Taxoid metabolism: Taxoid 14β-hydroxylase is a cytochrome P450-dependent monooxygenase

The production of the anticancer drug Taxol in Taxus (yew) cell cultures is often accompanied by the formation of side-route polyoxygenated taxoid metabolites bearing a 14β-hydroxyl group. The recent acquisition of several new semisynthetic taxoid intermediates enabled the screening of a family of Taxus cytochrome P450 cDNA clones for the 14β-hydroxylase and additional taxoid oxygenases. The candidate cytochrome P450 clones were functionally expressed in yeast and tested by in vivo feeding of radiolabeled 5α-acetoxy-10β-hydroxy taxadiene and 5α,13α-dihydroxy taxadiene. One clone efficiently and specifically transformed the 5α-acetoxy-10β-ol, but not the 5α,13α-diol, to a more polar product with the chromatographic properties of a taxoid triol monoacetate, and the identity of this product was confirmed by spectroscopic means as 5α-acetoxy-10β,14β-dihydroxy taxadiene. Microsome preparation from the transformed yeast allowed characterization of this new hydroxylase, which was shown to resemble other cytochrome P450 taxoid hydroxylases with pH optimum at 7.5 and a Km value for the taxoid substrate of about 50 μM. Because Taxol is unsubstituted at C14, the 14β-hydroxylase cannot reside on the pathway to the target drug but rather appears to be responsible for diversion of the pathway to 14-hydroxy taxoids that are prominent metabolites of Taxus cell cultures. Manipulation of this hydroxylase gene could permit redirection of the pathway to increase flux toward Taxol and could allow the preparation of 13α,14β-hydroxy taxoids as new therapeutic agents.