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Localization and Properties of Ribulose Diphosphate Carboxylase from Castor Bean Endosperm ¹

^a Thimann Laboratories, University of California, Santa Cruz, California 95064

A substantial portion of the ribulose 1,5-diphosphate carboxylase activity in the endosperm of germinating castor beans (Ricinus communis var. Hale) is recovered in the proplastid fraction. The partially purified enzyme shows homology with the enzyme from spinach (Spinacia oleracea) leaves, as evidenced by its reaction against antibodies to the native spinach enzyme and to its catalytic subunit. The enzyme from the endosperm of castor beans has a molecular weight of about 500,000 and, with the exception of a higher affinity for ribulose 1,5-diphosphate, has similar kinetic properties to the spinach enzyme. The castor bean carboxylase is inhibited by oxygen and also displays ribulose 1,5-diphosphate oxygenase activity with an optimum at pH 7.5.

³ Permanent address: Research Institute for Biochemical Regulation, Nagoya University Chikusa, Nagoya, Japan. Recipient of award from Japan-U.S. Cooperative Science Program (5R047).

¹ This work was supported by Atomic Energy Commission Contract AT(04-3)34.