Oxygenase and Carboxylase Activities of RuBP Carboxylase from Wheat Leaves

Abstract

The oxygenase reaction catalysed by RuBP carboxylase initiates a metabolic sequence responsible for photorespiration (Bowes et al. 1972). Studies with inhibitors and with mutants show that control of photorespiration can have beneficial effects only if it is achieved by decreasing the oxygenase activity of RuBP carboxylase (Servaites, Ogren, 1977; Somerville, Ogren, 1982). No corroborated evidence is available for carboxylation of RuBP without susceptibility to oxygenation. If oxygenase activity and photorespiration have a function in plants, then regulation by changing levels of metabolic intermediates might be expected. This was investigated in a test system containing purified RuBP carboxylase from wheat with RuBP and rate-limiting concentrations of HCO₃ ⁻ and O₂. The sensitivity of the ratio v_c:v_o to pH (CO₂ concentration), the presence of carbonic anhydrase (CO₂ availability) and the presence of Mn²⁺ is illustrated to validate the methods used.