Activity and properties of ribulose-1,5-bisphosphate carboxylase of sugarbeet plants grown under saline conditions

Activity and properties of sugar beet ( Beta vulgaris var. Polyrave) leaf ribulose-1,5-bisphosphate (RuBP) carboxylase were investigated following the exposure of plants to NaCl in the range of 45 to 270 m M for 7 days. An enhancement in RuBP carboxylase activity was found both in crude extracts and in purified preparations following plant exposure to 180 m M NaCl. Kinetic properties of the enzyme were significantly affected by salinity as determined by a 4.5 fold increase in K_m HCO^-₃] and K_m CO₂], and a V_max increase of 50%. Data based on polyacrylamide-gel-electrophoresis suggest that the molecular weight of the small subunit of RuBP carboxylase was reduced from 15,500 to 12,500 in plants grown under salinity. The large subunit was much less affected and no change was found in the whole enzyme. The enzyme isolated from plants exposed to salinity contained about 50% fewer titratable SH groups as compared with the control. The results indicate that in this plant, mild salt concentrations induced conformational changes in RuBP carboxylase which may be responsible for its tolerance to semi-salinity.