Effects of two different extracts of St. John's wort and some of their constituents on cytochrome P450 activities in rat liver microsomes.

We examined two commercially available St. John's wort extracts and some of their constituents for their potential in inducing rat cytochrome P450 (CYP450) enzyme activities after oral administration. None of the extracts or pure constituents tested enhanced the hepatic cytochrome content or the activity of cytochrome P450 isozymes in rat liver microsomes. Our results demonstrated that the reported interactions between St. John's Wort and various other drugs are not mediated by CYP 450 isoforms present in rat liver.