Oxygen- and Light-Induced Proteolysis in Isolated Oat Chloroplasts

The influence of light and O2 on the degradation of proteins in isolated oat chloroplasts was studied by SDS-polyacrylamide gel electrophoresis. Proteolysis increased as irradiance and the concentration of O2 increased. Dark treatments prevented protein degradation. Neutral and alkaline aminopeptidase activities and neutral endopeptidase activity decreased with the time and with the increase in the concentration of O2. However, acid endopeptidase activity increased as the level of O2 increased, accounting, at least in part, for the proteolysis observed under high irradiance and high concentrations of O2. Acid endopeptidase activity was strongly associated with thylakoid membranes. Treatment with H2O2 increased thylakoid-bound acid endopeptidase activity but had no effect on the solubilized activity. It is suggested that active species of oxygen generated in illuminated chloroplasts may enhance proteolysis by inducing alterations in membranes which, in turn, increase the endopeptidic activity.