Abstract
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 (Mg 2+-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. NH 4 + 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 Mg 2+ 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.
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