Models for the regulation of purine metabolism in rat hepatocytes: evaluation of tracer kinetic experiments.

Metabolic pathways are characterized by numerous regulatory mechanisms. Their study calls for the determination of the metabolite concentrations as well as the flux rates. Corresponding experiments using purified enzymes and an artificial environment frequently yield results that differ from findings for in vivo systems. To be more realistic, the tracer kinetic experiments presented here involved intact isolated hepatocytes. It is necessary to establish mathematical models to deduce the flux rates. With the presumption of metabolic steady-state conditions, the flux rates are determined by a possibly stiff system of linear differential equations. For the first time, the flux rate determination in the purine metabolism of rat hepatocytes was accomplished by applying a combination of a nonlinear least-square fit and a numerical integration. Because of the complexity of this pathway, it was necessary to use three different tracers requiring three partial models. By ensuring their compatibility and using a fit of high statistical quality, the experimental situation could be described adequately. Our flux rate analysis confirmed earlier experimental findings and also allows much more insight into the regulatory mechanisms of the metabolism studied.