Activation State of Ribulose Bisphosphate Carboxylase in Soybean Leaves

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Abstract
Conditions for extraction and assay of ribulose-1,5-bisphosphate carboxylase present in an in vivo active form (initial activity) and an inactive form able to be activated by Mg2+ and CO2 (total activity) were examined in leaves of soybean, G. max (L.) Merr. cv. Will. Total activity was highest after extracts had preincubated in NaHCO3 (5 mmol saturating) Mg2+ (5 mmol optimal) for 5 min at 25.degree. C or 30 min at 0.degree. C before assay. Initial activity was .apprx. 70% of total activity. Kact (Mg2+) and Kact (CO2) were .apprx. 0.3 mmol and 36 .mu.mol, respectively. The carry-over of endogenous Mg2+ in the leaf extract was sufficient to support considerable catalytic activity. Mg2+ was essential for both activation and catalysis, Mg2+ levels greater than 5 mmol were increasingly inhibitory of catalysis. Similar inhibition by high Mg2+ was also observed in filtered, centrifuged or desalted extracts and partially purified enzyme. Activities did not change upon storage of leaves for up to 4 h in ice water or liquid N before homogenization, but were .apprx. 20% higher in the latter. Activities were also stable for up to 2 h in leaf extracts stored at 0.degree. C. Initial activity quickly deactivated at 25.degree. C in the absence of high CO2. Total activity slowly declined irreversibly upon storage of leaf homogenate at 25.degree. C.

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