Cytochrome hepatic, characteristics

Because clearance at the whole-body level often is determined by metabolism at the cellular level, it is possible to use a variety of human-derived in vitro systems to determine rates of metabolism. These systems include pure human enzymes (such as cytochrome P450 enzymes) (13) and human liver subcellular fractions (S9 and microsomes) (14). However, with enzymes and subcellular fractions, some information is lost because the whole-cell integration of subcellular processes has been disrupted. The use of cultured human hepatocytes retains the whole-cell integration at the expense of greater experimental complexity (15). Each system provides a different window on the metabolic processes, is relatively easy to use, and can be obtained from commercial sources. Rates and pathways of metabolism may be compared with a series of discovery compounds to identify those with the greatest relative metabolic stability or with a benchmark compound of known human PK characteristics to provide a more absolute estimate of hepatic metabolic clearance. 

Hepatic metabolism (some to active metabolites). Induction of cytochrome P450s is characteristic and may lead to drug interactions. Because of T heme synthesis, they are contraindicated in porphyrias. 

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