Metabolic interaction between purine nucleotide cycle and oxypurine cycle during skeletal muscle contraction of different intensities: a biochemical reappraisal.

BACKGROUND: A substrate cycle is a metabolic transformation in which a substrate A is phosphorylated to A-P at the expense of ATP (or another "high energy" compound), and A-P is converted back to A by a nucleotidase or a phosphatase. Many biochemists resisted the idea of such an ATP waste. Why a non-phosphorylated metabolite should be converted into a phosphorylated form, and converted back to its non-phosphorylated form through a "futile cycle"? AIM OF REVIEW: In this Review we aim at presenting our present knowledge on the biochemical features underlying the interrelation between the muscle purine nucleotide cycle and the oxypurine cycle, and on the metabolic responses of the two cycles to increasing intensities of muscle contraction. KEY SCIENTIFIC CONCEPTS OF REVIEW: Nowadays it is widely accepted that the substrate cycles regulate many vital functions depending on the expense of large amounts of ATP, including skeletal muscle contraction, so that the expense of some extra ATP and "high energy" compounds, such as GTP and PRPP via substrate cycles, is not surprising. The Review emphasizes the strict metabolic interrelationship between the purine nucleotide cycle and the oxipurine cycle.