The errors listed are the standard error as calculated from a nonlinear regression fit of a single kinetic determination for one cell line with one substrate. The goodness of fit parameter is the correlation coefficient of the fit of the experimental data to the Michaelis-Menten equation. K_m values for UK-157,147 and UK-156,037 are listed as apparent K_m because bothK_m and V_max calculations are based on the sum of products formed. The data could not be processed for each product because it was not detectable at all substrate concentrations.