A kinetic description of the fate of chemicals in biosystems

A simple kinetic description of the fate of low-molecular-weight compounds in biosystems was derived using the mass action law. Michaelis-Menten kinetics of enzymatic reactions was considered with respect to its two boundary cases, namely first- and zero-order kinetics. Absorption, membrane accumulation, non-covalent protein binding, biotransformation, and excretion have been included in the model, with only the last two steps being considered as time-dependent on the pertinent time scale of hours and days. This time hierarchy allowed for simplification of the resulting expression. In accordance with the results of uptake experiments and contrary to previous approaches, transport of organic molecules into the cell was not considered as the rate-limiting step. The decisive compound properties were found to be hydrophobicity and the intrinsic rate parameters of biotransformation and excretion. The model was applied to the elucidation of the dependence of the observed biotransformation rate parameters on hydrophobicity. The resulting equations are consistent with literature data.