The enzymatic hydroxylation of 3,4-benzpyrene was not detected in human placentas obtained after childbirth from nonsmokers, whereas this enzyme activity was present in placentas obtained from individuals who smoked cigarettes. The degree of induction of benzpyrene hydroxylase caused by cigarette smoking varied in different individuals. Treatment of pregnant rats with benzpyrene increased the activity of this hydroxylase in the placenta.

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Cigarette smoking markedly increases the activity of enzymes in human placenta which hydroxylate 3,4‐benzpyrene and N‐demethylate 3‐methyl‐4‐monomethylaminoazobenzene. No detectable benzpyrene hydroxylase or aminoazo dye N‐demethylase activity was observed in 17 human placentas obtained after childbirth from nonsmokers, but these enzymes were found in the placentas obtained from 17 smokers. Considerable variability in benzpyrene hydroxylase activity was observed in placentas that were obtained from women who smoke the same number of cigarettes. Several polycyclic aromatic hydrocarbons which are present in cigarette smoke were tested for ability to induce benzpyrene hydroxylase in rat placenta. Treatment of pregnant rats with 3,4‐benzpyrene, 1,2‐benzanthracene, 1,2,5,6‐dibenzanthracene, chrysene, 3,4‐benzofluorene, anthracene, pyrene, fluoranthene, perylene, or phenanthrene increased benzpyrene hydroxylase activity in the placenta.