|Protein-Energy Interactions (International Dietary Energy Consultative Group - IDECG, 1991, 437 pages)|
|Nutrition of immune cells: The implications for whole body metabolism|
The advantage of a near-equilibrium reaction, in a metabolic pathway in vivo, is that the reaction may be very sensitive to small changes in concentrations of cosubstrate or coproduct. Consequently, large changes in flux can be transmitted through such a reaction without any requirement for complex regulatory properties. In general, this means that the activity of the enzyme can be measured relatively easily in crude extracts of the tissue; this ease of assay has, unfortunately, been used by some investigators as the only criterion for the selection of an enzyme to study the maximum flux through a metabolic pathway. This cannot be done. Metabolic logic tells us that these enzymes cannot be used as quantitative indices of flux but, despite this, they are still being used that way.
Enzymes that catalyse non-equilibrium reactions in a metabolic pathway provide directionality in that pathway and are usually subject to allosteric control (see above). Indeed, the control mechanisms may be complex. This means that knowledge of such control mechanisms must be available before a satisfactory assay method for measurement of the maximum activity can be developed; hence, knowledge of metabolic control is necessary to enable the enzyme activity to be adequately assayed in crude extracts of the tissue (see below).
There are at least two conditions that must be satisfied before an enzyme activity can be used to provide quantitative information. First, it is necessary to establish which enzymes in the pathway catalyse non-equilibrium reactions (see above). Second, it is necessary to demonstrate experimentally that the maximum activities of such enzymes in vitro can be used to indicate quantitatively the maximum flux through a reaction. This is done by comparison of the in vitro enzyme activity with the measured or calculated maximum flux through the pathway (NEWSHOLME and LEECH, 1983).