The evaluation of initial rates in the presence of background noise.

A three-parameter model was used to test a curvilinear regression for its gamma-intercept. It was applied in the kinetic analysis of a model reaction and of real experiments. The model reaction consisted of a dialysis experiment with increasing amounts of background added at t0. In the presence of this background, initial rates of solute exchange were determined precisely enough to be in agreement with Fick's law of diffusion. The biological experiments were concerned with the saturation kinetics of the acid-soluble radioactivity of cells exposed to 3H-labelled amino acids. Two models of regression were applied, facilitated and physical diffusion, both consistent with the assumption of a substantial amount of background. This background was assumed to be caused by adsorption of 3H-labelled amino acids to cell-surface proteins. Without further consideration it caused a systematic overestimation of the initial rates.