The 'double' Michaelis-Menten equation: estimation of parameters.

When the concentration dependence of an enzymic reaction or a transport process can be described by the sum of two Michaelis-Menten terms, reliable data that relate reaction rate and substrate concentration can be obtained even when as much as 70 per cent of substrate was consumed during the assay. Each data pair consists of the average reaction rate during an assay and the concentration where the instantaneous reaction rate was equal to the average rate. Although that concentration cannot be computed exactly (as it depends on the four kinetic parameters), it may be computed in a good approximation as if the reaction followed the simple Michaelis-Menten relationship. The relative error in the approximated concentration for 1 less than or equal to K2/K1 less than or equal to 10(5) and 10(-2) less than or equal to V2/V1 less than or equal to 10(2) did not exceed 5 per cent up to 50 per cent of substrate consumption, and did not exceed 10 per cent up to 70 per cent of substrate consumption.