Effect of SO32‐ on the activity of ribulose bisphosphate carboxylase from seedlings of Pinus silvestris

The effect of sulphite on ribulose bisphosphate carboxylase, extracted from needles of Pinus silvestris L., was studied in vitro at pH 8.15 and 25°C. 1 mM and higher concentrations of SO32‐ inhibited the enzyme. The enzyme was activated either in the assay medium (2.5 – 20 mM HCO3, 20 mM MgCl2) or in 10 or 20 mM HCO3‐ and 20–25 mM MgCl2. Linear reciprocal plots of the activity versus the substrate concentration were obtained, when the HCO3‐ concentration during activation was 4 mM or higher. When the enzyme was activated at high HCO3‐ and Mg2+ concentrations, the Km(CO2) was c. 27 μM. With respect to HCO3‐. SO32‐ inhibited the enzyme in a non‐competitive fashion. The inhibition was similar, whether SO32‐ was present during activation or not. Apparently. SO32‐ did not interfere with the binding of CO2 and Mg2+ at the activating site. The K1 was 11–13 mM SO32‐. With respect to ribulose bisphosphate the inhibition was also noncompetitive. Similar results with respect to HCO3‐ were obtained for spinach, Spinacia oleracea L., which is contrary to earlier reports.