Correction of the apparent Michaelis constant, biased by an unstirred layer, if a passive transport component is present.

An unstirred layer shifts the permeation curve due to a carrier-mediated transport system (non-passive component) to the right, so that a higher Km value is determined (apparent Michaelis constant biased by an unstirred layer). If a substance is transported simultaneously by a non-passive and a passive mechanism, and if the non-passive component can be inhibited, the permeation curve due to the non-passive component can be obtained by subtraction of the curve due to the passive component alone from the curve obtained with intact passive and non-passive component. But in the presence of an unstirred layer the difference curve lies always below the curve which would be obtained in the presence of the non-passive component alone. This error increases with increasing unstirred layer thickness and increasing magnitude of the passive transport component. By means of an appropriate equation the apparent Michaelis constant biased by an unstirred layer and determined from the difference curve can be corrected, if the unstirred layer thickness and area are known.