Direct enzyme-substrate affinity determination by real-time hyperpolarized 13C-MRSElectronic supplementary information (ESI) available. See DOI: 10.1039/c4cc05418k

A specialized kinetic analysis of real-time hyperpolarized [1,1,2,2-D 4 , 1- 13 C]choline 13 C-magnetic resonance spectroscopy enabled the determination of initial rates of metabolic enzyme activity (choline oxidase), enzyme-substrate affinity ( K m ), and inhibition. In a clinical MRI scanner, metabolite levels lower than 16 μM were detected at a temporal resolution of 1 s. A direct determination of K m , a fundamental measure of enzyme-substrate affinity, is now possible without sample manipulation by real-time hyperpolarized 13 C-MRS.