Effects of monosaccharides on the sodium activation curve of the intestinal (Na+-K+)-ATPase

1. 1. (Na+-K+)-ATPase was purified from the mucosa of rat small intestine; its final specific activity of about 1.2 I.U. The principal enzymatic properties of the enzyme were similar to those of (Na+-K+)-ATPases from other sources. 2. 2. In the presence of actively transported monosaccharides such as glucose, galactose, 3-O-methylglucose and mannose the K+-dependent transition of the hyperbolic Na+ activation curve into a sigmoidal one takes place at lower K+ concentrations than in its absence. 3. 3. Parallel to the change in kinetics an inhibitory effect of these monosaccharides on the (Na+-K+)-ATPase is observed, which is significantly dependent on the Na+ and K+ concentrations as well as on the Na+:K+ ratio. At low concentrations of these ions a total inhibition caused by these monosaccharides takes place.