Five caffeine metabolite ratios to measure tobacco-induced CYP1A2 activityand their relationships with urinary mutagenicity and urine flow

To choose a sensitive protocol to discriminate populations exposed and not exposed to inducers, five urinary metabolite ratios (MRs) [MR1 (17X + 17U)/ 137X, MR2 (5-acetylamino-6-formylamino-3-methyluracil [AFMU] + 1X + 1U)/17U , MR3 (17X/137X), MR4 (AFMU + 1X + 1U + 17X + 17U)/137X, and MR5 (AFMU + 1X + 1U)/17X] were calculated in 4-5 h and 0-24 h urine samples after caffein e intake. One hundred twenty-five healthy volunteers (59 nonsmokers and 66 smokers) were included in the study. All ratios showed a log-normal distrib ution. MR2 in the two time intervals was the only ratio nondependent on the urine flow. Differences between nonsmokers and smokers could be detected w ith all ratios at 45 h, However, only MR2 and, to a lesser extent, MR5 allo wed the discrimination of higher cytochrome P450 1A2 (CYP1A2) activity in s mokers in the 0-24 h sample. Although smokers had increased urinary mutagen icity in relation to nonsmokers, a significant association between MRs and urine mutagenicity was observed only with MR2 in the 4-5 h interval; this r atio/time schedule being that of higher association with tobacco consumptio n. The most flow-dependent ratios, MR1, MR3, and MR4, were closely correlat ed with each other at the two intervals. The flow dependency profile of eac h ratio may explain their different power to indicate both tobacco exposure and tobacco-derived mutagenicity, In conclusion, MR2 in the period of 45 h after caffeine intake seems preferable, especially at high urine flow rate s.