Recent research has shed more light on the biological and chemical consequences of exposure to cigarette smoke.

In the first study, reported in the October 1996 issue of Science (carcinogens from cigarette smoke. The P53 gene is called a tumor suppressor because, when it is deactivated, there is a significant increase in the incidence of tumors. It has been known for some time that cigarette smoke contains substantial amounts of benzo[a]pyrene, a potent carcinogen that is present in soot. The carcinogenicity of soot has been appreciated for more than 200 years: in 1775 it was shown to cause testicular cancer in “sweeping boys,” young boys who had been employed to clean the narrow chimneys of London ever since the Great Fire in 1666.

Benzo[a]pyrene itself is not carcinogenic; however, human tissues metabolize this substance by adding a number of oxygens to one of its rings. The resulting compound is known among environmental chemists as a “diol epoxide.” The purpose of this metabolism is to make the benzo[a]pyrene more soluble in water, and thus more easily excreted.

Unfortunately, this strategy goes awry in the case of benzo[a]pyrene, because some of the diol epoxide is not excreted, but finds its way into the cell nucleus, where it reacts with cellular DNA. (Epoxides react very rapidly with Lewis bases, and DNA contains a number of groups that are Lewis bases.) In the Science article, the researchers reported that the “diol epoxide” reacts with DNA in regions of the P53 gene that are known to be mutational hotspots; a significant number of lung cancers have mutations in exactly these regions of P53. The implication is that the benzo[a]pyrene in cigarette smoke is a direct cause of a mutation known to be linked to lung cancer.

Reference (2) contains an interesting summary of the relationship between human metabolism and cancer-causing substances.

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