The amino-terminal segment of the catalytic subunit of kidney Na,K-ATPase regulates the potassium deocclusion pathway of the reaction cycle.
Tryptic cleavage of the catalytic subunit of kidney Na,K-ATPase in the E1 conformation effects a change in kinetic behavior apparent at low ATP concentration. Thus, at < or = 10 microM ATP, K+ inhibits Na(+)-dependent ATPase activity of the undigested enzyme but activates activity of the digested enzyme. With time of trypsinolysis, a transient increase followed by a decrease in activity is observed at low [ATP], whereas at high [ATP] (1 mM), activity is progressively reduced. At low [ATP], the trypsin-treated/control activity ratio was > or = 3-fold higher with K+ compared to the ratio observed with the K+ congener Li+. Also, the relative Na/K exchange activity (22Na+ influx into K(+)-loaded inside-out vesicles from erythrocytes) with either 0.01 mM ATP or 1 mM CTP compared to 1 mM ATP was greater for the trypsin-treated than for the control enzyme. The kinetic change is correlated with the initial rapid cleavage of the N-terminal tryptic fragment (< or = 30 residues) from the catalytic subunit. It is concluded that this segment regulates the K+ deocclusion pathway of the reaction; removal of this fragment produces a modified active species having an increased rate of K+ deocclusion.
Department of Medicine, McGill University, Montreal, Canada.
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Wierzbicki
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Blostein
D000818:Animals; D001665:Binding Sites; D004285:Dogs; D004591:Electrophoresis, Polyacrylamide Gel; D007679:Kidney Medulla / Q000201:enzymology*; D007700:Kinetics; D008094:Lithium / Q000378:metabolism; D046911:Macromolecular Substances; D008962:Models, Theoretical; D010446:Peptide Fragments / Q000302:isolation & purification; D011188:Potassium / Q000378:metabolism*; D011189:Potassium Chloride / Q000494:pharmacology; D011487:Protein Conformation; D000254:Sodium-Potassium-Exchanging ATPase / Q000737:chemistry / Q000378:metabolism*; D014357:Trypsin
D046911:Macromolecular Substances; D010446:Peptide Fragments; D011189:Potassium Chloride; D008094:Lithium; D014357:Trypsin; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium