The non-gastric H,K-ATPase is oligomycin-sensitive and can function as an H+,NH4(+)-ATPase.
We used the baculovirus/Sf9 expression system to gain new information on the mechanistic properties of the rat non-gastric H,K-ATPase, an enzyme that is implicated in potassium homeostasis. The alpha2-subunit of this enzyme (HKalpha2) required a beta-subunit for ATPase activity thereby showing a clear preference for NaKbeta1 over NaKbeta3 and gastric HKbeta. NH4(+), K+, and Na+ maximally increased the activity of HKalpha2-NaKbeta1 to 24.0, 14.2, and 5.0 micromol P(i) x mg(-1) protein x h(-1), respectively. The enzyme was inhibited by relatively high concentrations of ouabain and SCH 28080, whereas it was potently inhibited by oligomycin. From the phosphorylation level in the presence of oligomycin and the maximal NH4(+)-stimulated ATPase activity, a turnover number of 20,000 min(-1) was determined. All three cations decreased the steady-state phosphorylation level and enhanced the dephosphorylation rate, disfavoring the hypothesis that Na+ can replace H+ as the activating cation. The potency with which vanadate inhibited the cation-activated enzyme decreased in the order K+ > NH4(+) > Na+, indicating that K+ is a stronger E2 promoter than NH4(+), whereas in the presence of Na+ the enzyme is in the E1 form. For K+ and NH4(+), the E2 to E1 conformational equilibrium correlated with their efficacy in the ATPase reaction, indicating that here the transition from E2 to E1 is rate-limiting. Conversely, the low maximal ATPase activity with Na+ is explained by a poor stimulatory effect on the dephosphorylation rate. These data show that NH4(+) can replace K+ with similar affinity but higher efficacy as an extracellular activating cation in rat nongastric H,K-ATPase.
Swarts
Herman G P
HG
Department of Biochemistry, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, 6500 HB Nijmegen, The Netherlands.
Koenderink
Jan B
JB
Willems
Peter H G M
PH
De Pont
Jan Joep H H M
JJ
D000251:Adenosine Triphosphatases / Q000032:analysis / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000641:Ammonia / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D016367:Baculoviridae; D001665:Binding Sites; D015153:Blotting, Western; D027682:Cation Transport Proteins; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D017506:H(+)-K(+)-Exchanging ATPase; D007093:Imidazoles / Q000494:pharmacology; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D009840:Oligomycins / Q000494:pharmacology*; D010042:Ouabain / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D011188:Potassium / Q000032:analysis / Q000378:metabolism* / Q000494:pharmacology; D011485:Protein Binding; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D017384:Sequence Deletion; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D018411:Spodoptera / Q000166:cytology / Q000821:virology; D014638:Vanadates / Q000494:pharmacology
D027682:Cation Transport Proteins; D004791:Enzyme Inhibitors; D007093:Imidazoles; D009840:Oligomycins; D021122:Protein Subunits; D011994:Recombinant Proteins; C035235:Sch 28080; D014638:Vanadates; D010042:Ouabain; D000641:Ammonia; D012964:Sodium; D000251:Adenosine Triphosphatases; C022761:potassium transporting ATPase; D017506:H(+)-K(+)-Exchanging ATPase; D011188:Potassium