Tris-Induced Cross-Linking of Thylakoid Peptides; Thiol Oxidation Catalyzed by Tris-Cu²⁺ Complexes as a Possible Mechanism

Abstract

After Tris (hydroxymethyl) aminomethane [Tris buffer]-treatment cross-linking of the chloroplast thylakoid peptides of 11, 13, 18, 43, 55, and 87 kdaltons was observed on SDS-polyacrylamide gel electrophoresis. Tris-induced disulfide formation was suggested by the decrease of thiol groups of the chloroplast thylakoids in Tris medium at pHs above 8. In addition to the finding that Tris coordinates to Cu²⁺ in the forms of Tris-Cu²⁺ and Tris₂-Cu²⁺ whose successive stability constants are 5.78×10³M^-1 and 4.46×10⁶M^-2, respectively, it was observed that Tris-Cu²⁺ complexes catalyze cysteine oxidation. Since release of copper from the chloroplast thylakoids is enhanced by increasing the concentration of Tris, oxidation of thiols by Tris-Cu²⁺ complexes formed in chloroplast thylakoids is a possible mechanism for the cross-linking of chloroplast thylakoid peptides.