The constant proportion enzyme group concept in the selection of reference enzymes in metabolism

Comparative analyses of enzyme activity patterns reveal groups of enzymes with constant proportions and enzymes with variable proportions of their maximum activities. Constant proportion groups comprise enzymes of unbranched metabolic sequences or functionally related pathways. Ratios of constant proportion groups reflect metabolic correlations, which may be used as discriminative magnitudes of metabolic specialization. Comparison of closely related muscles reveals that differences in the absolute levels of the constant proportion group enzymes of glycolysis parallel differences in maximum glycolytic flux rates. This holds for near-equilibrium as well as for non-equilibrium enzymes. In any case, maximum enzyme activities are significantly higher than maximum metabolic flux rates. Maximum enzyme activities therefore do not permit conclusions on maximum metabolic capacities to be drawn. They may, however, be compared in closely related tissues or different metabolic conditions of a given tissue as relative magnitudes of maximum flux rates. Reference enzymes of constant proportion groups as well as of other enzymes which are representative of distinct metabolic pathways may be used in this sense for the evaluation of enzyme activity patterns. Selection of appropriate enzymes depends on the aim of the intended study, on a thorough knowledge of their individual properties, and on the possibility of measuring their maximum activity under reproducible conditions.