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Development of Ribulose 1,5-Diphosphate Carboxylase in Nonphotosynthetic Endosperms of Germinating Castor Beans ¹

^a Texas A&M University, College Station, Texas 77843

Ribulose 1,5-diphosphate (RuDP) carboxylase has been partially purified from dark-grown nonphotosynthetic endosperms of germinating castor beans (Ricinus communis var. Hale). The Km values for RuDP, HCO₃^–, and Mg²⁺ are 0.51, 33, and 1.78 mM, respectively. The pH optimum for the carboxylation reaction is pH 7.5. Germination is required for the development of the carboxylase in the endosperms. The enzyme reaches a maximal activity in 4- to 5-day-old dark-grown seedlings (which have an endosperm weight of approximately 0.75 g fresh weight/bean) and then declines. Total endosperm carboxylase activity is 1230 nmoles/min·g fresh weight which is 25 and 50% of the total activity developed in soybean and maize leaves, respectively. Specific activity of the carboxylase in crude soluble endosperm preparations (which contain enzymic and storage protein) is 0.05 µmole/min·mg protein. This is 5 times greater than the specific activity of RuDP carboxylase in soluble preparations from etiolated leaves. During germination the V_max of the endosperm carboxylase for RuDP increases 10-fold. Development of the enzyme is inhibited 90% by the exposure of the endosperm to 2 µg/ml cycloheximide or 50 µg/ml chloramphenicol. Light (or phytochrome Pfr) is not required for the synthesis of the enzyme. Electron photomicrographs of dark-grown endosperm cells (with peak RuDP carboxylase activity) show proplastids with several invaginations of the inner membrane but no prolamellar-like structures.