Citrulline in proteins from the enzymatic deimination of arginine residues

Abstract

The occurrence of citrulline is restricted to specialized proteins whose tissues of origin contain the requisite enzyme for the conversion of the guanido side chain of certain arginine residues to the ureido side chains of citrulline. The tissues of importance are primarily vertebrate epidermis and the derived structures of it, although it has been reported that citrulline is present in a protein of nerve myelin. This chapter discusses the identification and estimation of citrulline in proteins. Citrulline gives a yellow color when allowed to react with Erhlich reagent, p-dimethylaminobenzaldehyde (1% w/v) in 0.5-1.0 M HCI. Citrulline can be estimated in acid hydrolysates of proteins and peptides using a standard amino acid analyzer. However, to elute the citrulline as a separate peak, it is necessary to alter the conditions of elution. The colorimetric determination of citrulline in proteins and peptides utilizes the reaction of ureido groups with diacetyl monoxime. Difficulties can be experienced with the quantitation of citrulline when colorimetric methods are applied directly to proteins. These difficulties include nonspecific color reactions and the appearance of trace amounts of suspended material in the reaction mixture. Consequently, it is advantageous to precede the colorimetric reaction by acid hydrolysis of the proteins.