The ion and cofactor optima of Na, K-ATPase in crude homogenates of H. nudes gills are shown in Fig. 2. Halfsaturating concentrations (K,,,) were estimated by Lineweaver- Burk plots (data not shown) and are: ATP, 0.60 mM; Mg2+, 2.6 mM; and K+, 1.0 mM.