Membrane inlet for mass spectrometric measurement of catalysis by enzymatic decarboxylases

Membrane inlet mass spectrometry (MIMS) uses diffusion across a permeable membrane to detect in solution uncharged molecules of small molecular weight. We point out here the application of MIMS to determine catalytic properties of decarboxylases using as an example catalysis by oxalate decarboxylase (OxDC) from Bacillus subtilis. The decarboxylase activity generates carbon dioxide and formate from the nonoxidative reaction but is accompanied by a concomitant oxidase activity that consumes oxalate and oxygen and generates CO2 and hydrogen peroxide. The application of MIMS in measuring catalysis by OxDC involves the real-time and continuous detection of oxygen and product CO2 from the ion currents of their respective mass peaks. Steady-state catalytic constants for the decarboxylase activity obtained by measuring product CO2 using MIMS are comparable to those acquired by the traditional endpoint assay based on the coupled reaction with formate dehydrogenase, and measuring consumption of O2 using MIMS also estimates the oxidase activity. The use of isotope-labeled substrate (13C2-enriched oxalate) in MIMS provides a method to characterize the catalytic reaction in cell suspensions by detecting the mass peak for product 13CO2 (m/z 45), avoiding inaccuracies due to endogenous 12CO2.