Metal-catalyzed photooxidation of histidine in human growth hormone.

Abstract Reports on nonenzymatic oxidation of human growth hormone (hGH) have been previously limited to methionyl residues (Met 14 and Met 125 ). We report on the oxidation of a histidyl residue in hGH treated with intense light. The photooxidation process is predominately site-specific to histidine at position 21, which forms a cation-binding site along with His 18 and Glu 174 . This site binds metal ions and, under intense light, catalyzes the oxidation of His 21 . Products are formed by the addition of one, two, or three atoms of oxygen to the histidyl residue.