Purification of Ribulose 1,5‐bisphosphate Carboxylase and Carbon Isotope Fractionation by Whole Cells and Carboxylase Fromcylindrothecasp. (Bacillariophyceae.)¹,²,³

Ribulose 1,5‐biphosphate carboxylase has been purified to homogeneity from extracts of Cylindrotheca sp. (strain N‐1 ), a marine, pennate diatom. The carboxylase has a molecular weight and structural composition similar to the enzyme from higher plants. When assayed in the presence of 1 mM NaHCO 3 the enzyme was stimulated nearly 40% by 1 mM aspartate and over 20% by 1 mM malate, and was inhibited to over 60% by 1 mM phosphoenolpyruvate. Similar experiments, using spinach carboxylase, failed to show activation by these metabolites. When assayed in the presence of 20 mM NaHCO 3 , 6‐phosphogluconate (1 mM) inhibited activity of ribulose bisphosphate carboxylase from Cylindrotheca by 60%, and higher concentrations of maiate (10 mM) inhibited activity by 25% Carbon isotope fractionation by ribulose bisphosphate carboxylase was ‐32.6% (ppt) when measured under N 2 using homogeneous enzyme, whereas maximum carbon isotope fractionation by the whole alga grown in 1% ‐C0 2 ‐in air averaged ‐ 16.8%. Carbon isotope fractionation by the whole alga varied with the density of the culture and was maximum at a low cell density (1.7 ± 10 6 cellslml). At higher densities, the fractionation decreased by 4.0%. Carbon isotope fractionation has been used previously to determine the pathway of carbon metabolism in other organisms; the results of this investigation seem to indicate that this strain uses both the reductive pentose phosphate pathway and the C 4 carbon pathway for primary CO 2 fixation .