Reassignment of the human aldehyde dehydrogenase ALDH8A1 (ALDH12) to the kynurenine pathway in tryptophan catabolism

The kynurenine pathway is the primary route for l-tryptophan degradation in mammals. Intermediates and side products of this pathway are involved in immune response and neurodegenerative diseases. This makes the study of enzymes, especially those from mammalian sources, of the kynurenine pathway worthwhile. Recent studies on a bacterial version of an enzyme of this pathway, 2-aminomuconate semialdehyde (2-AMS) dehydrogenase (AMSDH), have provided a detailed understanding of the catalytic mechanism and identified residues conserved for muconate semialdehyde recognition and activation. Findings from the bacterial enzyme have prompted the reconsideration of the function of a previously identified human aldehyde dehydrogenase, ALDH8A1 (or ALDH12), which was annotated as a retinal dehydrogenase based on its ability to preferentially oxidize 9-cis-retinal over trans-retinal. Here, we provide compelling bioinformatics and experimental evidence that human ALDH8A1 should be reassigned to the missing 2-AMS dehydrogenase of the kynurenine metabolic pathway. For the first time, the product of the semialdehyde oxidation by AMSDH is also revealed by NMR and high-resolution MS. We found that ALDH8A1 catalyzes the NAD⁺-dependent oxidation of 2-AMS with a catalytic efficiency equivalent to that of AMSDH from the bacterium Pseudomonas fluorescens Substitution of active-site residues required for substrate recognition, binding, and isomerization in the bacterial enzyme resulted in human ALDH8A1 variants with 160-fold increased K_m or no detectable activity. In conclusion, this molecular study establishes an additional enzymatic step in an important human pathway for tryptophan catabolism.