Models of the liver pentose cycle

Simple and complete models of the classical liver pentose cycle, and a model of Williams' proposed "L-type" pentose cycle, are compared. All extant experimental data on well-oxygenated whole cell systems can be fitted to the predicted output of the complete classical pentose cycle model; however, there are gross discrepancies between key experimental data and Williams' proposed scheme. The complete classical model allows isotopic reversibility in the non-oxidative segment of the cycle, but none of the reversible enzymes are extremely close to isotopic equilibrium. General approaches are presented to estimate the isotopic reversibility of most enzymic steps, without requiring isolation of the intermediates, present in some cases at very low concentrations. The isotopic reversibility of the non-oxidative pathway causes only minor errors in the equations used to estimate liver pentose cycle flux, which were based on simple unidirectional models.