Abstract: In certain embodiments of the present invention, kinetic parameters of I2S enzyme are determined. In some instances, a sample including I2S enzyme is incubated under defined conditions, with a series of determined amounts of I2S substrate including a detectable label. Following incubation, the reaction mixture can be analyzed, e.g., by a method including chromatography. A detection unit can be used to measure the presence of the detectable label. Data can be analyzed to determine kinetic parameters.