Measurement of arginase activity

Abstract

Current methods for determining arginase ( l-arginine amidinohydrolase, EC 3.5.3.1) activity involve the measurement of urea. The carbon dioxide evolved through the action of urease has been measured manometrically (1–3), although more frequently ammonia is determined by established but cumbersome procedures (4, 5). Urea also is measured colorimetrically. Its reaction with isonitrosopropriophenone in addition to requiring a one-half hour heating period leads to the formation of an unstable colored reaction product. Similarly, its reaction with xanthydrol (6, 7) results in the formation of a photolabile product, dixanthylurea. Recently a method was introduced which takes advantage of the different absorptivities of arginine, ornithine, and urea at 206 mμ (8). Urea and p-dimethylaminobenzaldehyde (Ehrlich reagent) react in dilute H 2SO 4 to form N-( p-dimethylamino)benzalurea, which is stable for a day or more. The measurement of urea in arginase reaction mixtures using this reaction is described.