Some Properties of Ribulose Bisphosphate Carboxylase Extracted from Tomato Leaves

Ribulose bisphosphate carboxylase (EC 4.1.1.39) activity was very low in tomato leaf extracts unless prepared in the presence of Mg²⁺, $HCO3-$ and polyclar AT. With young leaves, but not with fully-expanded leaves, the RuBP carboxylase activity extracted was increased by prolonged illumination of the leaves (≧2 h). The main effect of the light treatment was to increase the specific activity of the enzyme but there was also a small increase in RuBP carboxylase protein. Tomato leaf RuBP carboxylase in extracts had specific activities in the range 0.2–0–6 μmol CO₂ min⁻¹ mg^−-1 total protein extracted, or 0.5–1.2 μmol CO₂ min⁻¹ mg⁻¹ RuBP carboxylase, and an apparent K_m (CO₂) at 20 °C of 9.3 ± 1.2 μM (using a $pK1∞'$ of 6.407).