Analysis of metabolic control: new insights using scaled creatine kinase model.

The creatine kinase-adenylate kinase equilibria equations are given a dimensionless form by normalizing to total creatine concentration. Analysis with appropriate equilibrium and cation-binding constants identified two sharply separated phases of energy depletion. In the "buffering" phase, energy is derived from phosphocreatine. In the "depleting" phase, adenine nucleotides are the source of energy. Defining the state of the adenine nucleotide pool requires only pH, phosphocreatine, and creatine concentrations. Analysis of data from skeletal muscle, heart, brain, and smooth muscle demonstrated that the [free adenine nucleotide]/[total creatine] and [total phosphate]/[total creatine] are essentially constant over the greater than 20-fold concentration range among tissues and species. This result permits quantitative evaluation of cell energetics with data scaled to the total phosphate, as obtained with nuclear magnetic resonance studies, or to total creatine, as obtained in chemical analysis of freeze-trapped tissue. By applying the stability of the tissue parameters to the equations, it is demonstrated that unique identification of a hypothesis describing the recruitment of O2 uptake requires testing at several pH values.