An in vitro inhibitor of Na-K-ATPase present in an adenosinetriphosphate preparation.

An in vitro inhibitor of Na-K-ATPase was discovered in a commonly used preparation of ATP made by the Sigma Chemical Company, St. Louis, Mo. (Sigma grade ATP). As measured by a reliable and widely used assay system in which phosphate liberation in measured colorimetrically, Na-K-ATPase activity in the rat kidney, small intestine and colon was about 50% lower then Sigma grade ATP was used as substrate as compared to another Sigma Chemical Company product II ATP. Mg-ATPase and adenylate cyclase assays were unaffected by substituting Sigma grade for grade II ATP. The inhibitor of Na-K-ATPase could not be identified. Sigma grade ATP probably should not be used when measuring the activity of Na-K-ATPase in the rat kidney, small intestine, or colon.