[Pharmacokinetics of cytochrome P-450]

Pharmacogenetics of cytochromes P-450. Cytochromes P-450 are a large family of enzymes found in all living species whose function is the activation of molecular oxygen which, in turn, will oxidize an organic substrate. They are divided in two groups: one including the constitutive enzymes that intervene in vital processes such as cholesterol synthesis, cholesterol transfer into steroid and sex hormones, prostaglandin synthesis, etc.; the other group including the inducible enzymes, responsible of the metabolism of exogenous substances. Their concentration increases in the presence of specific substrates, like herbicides, cigarette smoke, hydrocarbons, insecticides, etc.. Of the latter group, the genetic polymorphism of two families is described. Family I is involved in the metabolism of polycyclic aromatic hydrocarbons: an allele codifying for a low activity cytoplasmic receptor (autosomic recessive inheritance) and a high affinity one (recessive inheritance) are present. The transformations carried out by the cytochromes P-450 give origin to intermediate reactive products, epoxides, that bonding to nucleoproteins or nucleic acids, can have either toxic or carcinogenic action. Therefore, the subjects with high affinity genes have an increased risk of cancer. This phenomenon, relating to pulmonary cancer, has been demonstrated in cigarette smokers. Family II is the group of greatest pharmacogenetic and clinical interest, since it is responsible of the polymorphism of the response to different drugs, such as halothane, (malignant hyperthermia), ethanol (alcohol intolerance), nitrosamine, (cancer), debrisoquine (hypotension), spartein (excessive uterine contractions). An increased or reduced ability to metabolize specific substances is the consequence: the pharmacological effects can therefore vary very much in the two classes of carriers of different alleles. Possible future applications of these polymorphisms in clinical practice are discussed.