Abstract

Kinetic rate equations which take into account the effects of depletion of free substrates and free enzymes by binding are derived for a model of cyclic enzyme systems. The reaction velocities predicted from these equations are compared with experimentally observed velocities in three different cyclic enzyme systems, i.e., pyruvic kinase and creatine kinase, pyruvic kinase and hexokinase, and pyruvic kinase and succinic thiokinase. The practical use of the last system for microdetermination of guanine nucleotides in tissue extracts is indicated.

Various applications of these equations are discussed: namely, a simple way to estimate the optimal ratio of enzymes for cycling techniques, a simple criterion for the determination of the rate-limiting enzyme in a cyclic system in vitro and in vivo, and the possible use for the determination of the molecular weight or the number of active sites per molecule of enzyme.