Comparison of red cell and kidney (Na+ +K+)-ATPase at 0 degrees C.

Human red cell and guinea pig kidney (Na+ +K+)-ATPase were phosphorylated at 0 degrees C. Using concentrations of ATP ranging from 10(-6) to 10(-8) M, ATP-dependent regulation of reactivity is observed with red cell but not kidney (Na+ +K+)-ATPase at 0 degrees C. In particular, with the red cell enzyme only, the following are observed: (i) the ratio of enzyme-bound ATP (E.ATP, measured by the pulse-chase method of Post, R.L., Kume, S., Tobin, T., Orcutt, B. and Sen, A.K. (1969) J. Gen. Physiol. 54, 306s-326s) to steady-state level of total phosphoenzyme (EP) decreases with decrease in ATP concentration and (ii) the apparent turnover of phosphoenzyme (ratio of Na+-stimulated ATP hydrolysis to level of total EP at steady state) also varies as a function of ATP concentration. In addition, when EP is formed at very low ATP (0.02 microM), and then EDTA is added, rapid disappearance of a fraction of EP occurs, presumably due to ATP resynthesis, only with the red cell enzyme. These differences in behaviour of the red cell and kidney enzymes are explained on the basis of the observed predominance of K+-insensitive EP in red cell, but K+-sensitive EP in kidney (Na+ +K+)-ATPase at 0 degrees C.