alpha-Bromo-4-amino-3-nitroacetophenone, a new reagent for protein modification. Modification of the methionine-290 residue of porcine pepsin.

A new coloured reagent for protein modification, alpha-bromo-4-amino-3-nitroacetophenone (NH2BrNphAc), was synthesized. The reagent was found to alkylate specifically the methionine-290 residue of porcine pepsin below pH 3 at 37 degrees C, which lead to a 45% decrease of enzyme's activity towards haemoglobin. The effect of this reagent as well as that of other phenacyl bromides on the activity of pepsin appeared to be a result of steric hindrance caused by the attachment of bulky reagent residue to the edge of the cleft harbouring the enzyme active site. Only marginal reaction with the co-carboxy group of aspartic acid-315 was found under the above conditions. More pronounced esterification of carboxy groups (up to one residue per enzyme molecule) occurred when the pH was shifted to 5.2. The latter modification had no noticeable effect on enzyme activity, thus disproving a previously held assumption that pepsin inactivation by phenacyl bromide is due to the carboxy-group esterification. alpha-Bromo-4-amino-3-nitroacetophenone forms derivatives with characteristic u.v. spectra when it reacts with methionine, histidine, aspartic and glutamic acid residues, and may be recommended as a reagent for protein modification.