A carrier enzyme basis for ammonium excretion in teleost gill. NH⁺₄-stimulated Na-dependent ATPase activity in Opsanus beta

1. 1. Branchial Na⁺ K⁺-ATPase specific activity is some 20% greater in hyposahne adapted Opsanus beta than in SW specimens. 2. 2. Ouabain insensitive ATPase (Mg²⁺-ATPase) specific activities were similar, while whole body-activity differences in low salinity and SW adapted fish could be accounted for by the 30% difference in extractable gill protein. 3. 3. NH₄⁺ ion was 15% more effective at dephosphorylation of the microsomal Na-dependent phosphoenzyme than either Rb⁺ or K⁺. and revealed a maximal ATPase affinity (K_m = 0.2 mM) within the physiological range of blood [K⁺]. 4. 4. Similar properties as pH optima, ATP and Mg²⁺ K_m's. ouabain sensitivity, percent recoveries and subcell distribution indicated that the NH⁺₄-stimulation acts through the Na⁺ K⁺ -ATPase carrier enzyme and may be resonsible for the Na⁺/NH⁺₄ exchange in Opsanu beta.