The reaction catalysed by creatine kinase was studied in both directions by quenched-flow techniques to follow the initial product formation in the millisecond range. In both directions the amount of product formed increases linearly with time, and the turnover number corresponds to the steady-state value. Extrapolation to zero time indicates the absence of either a large transient phase or a large lag phase in both directions. This indicates that the actual chemical reaction is rate-limiting, and that all possible isomerizations before or after the chemical step must be much more rapid.
