azo dyes

The carcinogenicity of most aromatic amines and azo dyes is exerted mainly in the liver, where the "ultimate carcinogen" is formed by the action of the cytochrome P-450 oxygenase systems.

Thus, fed to rats, acetylaminofluorene and the azo dyes induce hepatocellular carcinomas (but not cancers of the gastrointestinal tract).

An agent implicated in human cancers, β-naphthylamine, is an exception. In the past, it was responsible for a 50-fold increased incidence of bladder cancer in heavily exposed workers in aniline dye and rubber industries. After absorption, it is hydroxylated into an active form, then detoxified by conjugation with glucuronic acid. When excreted in the urine, the nontoxic conjugate is split by the urinary enzyme glucuronidase to release the electrophilic reactant again, thus inducing bladder cancer.

Regrettably, humans are one of the few species to possess urinary glucuronidase.

Some of the azo dyes were developed as food coloring (e.g., butter yellow to give margarine the appearance of butter and scarlet red to impart the seductive coloration of certain foods such as maraschino cherries). These dyes are now federally regulated in the United States because of the fear that they may be dangerous to humans.