Enzymic Fractionation of the Stable Carbon Isotopes of Carbon Dioxide by Ribulose-1,5-bisphosphate Carboxylase

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Abstract
The enzymic fractionation of the stable carbon isotopes of CO2 (Δco2) was determined using a purified preparation of ribulose-1,5-bisphosphate (RuBP) carboxylase isolated from cotton (a C3 plant) leaves. The bicarbonate concentration in the reaction mixture saturated the enzyme and furnished an infinite pool of 12CO2 and 13CO2 for enzyme fractionation. The RuBP was 96 to 98% pure. The phosphoglycerate synthesized in the reaction mixtures was purified free of RuBP, phosphoglycolate, and other phosphate esters by column chromatography on Dowex 1-Cl− resin. The average Δco2 value of −27.1% was determined from five separate experiments. A discussion of the isotope fractionation associated with photosynthetic CO2 fixation in plants shows that the enzymic fractionation of stable carbon isotopes of CO2 by RuBP carboxylase is of major importance in determining the δ13C values of C3 plants.