In cytochrome P450, P stands for:

Cytochrome P450: The main reaction involved in phase 1 metabolism is hydroxylation, catalyzed by a family of enzymes known as monooxygenases or 'mixed-function oxidases.'  There are at least 57 cytochrome P450 genes in the human genome.  Cytochrome P450 is a heme enzyme.  They exhibit an absorption peak at 450 nm.  Approximately 50% of the common drugs that humans ingest are metabolized by isoforms of cytochrome P450.  They also act on steroid hormones, carcinogens and pollutants.  In addition to their role in metabolism of xenobiotics, cytochrome P450 are important in the metabolism of a number of physiological compounds — for example, the synthesis of steroid hormones and the conversion of vitamin D to its active metabolite, calcitriol.  NADPH is required to reduce cytochrome P450.  Lipids which are components of cytochrome P450 is phosphatidyl choline.