Fast regulation of cytochrome P450 activities by phosphorylation and consequences for drug metabolism and toxicity.

In contrast to the wellknown regulation of cytochrome P450 (CYP) activity by enzyme induction, which represents a process with slow onset and slow offset, more recent studies revealed phosphorylation as a fast (within observation instantaneous) and isoenzymeselective regulation. The phosphorylated enzyme (investigated isozyme: CYP2B1) was fully inactive. The phosphorylation is mediated by PKA and hence under control of hormones and drugs that alter cellular cAMP levels. The consequences for the metabolic control of toxic species derived from drugs and environmental carcinogens are discussed. This information will help to improve therapy with drugs metabolized by CYPs which are phosphorylated by PKA, especially if these drugs possess a narrow window between required effectiveness and unacceptable toxicity.