Limitations of Michaelis-Menten kinetics in presence of intestinal unstirred layers.

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Abstract
Experimental results of intestinal transport are often analyzed by use of 1 of the 3 linear transformations of the Michaelis-Menten equation. This approach may be invalid because it fails to account for the effective resistance of the intestinal unstirred water layer (UWL). Biologically relevant values for the UWL resistance affinity constant Km, maximal transport rate Jdmax, and the distribution of the transport sites along the villus were substituted into a newly derived equation that describes unidirectional flux (Jd) in the presence of an UWL. This series of curves was then redrawn using the plot of C1/Jd vs. C1, where C1 is the concentration of the probe molecule in the bulk phase. A linear relationship was observed only under special circumstances. The discrepancy between the value of the apparent (Km*) and true (Km) affinity constants increased with high UWL resistance, low values of Jdmax and failure to correct for passive permeation. Although the difference between the true (Jdmax) and apparent .**GRAPHIC**. maximal transport rates was increased when the Km was high, when Jdmax was low, or when most transport occurred from near the tip of the villus, the numerical value of the discrepancy was small and this plot provided an acceptable method of estimating Jdmax. This plot does provide a qualitative method for the assessment of the adequacy of the correction for passive permeation and for the determination of whether the Km of the carrier and the resistance of the overlying UWL vary at different sites along the villus.