Reductive activation of fructose-1,6-bisphosphatase and the peroxide effect on chloroplast photosynthesis

Abstract

Studies of isolated intact spinach chloroplasts reveal most of the available sulfhydryl groups are associated with stromal protein as opposed to a thylakoid membrane fraction under nondenaturing conditions. Increases in sulfhydryl content of about 45% occurred with illumination and could be correlated with reductive activation of fructose-1,6-bisphosphatase during CO 2 assimilation. The omission of catalase inhibited net photosynthesis and caused a lower sulfhydryl number along with diminished fructose-1,6-bisphosphatase activity. Subsequent additions of 0.3 m m NH 4Cl accelerated oxygen evolution while both the sulfhydryls and fructose-1,6-bisphosphatase rate were restored to their control values. Peroxide additions extended the initial lag period before the onset of photosynthetic oxygen production and also inhibited the rate if added during continuous illumination. In both cases, fructose-1,6-bisphosphatase activity was similar to that observed in the absence of catalase. The relative sulfhydryl content also remained unchanged. Preincubation of extracts from illuminated chloroplasts in 0.66 m m peroxide did not significantly alter fructose-1,6-bisphosphatase rates whereas CuSO 4 drastically inhibited activity and diminished the sulfhydryl number. These results suggest it is unlikely that peroxide alone induces re-formation of disulfide bridges in fructose-1,6-bisphosphatase and causes photosynthetic inhibition. However, the light-activated form of the enzyme is quite sensitive to a mild copper-induced oxidation in vitro and appears to be similarly affected within the chloroplast when both peroxide and the metal catalyst are present.