A substance similar in action to catalase was found to be present in an extract of the chloroplast stroma. Subsequent investigation with catalase suggested this enzyme served as a radical scavenger for the removal of hydroxyl radicals formed as a result of the oxidation of water by the irradiated thylakoids. Catalase may, under certain conditions, demonstrate peroxidase like activity and therefore this enzyme's effect on the irradiated thylakoids was also examined. The distinctly different biphasic nature of the current time curves suggested that peroxidase inhibits the normal rate of electron flow to an extent which permits visualization of the individual photosystems.
