The Enzymic Derivation of Citrulline Residues from Arginine Residues In Situ During the Biosynthesis of Hair Proteins Cross-Linked by Isopeptide Bonds

The study described an enzymatic activity in hair follicles that converted arginine residues to citrulline residues in proteins in situ. This Ca2+ dependent enzyme activity was found in hair follicle extracts but not in serum, liver, or brain extracts. The enzyme acted on proteins other than hair proteins, and the citrulline produced was quantifiable through a colorimetric procedure. The formation of citrulline was confirmed by amino acid analysis and was not related to the formation of isopeptide linkages, which were catalyzed by transamidase in hair follicles.