Population distribution of placental benzo(a)pyrene metabolism in smokers

Human placental microsomes isolated from term placentas derived from nonsmoking women and women smoking 1 to 40 cigarettes a day were analyzed for the metabolism of benzo(a)pyrene measured as various metabolites by HPLC and/or as aryl hydrocarbon hydroxylase (AHH)6 activity. In accordance with other reports, AHH activity was several times higher in smokers than in nonsmokers. Regression analysis on 13 different placental tissues from women smoking from 1 to 40 cigarettes demonstrated a high correlation (r = 0.8 to 0.9) between AHH activity (or the formation of benzo(a)pyrene phenols resolved by HPLC) versus the formation of the procarcinogenic benzo(a)pyrene-7,8-diol. Subsequent studies on placentas derived from 67 women who smoked 10 to 40 cigarettes per day demonstrated a definite dose-response relationship between AHH activity and the number of cigarettes smoked/day. The dose-response curve was sigmoidal in shape; however, when the data were plotted on a semi-log scale the curve assumed a linear shape, reaching saturation of AHH induction beyond 20 to 25 cigarettes/day. While mean AHH activity was dependent upon the number of cigarettes smoked/day, considerable interindividual variability in AHH (ranging more than 1,000-fold in some cases) was observed among individuals with comparable smoking histories, i.e. smoking the same number of cigarettes. Population distribution suggested clustering of the population in the low-AHH-activity region while cord-blood thiocyanate analysis and twin studies suggested that genetic factors contributed to a major portion of the inter-individual variability in AHH activity observed among smokers.