Application and potential of coenzyme labelling

Abstract

Reaction rates and equilibria in intermediary metabolism are regulated by several factors such as substrate concentrations, allosteric effects and feed-back control of enzyme synthesis. Another factor that may be of importance is the interaction between reactions that utilise the same coenzyme. A typical example is the decreased conversion of lactate to pyruvate during ethanol oxidation, mediated by the increased ratio between the concentrations of free (i.e. not protein-bound) NADH and NAD⁺ in the liver cells. Compartmentation of redox reactions and coenzymes may determine the degree of interaction. For example, the impermeability of the mitochondrial membrane to pyridine nucleotides results in the well-known existence of cytosolic and mitochondrial pools of NAD with widely different redox states. The diffusion rate in the cytosol is generally considered to be sufficiently high to prevent the formation of several coenzyme pools in this compartment. However, the equilibration of NAD bound to enzymes may be limited by slow dissociation rates. Another possibility for compartmentation of coenzymes when whole organs are studied is the existence of functionally different cell types in the organ.