Studies on the Reconstitution of O 2 -Evolution of Chloroplasts

Abstract

Extraction of spinach (Spinacia oleracea L.) chloroplasts with cholate-asolectin in the absence of Mg(2+) results in the rapid and selective inactivation of O(2) evolution and a partial (30 to 40%) loss of photosystem II (PSII) donor activity without extraction of thylakoid bound Mn ( approximately 5 to 6 Mn per 400 Chlorophyll). Inclusion of ethylene glycol in the extractions inhibits loss of O(2) evolution and results in quantitative and qualitative differences in proteins solubilized but does not significantly inhibit the partial loss of PSII donor activity. Similarly, in two stage experiments (extraction followed by addition of organic solvent and solubilized thylakoid protein), O(2) evolution (V and V(max)) of extracted chloroplasts is enhanced approximately 2.5- to 8-fold. However, PSII donor activity remains unaffected. This reversal of cholate inactivation of O(2) evolution can be induced by solvents including ethanol, methanol, 2-propanol, and dimethyl sulfoxide. Such enhancements of O(2) evolution specifically required cholate-solubilized proteins, which are insensitive to NH(2)OH and are only moderately heat-labile. NH(2)OH extraction of chloroplasts prior to cholate-asolectin extraction abolishes reconstitutability of O(2) evolution. Thus, the protein(s) affecting reconstitution is unlike those of the O(2).Mn enzyme. The specific activity of the protein fraction effecting reconstitution of O(2) evolution is greatest in fractions depleted of the reported Mn-containing, 65-kilodalton, and the Fe-heme, 232-kilodalton (58-kilodalton monomer), proteins. Divalent ( approximately 3 millimolar) and monovalent ( approximately 30 millimolar) cations do not affect reconstitution of PSII donor activity but do affect reconstitution of O(2) evolution by decreasing the protein(s) concentration required for reconstitution of O(2) evolution in nonfractionated, cholate-asolectin extractions. The data indicate a reconstitution of the PSII segment linking the PSII secondary donor(s) to O(2)-evolving centers.