Metal-catalyzed photooxidation of histidine in human growth hormone.

Reports on nonenzymatic oxidation of human growth hormone (hGH) have been previously limited to methionyl residues (Met14 and Met125). 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 His18 and Glu174. This site binds metal ions and, under intense light, catalyzes the oxidation of His21. Products are formed by the addition of one, two, or three atoms of oxygen to the histidyl residue.