PHOTOSYNTHESIS BY ISOLATED CHLOROPLASTS .4. REGULATION BY FACTORS FROM LEAF CELLS

The rate of photosynthesis by isolated spinach chloroplasts is strongly affected by the presence of clarified juice from spinach leaves. With chloroplasts photosynthesizing under otherwise optimal conditions (which include the presence of 5 mM pyrophosphate), the presence of juice in an amount which corresponds to 1 4 of the calculated ratio in vivo of juice/chloroplasts causes a 20% stimulation in the rate, whereas 3 4 of the full calculated amount causes 90% inhibition. The degree of inhibition or stimulation is a complex function of the ratio of added pyrophosphate to added juice. With a wide range of concentrations of added juice, additions of small amounts of pyrophosphate cause the rate of photosynthesis to decrease, and this inhibition increases with increasing pyrophosphate concentration, becoming and remaining severe until a threshold ratio of pyrophosphate to juice is reached. This threshold ratio is nearly constant over a 12-fold range of added juice. Beyond the threshold ratio, increasing pyrophosphate (at a given level of juice) reverses the inhibition, and with roughly 3 times the threshold ratio, the control rate is surpassed and stimulation is observed. Gel filtration of the juice produces two well-separated inhibitory fractions. One inhibitory fraction contains a compound of high molecular weight complexed with a small molecule, while the second inhibitory fraction contains the unassociated small molecule. The complex is inhibitory only in the presence of pyrophosphate, whereas the small unassociated molecule has the same activity whether pyrophosphate is present or not. The complex is stabilized in the presence of pyrophosphate. Without pyrophosphate present, the complex dissociates almost completely into its constituents. Dialysis of the dissociated complex gives an inactive large molecule in the retentate. The activity of the small molecule is greatly amplified when it is added back to the inert large molecule. The inhibitory fractions are found, at least in part, in previously isolated chloroplasts. These findings and other results described in the text strongly suggest a role for these factors in the regulation in vivo of photosynthesis. © 1969.