Urocanic acid assay

Because of the high selectivity and sensitivity of the postcolumn fluorescence detection of histidine with OPA, the present HPLC method is applicable to a specific and rapid assay of histidine in human semm, blood, and urine after simple pretreatment. A recent paper demonstrated that the postcolumn detection with OPA was applicable to the simultaneous assays of histidine and its major metabolites cis- and frawi-urocanic acids) in human stratum corneum. " The postcolumn detection system was also applicable to the flow injection analysis (FIA) method for the assay of histidine in semm and urine. The FIA method enabled us to determine histidine in blood after pretreatment of the sample with A-ethylmaleimide (masking reagent of glutathione).These methods are useful in the diagnosis of histidinanemia, one of hereditary metabolic disorders characterized by a virtual deficiency of histidine ammonia-lyase. 

In histidinemia, the missing enzyme is histidase. This was established in two ways administration of urocanic acids, which is followed by histidinemia and his-tinuria and direct assay for the enzyme. The metabolic block leads to histidine accumulation in the blood and excessive excretion in the urine. 

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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