Aromatic aldehydes as fluorogenic indicators for human aldehyde dehydrogenases and oxidases: substrate and isozyme specificity

Substrate properties of a number of potentially fluorogenic aromatic aldehydes of naphthalenes, phenanthrenes and anthracenes and of some coumarin aldehydes towards various forms of the human and rat aldehyde oxidase and dehydrogenase were examined using absorption and emission spectroscopy. It was demonstrated that recombinant human class 1 aldehyde dehydrogenase (ALDH-1) readily oxidizes naphthalene (except for those ortho-substituted), phenanthrene and coumarin aldehydes, whereas the class 3 enzyme (ALDH-3) from human saliva is active only towards 2-naphthaldehyde derivatives. The observed reaction rates in both cases are comparable to those of the best known substrates, and the Km values are typically in the sub-micromolar range. Aldehyde oxidases (AlOx), which are present in mammalian liver, reveal much broader substrate specificity, oxidizing nearly all the compounds examined, including those of the anthracene series, with maximum activity in the micromolar range of substrate concentration. In rat liver, nearly all AlOx activity was located in the cytosolic fraction.