31P NMR saturation transfer measurements of the steady state rates of creatine kinase and ATP synthetase in the rat brain

There are two theories concerning the function of phosphocreatine (PCr) in cellular energy metabolism. In one PCr is seen as a reservoir, functioning to maintain constant cytoplasmic ATP concentrations. The alternate theory postulates that PCr acts as an energy shuttle between the site of ATP production in the mitochondria and the site of utilization in the cytoplasm (reviews [ 1,2]). Both theories require that the exchange flux in the creatine kinase (CPK) reaction be considerably greater than the rate of ATP hydrolysis. This is necessary in order to stabilize ATP concentrations during transitions in workload, or during periods when ATP synthesis is impaired, such as ischaemia. Calculations of cytoplasmic phosphorylation potential [3] and measurements of intracellular pH [4] have been based on the assumption that the CPK reaction is near equilibrium. It is now possible to test this assumption in vivo using the technique of 31P NMR saturation transfer; the flux through the CPK reaction was 5 times the steady state rate of ATP turnover in the Langendorff perfused rat heart 151. The total extractable activities of CPK in the rat brain and heart are similar; 500 pmol . g-’ . min-’ in heart [6] and 200 pmol . g-’ min-’ in brain [7], measured at 25’C. The mitochondrial form of the enzyme accounts for 30-40% of the total activity in the heart [l] and 5% in brain [7]. However, mitochondrial CPK activity in both organs is very similar on a per gram mitochondrial protein basis [8]. Despite these similarities, the metabolic response to a decrease in the efficiency of cellular energy transduction,