Identification of a creatine kinase inhibitor in human serum

Human skeletal muscle creatine kinase is inhibited by a dialyzable factor in human serum. The inhibitor was fractionated from other serum components by ultrafiltration and chromatography on Sephadex G-10 and was identified as uric acid by spectral, chromatographic, and kinetic methods. Its quantitative kinetic properties are identical to those of uric acid, both as a substrate for uricase and as an inhibitor of creatine kinase. Inhibition of creatine kinase by the natural inhibitor and by known uric acid is completely reversible by addition of thiol compounds.