Photosynthesis of isolated chloroplasts. V. Effect on fixation rate and metabolite transport from the chloroplasts caused by added fructose-1,6-diphosphatase

Abstract

Addition of fructose-1,6-diphosphatase (D-fructose-1,6-diphosphate 1-phosphohydrolase, EC 3.1.3.11) plus Mg 2+ in increasing concentrations causes stimulation and then inhibition of photosynthesis in isolated spinach chloroplasts. The effects are similar to those previously found with leaf extract additions, including modifications of the effects caused by added PP i. Under the conditions of pH, Mg 2+ concentration, and PP i concentration in which these effects are seen, the added enzyme has only negligible activity as fructose-1,6-diphosphatase. Thus the enzyme appears to be exerting its effect through some function other than as an fructose-1,6-diphosphatase. It is tentatively suggested that this other function is in the selective diffusion of metabolites out through the chloroplast outer membrane. This hypothesis is supported by the following observations: 1. 1. Addition of fructose-1,6-diphosphatase plus Mg 2+ to chloroplasts photosynthesizing with 14CO 2 causes an increase in the total labeled Fru-1,6- P 2 and dihydroxyacetone- P (which appear mostly in the suspending medium), and a decrease in the total labeled Fru-6- P and of all compounds following Fru-1,6- P 2 in the carbon reduction cycle, including the carboxylation substrate, Ribul-1,5- P 2. Furthermore, the amounts of all metabolites retained in the chloroplasts decrease. 2. 2. Addition of Mg 2+ alone, in concentration slightly higher than PP i concentration, causes some loss of all compounds from the chloroplasts to the medium. 3. 3. Addition of 5 mM PP i decreases the loss of hexose and heptose monophosphates and of Ribul-1,5- P 2, but does not decrease the loss of Fru-1,6- P 2 and of dihydroxyacetone- P from the chloroplasts to the medium.