CHEMICAL DERIVATIVES OF CHYMOTRYPSINOGEN II. REACTION WITH 0-METHYLISOUREA* BY CHARLES H. CHERVENKAt

In Paper I (1) we have described the reaction of chymotrypsinogen with carbon disulfide and the preparation of a monosubstituted derivative in which the single a-amino group of the protein has been converted to a dithiocarbamate group. A study of the reaction of 0-methylisourea with chymotrypsinogen is reported in this paper. In 1935 Greenstein found that 0-methylisourea reacts with lysine to form the amino acid, homoarginine (2). From a quantitative study of the reaction by the Sakaguchi method, Greenstein found that the free a-amino group of peptides was not substituted by the reagent under the mild conditions employed. Thus, a specificity of this reagent for e-amino groups of peptides was indicated. Other investigators obtained similar results (3). 0-Methylisourea was first applied to proteins by Hughes, Saroff, and Carney (4) who obtained a crystallizable derivative of human serum albumin of greatly decreased solubility. The decrease in Van Slyke nitrogen indicated that 54 to 57 amino groups had reacted, and this change was paralleled by an equivalent decrease in the amount of 0-methylisourea in the reaction mixture. By means of electrophoretic and ultracentrifugal studies, the crystalline product was found to be no more heterogeneous than the starting material. No further chemical characterization of the product was made, and no evidence concerning the reactivity of the a-amino groups was available. The small amount of Van Slyke nitrogen obtained from the product even after the most extended treatment with the reagent suggested that the a-amino groups had not been substituted. Recently Roche, Mourgue, and Baret (5) reported on the reaction of 0-methylisourea with several proteins, including bovine serum albumin, casein, and ovalbumin. Paper chromatograms of acid hydrolysates revealed spots corresponding to the reaction product of lysine with O-methylisourea, homoarginine. No evidence was found for the a-guanidinocar-