Affinity of RuBP Carboxylases for Carbon Dioxide and Inhibition of the Enzymes by Oxygen

Ribulose bisphosphate carboxylase (E.C.4.1.1.39) was purifiedfrom leaves of Triticum aestivum, Hordeum vulgare, Spinaceaoleracea, Petroselinum crispum, salad mustard-most likely Brassicanapus, Helianthus annuus, Solanum tuberosum, Beta vulgaris,Lolium perenne, Equisetum arvense, Zea mays, Ginkgo biloba,Pteris aquilina, Salix babylonica, Chamaecyparis lawsonianaand Atrichum undulatum by density gradient centrifugation andgel filtration or by ammonium sulphate fractionation, densitygradient centrifugation, ion-exchange chromatography and gelfiltration. Purified enzymes were freeze-dried and then storedat 0 °C to 4 °C. Portions of each enzyme preparationwere reactivated at 25 °C for 5 h in the presence of 10mM HCO₂ and 20 mM MgCl₂-RuBP carboxylase activities were measuredat four different concentrations of CO₂ at 25 °C and pH8.2 in solutions equilibrated with pure nitrogen or air (21%O₂, 79% N₂). K_m(CO₂), V_max and K₁(O₂) values were computed fromthe results. Significant differences were found in the K_m(CO₂)values for enzymes isolated from different species. Sensitivityof the enzymes to oxygen was less variable.