Functional features of human cytochrome P450 1A2 : with special focus on caffeine and melatonin metabolism

Abstract: CYP1A2 is one of the major cytochrome P450 enzymes in the liver and metabolises drugs such as caffeine, clozapine, tacrine, and theophylline. CYP1A2 also has a role in chemical carcinogenesis due to its activation of procarcinogens, such as heterocyclic amines and arylamines. Caffeine is extensively used to assess CYP1A2 activity. A simplified phenotyping methodology based on dietary caffeine and random urine collection was validated against a standardised methodology. The correlation coefficient between the two estimates of the CYP1A2 phenotype was 0. 9 1. The random phenotyping method was used to investigate CYP1A2 activity as risk factor for normal karyotype spontaneous abortion, and whether it would modify the effect of caffeine on abortion risk. The findings indicate that high CYP1A2 activity may increase the risk of spontaneous abortion, independently or by modifying the effect of caffeine. The random CYP1A2 phenotyping method was also used to study the inducing effect of the IF-allele on CYP1A2 activity in non-smoking and smoking pregnant women. The results did not confirm an increased induction by the mutation on CYP1A2 activity in pregnant women. It is of interest to find additional probe drugs for assessment of CYP1A2 activity. Melatonin has been suggested as an alternative. Its 6-hydroxylation was validated in vitro as a marker reaction for CYP1A2. The result showed that melatonin has a high affinity for CYP1A2 and that the contribution of other P450 enzymes was negligible. This supports earlier studies that melatonin could serve as an alternative probe for CYP1A2 activity. To evaluate the usefulness of cryopreserved human liver slices for drug metabolic studies, the intrinsic formation clearance of paraxanthine and 6-hydroxymelatonin were studied in slices and microsomes and the data were scaled to predict the in vivo clearance. Data from human liver microsomes underestimated the in vivo clearance of melatonin and this was also true for the cryopreserved liver slices. For caffeine there was a good agreement between the data generated in microsomes and in vivo clearance. These studies show that a simplified phenotyping methodology can be used in larger studies to assess CYP1A2 activity. It seems that melatonin may serve as an alternative to caffeine as a marker reaction for CYP1A2 although in vivo studies are needed to confirm this. Cryopreserved liver slices as an in vitro system are promising but require further development.