Photosystem II complexes were solubilized with the detergent sulfobetaine 12 from thylakoid membranes of the thermophilic cyanobacterium Synechococcus sp. and purified by two sucrose gradient centrifugations and by chromatography on a Mono Q column. In such photosystem II complexes having a photosynthetic O2, evolving activity of 2938 μmol O2 evolved/mg chlorophyll x h, an ʟ-arginine metabolizing activity leading to ornithine and urea as major products, could be shown to be present. Besides ornithine and urea, a product (or products) of yet unknown structure is formed in addition - especially under aerobic conditions. This activity remained associated with photosystem II complexes even after substantial additional treatments to remove loosely bound proteins. On chlorophyll basis the maximal activity obtained under optimal assay conditions corresponded to 94 μmol ornithine formed/mg chlorophyll x h. This PS II associated, ʟ-arginine metabolizing enzyme was isolated (utilizing a manganese charged chelating Sepharose 6 B column) and partially characterized. It could be shown that this enzyme requires manganese and chloride for its ʟ-arginine metabolizing activity and that manganese becomes totally lost during purification indicating that manganese is bound to a fairly exposed site on the protein. Since it is rather unlikely that two different manganese and chloride binding proteins are present in such highly purified photosystem II complexes, the possibility of this protein being the water oxidizing enzyme will be discussed. Whether the manganese and chloride requiring ʟ-arginine metabolizing activity of this protein which provided a suitable assay for its isolation from photosystem II complexes, has any physiological significance, can not be answered at the present time.
