Imidazolone propionic acid

More recently Brown and Kies (S65) reported the formation from histidine and the excretion in the urine of hydantoin- propionic acid, and also its formation by liver extracts of guinea pig and the rat. The substrate for the oxidation was shown very probably to be imidazolone-propionic acid. No oxidation or formation of hydantoinpropionic acid could be demonstrated if the urocanase activity was first destroyed. The L-hydantoin-5-propionic acid was isolated by chromati raphy, and crystallized. Its identity was unequivocally established. 

Imidazolone propionate hydrolase catalyzes the enzymatic cleavage of the imidazole ring to yield formi-minoglutamate. The rat liver enzyme has been partially purified. In addition to the enzymic conversion, two nonenzymic spontaneous reactions yield N-formyl-isoglutamine and 4-oxoglutamic acid. In addition to the oxidative pathways for histidine, there exist three other pathways for its use protein synthesis, decarboxylation to yield histamine (see Inflammation), and transaminase. The activity of histidine pyruvic transamination in rat liver is three times that of histidase. The product of the transaminase reaction is imidazole pyruvic acid, which in turn is converted to imidazole acetic acid. 

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. 

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. 

Several other proteins show a low, but significant amino acid identity with atzA (Table 22.4). All of these, urease-alpha subunit (urea amidohydrolase), cytosine deaminase, and imidazolone-5-propionate hydrolase, catalyze hydrolytic reactions with substrates involved in the metabolism of nitrogenous compounds (Sadowsky et al 1998). A Rhizobium strain capable of atrazine dechlorination has been isolated from a soil that was previously treated with atrazine (Bouqard et al., 1997). This bacterium could not mineralize atrazine, and it accumulated hydroxyatrazine as the sole metabolite after long-term incubations. Interestingly, 22 of the 24 identified amino acids at the N-terminus of the atrazine halidohydrolase from Rhizobium were identical with atzA from Pseudomonas strain ADP. 

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