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NAD+ functions not as a redox reagent but as an electrophile in the second step in the degradation of histidine. In this as yet unique enzymatic reaction, water is added to urocanic acid (1), leading to the formation of 2. The proposed mechanism has now been substantiated by NMR spectroscopic results on the 13C-labeled adduct 3. R = ADP-ribosyl, • = 13C.