Carcinogen

A carcinogen is a cancer causing substance. Examples of this are substances seen in tobacco smoke. These includes tar, nicotin, carbon monoxide, etc.

Carcinogens arise in the body after metabolic activation from enzymes such as the p450 enzymes of environmental chemicals. These environmental chemicals remain biologically inert until they are taken up into tissue when they then become activated, becoming a carcinogen. The carcinogen then spontaneously becomes a reactive intermediate, forms DNA adducts and causes genetic damage resulting in cancer.

The body has defensive mechanisms at each of these steps to prevent the carcinogen leading to cancer such as metabolic detoxification by the GST (glutathione-S-transferase) enzymes, DNA repair, and at the final step cell apoptosis. However if this defence fails and there are not enough metabolic detoxification enzymes or the DNA is replicated with the wrong base order, cancer occurs.