Comparison of computer simulations of the F‐type and L‐type non‐oxidative hexose monophosphate shunts with ³¹P‐NMR experimental data from human erythrocytes

Abstract

Mathematical modelling was used to predict the behaviour of the two most favoured schemes for the operation of the non-oxidative hexose monophosphate shunt (HMS), the F-type and the L-type pathways. The models simulate the time courses of sugar-phosphate concentrations when various substrates are metabolized via each pathway. A 31P-NMR technique, with which to observe time courses of concentrations of sugar phosphates in a human red cell lysate, was developed. The accuracy of each hypothesised scheme was then evaluated by comparing predicted with observed data.The results were more consistent with time courses of sugar-phosphate levels predicted by the F-type (classical) pathway than those predicted by the L-type model. However, the accumulation of sedoheptulose 1,7-bisphosphate when a haemolysate was incubated with ribose 5-phosphated showed that the F-type pathway is not a complete description of the system of reactions. Transaldolase was demonstrated to be essential for the normal metabolism of sugar phosphates by haemolysates. The effects of the heat-inactivation of transaldolase on the metabolism of sugar phosphates were accurately predicted by the F-type model.The relevance of attempting to describe the reaction of the non-oxidative HMS as a distinct 'pathway' or 'cycle' is discussed.Two major roles of the hexose monophosphate shunt are to generate reducing equivalents in the form of NADPH, and pentose sugars for nucleotide synthesis. The first function is carried out by the oxidative reactions : the non-oxidative Correspondence to P. W. Kuchel,

Nuclear magnetic resonance studies of the regulation of the pentose phosphate pathway

“…Other reviews suggest that the majority ofradiotracer studies are in accord with the classical formulation (18), and there is to this date a controversy concerning the operation of the pathway in the liver. More recently, a comparison of NMR data in red blood cells to computer simulations of the Ltype and F-type pathways showed that the results were more consistent with the F-type than with the L-type formulation (19). The model we use in the development of our method is the classical formulation of the pathway and our data are also consistent with this formulation.…”

Model of 2,3-bisphosphoglycerate metabolism in the human erythrocyte based on detailed enzyme kinetic equations: equations and parameter refinement

“…Glc(1,6)P # is an inhibitor of HK and an activator of PFK and PK in itro (see the Appendix). In addition, the work reported in [21] indicates that the free concentration of Glc(1,6)P # is very sensitive to pH and oxygenation state. Thus this compound may be an important modulator of erythrocyte metabolism in i o.…”

“…Membrane transport of metabolites, the binding of metabolites to haemoglobin (Hb) and Mg# + [21], as well as pH effects on key enzyme reactions and binding processes, are also included. The model (at this stage) does not include osmotic and electric (membrane potential) effects.…”

“…The kinetic equations describing the formation of Mg# + and Hb complexes are presented in [21]. A system of 108 differential equations was constructed from these kinetic equations.…”

“…Also, in [19], k $ should have the value 10* M −" :s −" and not the reported 10( M −" :s −" . The kinetic equations used to model the non-oxidative PPP (R5PI, Ru5PE, TA and TK) were from McIntyre et al [21]. TK catalyses two reactions at a common active site and hence it proved simpler to model its operation with elementary rate equations rather than a steady-state rate equation.…”