Protein scission by metal ion-ascorbate system.
About 14 proteins were tested for specific oxidative scission catalyzed by metal ions in the presence of ascorbate and oxidizing agents (O(2) or hydrogen peroxide). Only four of them were degraded by Fe(3+)/Fe(2+)- ascorbate, twelve - by Cu(2+)/Cu(+)-ascorbate and two proteins (alpha- and beta-caseins) were degraded by Pd(2+) ions. The rate and the intensity of degradation are very different for various proteins. For the most of tested proteins only a small fraction of molecules was degraded. None of them was degraded completely. Two possible reasons of protein stability against oxidative degradation may be proposed as follows: either there is no metal binding site in a protein molecule, or metal binding ligands of protein undergo a rapid oxidative modification and the metal ion is released from the binding site. Human growth hormone was cut specifically at two sites by Cu(2+)/Cu(+)-ascorbate system. At least one of amino acid residues of this protein was modified by formation of reactive carbonyl.