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 concentrat ions as well as the flux rates. Corresponding experiments using purifi ed enzymes and an artificial environment frequently yield results that differ from findings for in vivo systems. To be more realistic, the t racer kinetic experiments presented here involved intact isolated hepa tocytes. It is necessary to establish mathematical models to deduce th e flux rates. With the presumption of metabolic steady-state condition s, the flux rates are determined by a possibly stiff system of linear differential equations. For the first time, the flux rate determinatio n in the purine metabolism of rat hepatocytes was accomplished by appl ying a combination of a nonlinear least-square fit and a numerical int egration. Because of the complexity of this pathway it was necessary t o use three different tracers requiring three partial models. By ensur ing their compatibility and using a fit of high statistical quality, t he 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 .