Activation of Ribulose Bisphosphate Carboxylase Purified from Wheat Leaves

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Abstract
A stable freeze-dried powder was prepared of partly purified ribulose bisphosphate carboxylase from wheat leaves. As with preparations from other leaves it is necessary to incubate the enzyme with Mg2$ and CO2 to achieve maximum activity. At 25 °C this activity was 0.75 IU mg−1 protein for a preparation activated at 50 °C for 10 min; the Km for CO2 was 15 μM. The time for reactivation of enzyme that had been inactivated through the absence of CO2 and Mg2$ was influenced by the length of the inactivating treatment. After a short inactivation period the enzyme was reactivated within a few minutes, whereas after a longer period several hours were needed. Enzyme in the latter state had some properties in common with enzyme inactivated by lower temperatures but in the presence of CO2 and Mg2$. The enzyme kinetic characteristics are similarly affected by both kinds of inactivation; the maximum velocity is decreased but the affinity for CO2 is not affected. Reactivation following a long inactivating treatment becomes more dependent on Mg2$ concentration as the temperature is increased from 0 to 20 °C.