Photosynthetic Assimilation of Carbon Dioxide and Acetate by Isolated Chloroplasts

Chloroplasts isolated by methods similar to that described by Arnon et al. (1) are capable of incorporating acetate carbon into both lipid and n)on-lipid materials when fortified with appropriate cofactors (2, 3,4, 5). The actual rate of incorporation of acetate carbon in these preparations was, however, very small compared with the normal rate of photosynthesis (1-2 jumoles/mg chlorophyll'hr) by such chloroplasts (1). In the few cases where the rate of incorporation of acetate carbon can be computed (2, 5) an upper limit of approximately 6 mpumoles/mg chlorophyll-hour was found. Concurrent rates of COO fixation were not published. Jensen and Bassham (6, 7) have recently prepared chloroplasts capable of fixing CO2 at rates in excess of 100 ,moles/mg chlorophyll-hour. We have looked for acetate utilization in chloroplasts prepared by this method and have fractionated the products into water soluble and water insoluble and lipid fractions. The products of photosynthesis using 14C-labeled bicarbonate have been fractionated in a similar way in order to compare acetate and CO, as substrates for lipid synthesis, in the same chloroplasts. No attempt in either case was made to fractionate further the lipid fraction. Where this has been done in previous work with broken chloroplasts (5, 6) more than half of this isotope incorporated from labeled acetate was shown to be in fatty acids, with a small fraction in glvcerides. In our experiments, chloroplasts were capable of CO, fixation at a maximal rate of 96 ,umoles/mg chlorophyll*hour under No and were able to continue photosynthesis for up to 1 hour at slightly reduced rates (table I). Air inhibited the rate of photosynthesis by about 25 % in the presence and absence of acetate but had no effect on the distribution of 14C assimilated. Furthermore, we found that the presence of 1 mM acetate did not affect either the uptake of 1"CO, or the distribution of isotope arising from the fixation of 14CO2. On the other hand, 10 mM acetate inhib-