Extending the quasi-steady state concept to analysis of metabolic networks.

A means is proposed for evaluating enzyme effectiveness in vivo via a simplified dynamic description of the metabolic reaction network within which the enzyme operates. The basis of the method is application of sensitivity analysis to a quasi-steady approximation of a complete dynamic model, and its implementation centers on interpreting the transient relations of selected intermediates following a perturbation to the system of interest: for many important situations such relations can be simply interpreted to give a useful global measure of enzyme effectiveness. This method is found to be successful for estimating phosphofructokinase and pyruvate kinase activity in the human red cell, and it appears promising as a basis for developing a means for detecting enzyme abnormalities caused by environmental or genetic factors. This method may also prove useful for comparative studies of glycolysis in different types of cells. The analysis presented is based on available models of red cell glycolysis, but the results are not highly sensitive to ambiguities in the system model. The approach suggested appears to provide an effective means for describing system dynamics and determining the behavior of an individual enzyme in an intact system by making a first-order allowance for interaction with the system as a whole. Requirements for success of this approach remain to be identified in detail, but effective time-scale separation is probably the key.