Ribulose Bisphosphate Carboxylase/Oxygenase Content Determined with [ 14 C]Carboxypentitol Bisphosphate in Plants and Algae

Abstract

As is the case with spinach ribulose bisphosphate carboxylase/oxygenase (Rubisco), [(14)C]carboxyarabinitol bisphosphate (CABP) bound to purified Chlorella Rubisco with a molar ratio of unity to large subunit of the enzyme. The concentration of binding sites in extracts of photosynthetic organisms was determined by reacting the extracts with [(14)C]-carboxypentitol bisphosphate (CPBP) and precipitating the resultant Rubisco-[(14)C]CABP complex with a combination of polyethylene glycol-4000 and MgCl(2). Plots of the relationship between concentrations of [(14)C] CPBP in the reaction mixture and the precipitated [(14)C]CPBP gave a straight line and the concentration of binding sites were estimated by extrapolation to zero [(14)C]CPBP since the dissociation constant of CABP with Rubisco is 10(-11) molar. Spinach, pea, and soybean leaves contained 6.4 to 6.8 milligrams Rubisco per milligram chlorophyll, corresponding to 92 to 97 ribulose bisphosphate-binding sites per milligram chlorophyll. The Rubisco content of sunflower and wheat leaves was 5.3 to 5.5 milligrams per milligram chlorophyll. The concentrations in C(4) plants were not uniform and corn and Panicum miliaceum leaves contained 3 and 7 milligrams Rubisco per milligram chlorophyll. The Rubisco content of green algae was one-fifth to one-sixth that of C(3) plant leaves and was affected by the CO(2) concentration during growth. The content of Euglena and blue-green algae is also reported.