P-450 Level and Taurochenodeoxycholate 6β-Hydroxylase System of Rat Liver Microsomes

Summary

The level of cytochrome P-450 and the activity of taurochenodeoxycholate 6β-hydroxylase of rat liver microsomes were determined after the administration of phenobarbital, cholestyramine, thyroxine and chenodeoxycholate. Treatment with phenobarbital caused increases in both the 6β-hydroxylase activity and P-450 level, while the other compounds caused decreases in both. Quantitatively there was a lack of parallelism between the hydroxylase activity and the level of P-450, indicating that P-450 is not a rate-limiting factor in the 6β-hydroxylase system. An extract of the microsomes was prepared in 1.0 M phosphate (pH 7.6) as previously described. The extract was active in 6β-hydroxylation, with an apparent Michaelis-Menten constant towards taurochenodeoxycholate of 1.2 × 10^-4M, and a maximum rate of hydroxylation of approximately 1 nmole/min/mg protein. The ratio of P-450 to protein of the extract was 2 to 2.5 times greater than that in the microsomes.