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

^a Department of Plant Sciences, Texas A&M University, College Station, Texas 77843

The enzymic fractionation of the stable carbon isotopes of CO₂ (co₂) 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 co₂ 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 ¹³C values of C₃ plants.