Urocanic acid isolation

Isoguanine (94), which is found in Prioneris wings along with hypoxanthine (92), xanthine (91), and uric acid (95), shows antineoplastic activities (Table VI) (57). Isolation of urocanic acid (137) from this wing material represents the first detection of this histidine derivative in an arthropod (Table VIII). A nonpteridine pigment, xanthommatin (58) (cf. Section II,B,l,b), is found in the larval body of Pieris brassicae (Table V). 

Urocanic acid has been isolated from the urine of various animals fed L-histidine (see for older references). None was obtained from D-histi-dine. Reports that urocanic acid was excreted following subcutaneous injection of histidine could not be confirmed. Furthermore, the older work has been criticized because of inadequate isolation methods. 

Mehler and Tabor B48) performed a particularly careful experiment to demonstrate that urocanic acid is an intermediate in the catabolism of histidine in the liver. This was done by incubating histidine-2-C with crude liver preparations in the presence of twenty times the histidine concentration of unlabeled urocanic acid. From samples taken at zero time and at the end of the incubation period it was estimated that most of the radioactivity disappearing from the histidine appeared as urocanic acid. To rule out the possibility that the C in the urocanic acid was derived by some exchange reaction, while net degradation of histidine could occur by a different reaction, an experiment was performed with unlabeled histidine and C -urocanate. The isolated histidine was isotope-free. This result demonstrated that the conversion of histidine to urocanate is not reversible and that there is no chemical exchange reaction of an unknown nature. 

Subsequently crystalline formiminoglutamic acid was isolated 251, 252), its composition determined, and its identity proved by synthesis 253, 254). Synthesis was carried out by two different routes and by methods designed to avoid likelihood of cyclization 253, 254). This is because the elementary analysis of formiminoglutamic acid is identical with that of imidazolonepropionic acid, which also is an intermediate in the degradation of urocanic 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. 

Two complex sulfur-containing 5 -deoxyribosyl nucleosides were isolated from the Australian species Atriolum rohustum collected at Heron Island, along with three new amino acid derivatives. Most of these compounds derive from urocanic acid Kehraus et al, 2004). 

Study of the reaction with these enzyme preparations have indicated a ratio of oxygen absorption to urocanate decomposition of mole per mole. Products of the oxidative reaction reported to have been isolated are succinic monoureide, hydantoinacrylic acid, and a-ketoglutaric acid amide (269, 270). The authors reached the conclusion that the first step in the reaction sequence was the hydrolytic formation of imidazolonepropionic... 

---