P450PCN protein levels and nifedipine oxidase activities were quantitated in 12 human livers and were shown to be highly correlated. Antibody against rat P450PCN1 completely inhibited all nifedipine oxidase activity in three human liver samples. These results suggest that a human P450 related to rat P450PCN1 is the major form of P450 catalyzing nifedipine oxidation. The cDNA for a human P450, designated phPCN1, was isolated from a human liver lambda gt11 cDNA library, and sequenced completely. The deduced amino acid sequence is 77% similar to rat P450PCN1. By use of the adenovirus- and SV40-based expression vecotr p91023(B), the phP450PCN1 cDNA was expressed in COS cells and had high nifedipine oxidase activity, providing conclusive evidence that this P450 is the primary enzyme associated with metabolism and inactivation of this important drug. Using somatic cell hybrids, the P450PCN gene was localized to human chromosome 7.