Differences in carbon isotope discrimination of three variants of D-ribulose-1,5-bisphosphate carboxylase/oxygenase reflect differences in their catalytic mechanisms.

The carboxylation kinetic (stable carbon) isotope effect was measured for purified d-ribulose-1,5-bisphosphate carboxylases/oxygenases (Rubiscos) with aqueous CO(2) as substrate by monitoring Rayleigh fractionation using membrane inlet mass spectrometry. This resulted in discriminations (Delta) of 27.4 +/- 0.

Measurement of (carbon) kinetic isotope effect by Rayleigh fractionation using membrane inlet mass spectrometry for CO-consuming reactions.

Methods for determining carbon isotope discrimination, Δ, or kinetic isotope effects, α, for CO-consuming enzymes have traditionally been cumbersome and time-consuming, requiring careful isolation of substrates and products and conversion of these to CO for measurement of isotope ratio by mass spectrometry (MS). An equation originally derived by Rayleigh in 1896 has been used more recently to good effect as it only requires measurement of substrate concentrations and isotope ratios. For carboxylation reactions such as those catalysed by d-ribulose-1,5-bisphosphate carboxylase / oxygenase (RuBisCO, EC 4.