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. 1984 Mar;74(3):681–686. doi: 10.1104/pp.74.3.681

Activation State of Ribulose Bisphosphate Carboxylase in Soybean Leaves 1

Jerome C Servaites 1, Rebecca S Torisky 1
PMCID: PMC1066746  PMID: 16663481

Abstract

Conditions for extraction and assay of ribulose-1,5-bisphophate 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, Glycine max (L.) Merr. cv Will. Total activity was highest after extracts had preincubated in NaHCO3 (5 millimolar saturating) and Mg2+ (5 millimolar optimal) for 5 minutes at 25°C or 30 minutes at 0°C before assay. Initial activity was about 70% of total activity. Kact (Mg2+) and Kact (CO2) were approximately 0.3 millimolar and 36 micromolar, respectively. The carry-over of endogenous Mg2+ in the leaf extract was sufficient to support considerable catalytic activity. While Mg2+ was essential for both activation and catalysis, Mg2+ levels greater than 5 millimolar 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 hours in ice water or liquid nitrogen before homogenization, but were about 20% higher in the latter. Activities were also stable for up to 2 hours in leaf extracts stored at 0°C. Initial activity quickly deactivated at 25°C in the absence of high CO2. Total activity slowly declined irreversibly upon storage of leaf homogenate at 25°C.

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Selected References

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