan.
Mizukami
T
T
Huang
Y
Y
Masubuchi
Y
Y
Suzuki
T
T
D000319:Adrenergic beta-Antagonists / Q000378:metabolism*; D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003647:Debrisoquin / Q000494:pharmacology; D005260:Female; D006900:Hydroxylation; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D011412:Propanolamines / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011802:Quinidine / Q000494:pharmacology; D011803:Quinine / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D017208:Rats, Wistar; D013045:Species Specificity; D013237:Stereoisomerism
D000319:Adrenergic beta-Antagonists; D011412:Propanolamines; C036344:4-hydroxybunitrolol; D003577:Cytochrome P-450 Enzyme System; D011803:Quinine; C084811:bunitrolol; D011802:Quinidine; D003647:Debrisoquin