The estimation of arginine.

Arginine has been estimated by precipitation as the picrate and flavianate complexes, enzymically by the action of arginase followed by the estimation of the urea formed, microbiologically and colorimetrically (cf. Block & Boiling, 1947). The colorimetric method developed by Sakaguchi (1925a, b) has been most widely used. This method relies upon the formation of a red-coloured complex produced by arginine and other monosubstituted guanido compounds in the presence of a-naphthol and alkaline hypobromite or hypochlorite. Although this method will qualitatively detect small quantities of arginine, it is unreliable quantitatively, as evidenced by the large number of modifications that have been suggested, and which have been reviewed recently by G6mez & Marenzi (1953). A colorimetric method for the estimation of both monoand di-substituted guanidines was introduced by Eggleton, Elsden & Gough (1943). These compounds give a red-coloured complex with diacetyl and a-naphthol in alkaline solution. The authors found that monosubstituted guanidines such as arginine and glycocyamine produced only one-ninth of the colour given by asymmetrically disubstituted guanidines such as creatine. This observation has been confirmed by Ennor & Stocken (1953) and by Roche, Thoai & Hatt (1954). Although this method is less sensitive than the Sakaguchi method, it is simpler and more reliable. This paper describes a modification of the above reaction. When the colour is developed in the presence of n-propanol and an increased concentration of x-naphthol, the sensitivity of the method for monosubstituted guanidines is equal to that previously obtained with disubstituted guanidines. Thus it is possible to estimate arginine in amounts ranging from 10 to 100 ,ug.