2.1. Cytochrome P450 (CYP) enzymes

Cytochrome P450s constitute a superfamily of enzymes crucial for the oxidative, peroxidative, and reductive metabolism of a diverse group of compounds, including endobiotics, such as steroids, bile acids, fatty acids, prostaglandins, and leukotrienes, and xenobiotics, including most of the therapeutic drugs and environmental pollutants (Nelson et al. 1996, Bertz & Granneman 1997). The first report on the existence of a CYP enzyme or a “microsomal carbon monoxide-binding pigment”, as it was called at that time, was published in 1958 by Klingenberg et al. This enzyme gave a unique 450-nm optical absorption peak, and when its hemoprotein nature was recognized, it was given the name cytochrome P450 (Omura 1999).

CYP enzymes are expressed ubiquitously in different life forms: they have been found in animals, plants, fungi, and bacteria (Nelson et al. 1996). They seem to be indispensable for eukaryotic species, but not for prokaryotes, since some bacteria lack CYP enzymes (Nelson 1999). Eukaryotics need CYPs for the biosynthesis of sterols, which are constituents of plasma membrane (Omura 1999). Eukaryotic CYP enzymes are membrane-bound, mostly localized to the endoplasmic reticulum, but some CYPs are also present in mitochondrial inner membranes. In order to function, cytochrome P450s require an electron transfer chain. In the endoplasmic reticulum, this source is NADPH-cytochrome P450 reductase, previously called NADPH-cytochrome c reductase (Omura 1999). In mitochondria, electrons are transferred from NADPH by redoxin reductase to redoxin and then to CYP (Gonzalez 1990). Despite their occasionally minimal sequence similarity, all CYPs have a similar structural fold with a highly conserved core (Graham & Peterson 1999).

Humans have been estimated to have at least 53 different CYP genes and 24 pseudogenes (Nelson 1999) (Dr. Nelson homepage: http://drnelson.utmem.edu/CytochromeP450.html). The notable diversity of CYP enzymes has given rise to a systematic classification of individual forms into families and subfamilies. The protein sequences within a given gene family are at least 40% identical (e.g. CYP2A6 and CYP2B6), and the sequences within a given subfamily are > 55% identical (e.g. CYP2A6 and CYP2A7) (Nelson et al. 1996). The italicized names refer to genes, e.g. CYP2A13. There are 17 different families currently known in humans. The enzymes in the families 1-3 are mostly active in the metabolism of xenobiotics, whereas the other families have important endogenous functions (Table 1). Inactivating mutations in the CYPs with physiological functions often lead to serious diseases, whereas similar mutations in xenobiotic-metabolizing CYPs rarely do, although they affect the host’s drug metabolism and susceptibility to some diseases, without directly causing disease (Nelson 1999).