Effects of Intermediates of the Photosynthetic Carbon Reduction Cycle on Carbon Metabolism in Spinach Chloroplasts Illuminated on Filter Paper Discs

Suspensions of chloroplasts isolated from leaves of spinach or peas and illuminated in suitable aqueous media are capable of both 02 evolution and CO2 fixation at rates comparable with those which occur in vivo (8, 11). Under these conditions the major radioactive products of photosynthetic "4CO2 assimilation are glycerate-3-P and dihydroxyacetone-P, a considerable proportion of which may be recovered from the medium outside the chloroplasts (3). It is still not clear what prevents metabolism of these early products of photosynthesis to hexose phosphates and compounds such as amino acids and sucrose. Recently we have suggested (6) that glycerate-3-P and DHAP1 accumulate in chloroplast suspensions simply because they diffuse from the plastids into the external medium where they cannot be metabolized further. We have shown that these compounds were not accumulated if the volume of medium in the experimental sample was reduced to a minimum by layering the chloroplasts on filter paper discs. Under these conditions the chloroplasts accumulated hexose-monoP and amino acids such as glycine and alanine but did not form significant amounts of sucrose. Factors controlling the biosynthesis of sucrose from CO2 in chloroplasts remain unknown. It is possible that the nature of the products formed by isolated chloroplasts could depend on the relative concentrations of intermediates of the photosynthetic carbon reduction cycle within the plastids. We have therefore investigated effects of preincubating chloroplasts with varying concentrations of glycerate-3-P and sugar phosphates on the distribution of radioactivity into the products of 14CO2 assimilation by spinach