Two different phosphorylation-dephosphorylation cycles of Na,K-ATPase proteoliposomes accompanying Na+ transport in the absence of K+.

Phosphate from the phosphointermediate (EP) of the human red blood cell Na/K pump is coeffluxed with Na, in the absence of external K.

“…Values of Na efltux (Table IX) for the indicated conditions are repeated in Table X for ease of comparison. Although the stoichiometry ofADP-independent Nai for Nao exchange may be 1 to 1 or 3 to 2 (see Blostein, 1983;Yoda and Yoda, 1987), the expected stoichiometry for ADP-dependent Nai for Nao exchange is 1:1 (Garrahan and Glynn, 1967a). For the results presented in Table X it is evident that in both of the above kinds of Nai/Nao exchange effiux exceeds influx.…”

“…The extent of this inhibition of (or the level of the residual) uncoupled Na efflux can be estimated by subtracting the amount of the Nai/Nao exchange efflux from the total Na efflux. The amount of the exchange efflux is estimated by using the Nao influx values and assuming a Nao for Nai exchange stoichiometry of either 1 for 1 or 2 Nao for 3 Nai (Lee and Blostein, 1980;Blostein, 1983;Yoda and Yoda, 1987). If the Nao/Nai exchange stoichiometry is 1 for 1 then for experiment 3A (Table X) the corrected efflux is 0.475-0.177 = 0.298 mmol Na/liter ghosts • h; if the stoichiometry is 3 Nao/2 Nai then the corrected efflux is 0.475-0.266 = 0.209 mmol Na/liter ghosts x h. Since in this experiment, uncoupled Na efflux at zero Nao (see Table IX) was 0.575 mmol Na/liter ghosts x h, the inhibition was 48 and 64%, respectively, for the above stoichiometries.…”

“…It could be, for instance, that deoccluded Na could not be released to the outside before the accompanying 32p is released. It is also possible that some but not all (that is, the first of two if the stoichiometry is 2Na/PO4 as in Table VIII) of the deoccluded Na is released before 32p release (see Yoda and Yoda, 1987) with the remainder Na being released afterwards. On the other hand, if type IB uncoupled Na efflux is electrogenic, for which there is some evidence (see Trace B in Figs.…”

“…must leave in order to provide for occupancy of K, in its sites. Evidence that K,, can bind before all of the Na is released has been discussed by Kennedy et al (1986) and Yoda and Yoda (1987). But one should be aware that all of the above considerations of ligand interactions relative to order are also dependent upon the actual stoichiometric value of Na to Pi and the average valence of the transported Pi, both of which have yet to be accurately established.…”

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“…Later work has shown that the outward release of potassium ions from the occluded-potassium form of the enzyme, when orthophosphate is added, is an ordered release (85)(86)(87) and that the outward release of sodium ions from the occluded-sodium form of the enzyme occurs in two stages (88)(89)(90). Despite these complications, the working cycle remains an elaboration of the old Albers-Post ping-pong scheme.…”