Volume changes associated with hydrolysis of ATP by teleost branchial (Na+ + K+)‐ATPase and Mg2+‐ATPase

Abstract

AbstractThe effects of hydrostatic pressure on branchial NaK‐ATPase and Mg‐ATPase activities of Scorpaena guttata (shallow‐water fish) and Anoplopoma fimbria (deep‐water fish) were used to calculate volume changes that occur during catalysis. The apparent activation volumes (ΔVs) of the NaK‐ATPase, determined at test pressures between 68 and 340 atm, were positive and not significantly different in the two species. ΔV for the Mg‐ATPase of S. guttata was also positive but significantly lower than the ΔV value for the NaK‐ATPase. Negative values for ΔV were obtained for the Mg‐ATPase of A. fimbria. Solubilization of the enzymes from gill microsomes of A. fimbria with Lubrol WX had no effect on ΔVs. NaI treatment of gill microsomes resulted in significant changes in ΔV for both enzymes in A. fimbria and in the Mg‐ATPase of S. guttata. The effect of hydrostatic pressure on Kmfor ATP was used to estimate apparent ATP binding volume (ΔVbind.). The Km values obtained at 1 atm for the Mg‐ATPase and NaK‐ATPase of A. fimbria (0.030 and 0.043 mM, respectively) were approximately an order of magnitude lower than those of S. guttata(0.24 and 0.28 mM, respectively). High hydrostatic pressure (340 atm) produced an increase in Km in the NaK‐ATPase of both species, indicating that ΔVbind. is positive. The Km of the Mg‐ATPase in S. guttata was relatively insensitive to pressure (small positive value for ΔVbind.) where it decreased at high pressure in A. fimbria, yielding a large negative value for ΔVbind..