A Second Ouabain-sensitive Sodium-dependent Adenosine Triphosphatase in Brain Microsomes

Abstract

Abstract The hydrolysis of AT32P by calf brain microsomes appears to be catalyzed by two different enzymatic sites that are inactivated by ouabain. The first, Site I, is a Na+- and K+-activated ATPase (Km = 10-4 m ATP; Vmax = 1.5 µmoles of Pi released per mg of protein per min) resembling those known to be in other microsomal preparations. The second, Site II (Km = 2 x 10-7 m ATP; Vmax = 0.07 µmoles of Pi released per mg of protein per min) is a Na+-activated ATPase inhibited by K+. At concentrations of K+ sufficient to activate Site I, but too low to completely inhibit Site II, these enzymatic sites operate independently and simultaneously; this is most apparent at low concentrations of ATP when their rates are comparable. The Na+-K+-ATPase has been implicated in the mechanism for Na+ and K+ transport across biological membranes. The Na+-ATPase may be related to certain ouabain-sensitive transport systems that require Na+ but not K+.