Formation of Urocanic Acid

A new procedure for the determination of urocanic acid has been developed, based on the fact that this compound exhibits a strong absorption in the ultraviolet in the region of 240-280 m/i. This can be employed both to demonstrate the formation of urocanic acid and as a method of assay for the enzyme deaminating histidine to urocanic acid. With this test method it has been shown that urocanic acid accumulates when histidine is incubated with liver extract or with extracts of acetone-dried liver powder. Mehler and Tabor determined that the activity of the enzyme forming urocanic acid appeared to be sufficient to account for the total histidine degradation of liver extracts, as judged by the rate of formation of urocanic acid. 

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The main pathway of L-histidine degradation includes formation of urocanic acid and leads to glutamic acid (Fig. 241). Histidine ammonia-lyase (like phenylalanine ammonia-lyase, D 22.2.1) catalyzes the transelimination of the NH2-group and a jff-hydrogen atom. Of minor importance in L-histidine degradation is the formation and further degradation of imidazole pyruvic acid. 

Study of the enzymic reactions of histidine catabolism was aided tremendously by the observation that urocanic acid exhibits a strong absorption in the ultraviolet region with a maximum at 277 m/u and a molar extinction coefficient of 18,000 (S4S, XS6). This property renders it very convraiient to follow both the formation of urocanic acid from histidine and its disappearance spectrophotometrically. The observation that in the enzymic decomposition of urocanic acid a product with an absorption peak at 264 m i is formed also is of extreme importance in following the further sequence of the reactions of histidine catabolism 257, 258). 

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