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

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 (K_m = 10^-4 m ATP; V_max = 1.5 µmoles of P_i released per mg of protein per min) resembling those known to be in other microsomal preparations. The second, Site II (K_m = 2 x 10^-7 m ATP; V_max = 0.07 µmoles of P_i 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⁺.