Urocanase reaction

Bound NAD+ is also present in S-adenosylhomo-cysteine hydrolase,119 120 which catalyzes the irreversible reaction of Eq. 15-14. Transient oxidation at the 3 position of the ribose ring facilitates the reaction. The reader can doubtless deduce the function that has been established for the bound NAD+ in this enzyme. However, the role of NAD in the urocanase reaction (Eq. 15-15) is puzzling. This reaction, which is the second step in the catabolism of histidine, following Eq. 14-44, appears simple. However, there is no obvious... 

Uridylate kinase 655 5 -Uridylic acid. See UMP Urocanase reaction 778 Urocanic acid 755,756s Urokinase 634 Uronic acid 164 Uroporphyrin(s) 843 Uroporphyrin I, 845s Urothione 804s, 891 Urticaria 385 Usher protein 364... 

Rao, D.R., Greenberg, D.M. Studies on the enzymic decomposition of urocanic acid. II. Properties of products of urocanase reaction. Biochim. biophys. Acta (Amst.) 43, 404-418 (1960)... 

The evidence that the product of the urocanase reaction is 4(6)-imidazolone-5(4)-propionic acid is based primarily on the rimilarity of its spectral characteristics in acid, alkaline, and neutral solutions to synthetic 4(5)-imidazoline and 4(5)-imidazolone-5(4)-acetic acid, which have been prepared by Witkop and co-workers 262, 263). These compounds are degraded by bacterial enzyme extracts to formiminoglycine and formi-minoaspartic acid in a manner rimilar to the formation of formimino-glutamic acid in the reaction with crude urocanase preparations. 

Urocanic acid is degraded by urocanase. Although this enzyme has been purified from both animal and bacterial sources,its mode of action remains obscure. The first product to accumulate is the open-chain formiminoglutamic acid. There is no evidence for more than one enzyme participating in this reaction which involves the addition of two water molecules and opening the imidazole ring (IV). No dissociable cofactor has been detected. The enzyme that removes the 277 mp absorption has a pH optimum near 7. 

More recently it has been demonstrated that urocanase preparations consist of at least two enzymes 57, 58). A separation of these can be effected so that formiminoglutamate is no longer produced in the reaction. While the product formed by the first enzyme, for which the name urocanase is retained, has not been crystallized and its structure definitely proved, there exists a substantial body of evidence that this compound is 4(5)-imidazolone-5(4)-propionic acid. 

When a crude liver extract is added to the purified urocanase, decomposition of urocanic acid proceeds to W-formimino-L-glutamic acid. The enzyme carrying out this stage of the reaction has been named imidazolonepropionic acid hydrolase. In view of the fact that im-idazolonepropionic acid has not been isolated and study of the hydrolysis reaction can only be performed in incubation mixtures in which urocanic acid is reacted with purified urocanase very little is known about the properties of the hydrolase enzyme. This enzyme has been purified about tenfold by ammonium sulfate fractionation in our laboratory (266). The enzyme activity is shown by an acceleration in the rate of the decomposition of the 264 my absorption. The product formed was shown to be formimino glutamic acid by testing with purified formiminotransferase which is specific for this compound. 

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