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At least 19 recordsLinked to original sources
PubMed1983

Inhibition of liver surface membrane Na+, K+-adenosine triphosphatase, Mg+2-adenosine triphosphatase and 5'-nucleotidase activities by protoporphyrin. Observations in vitro and in the perfused rat liver.

To clarify the pathogenesis of protoporphyrin-induced cholestasis, liver surface membrane enzyme activities were determined after (a) isolated rat liver perfusion with protoporphyrin administered by bolus (1.0 mumol) or bolus plus constant infusion (1.0 mumol + 0.05 mumol/min) and (b) combination of liver surface membrane with protoporphyrin (0.9-53.4 nmol/ml) in vitro. The perfusion studies showed that protoporphyrin significantly inhibited Na+, K+- and Mg+2-adenosine triphosphatase activities. In vitro, these adenosine triphosphatase activities and the 5'-nucleotidase activity were inhibited linearly by protoporphyrin up to a concentration of approximately 18 mumol/ml; thereafter, enzyme activity was maintained. Greater inhibition of the adenosine triphosphatase activities occurred with protoporphyrin than was reported for chlorpromazine at similar molar concentrations. This effect was independent of the quantity of membrane protein analyzed and was not reversible with a 50:1 dilution. The inhibitory effect of protoporphyrin on surface membrane enzyme activities was also nonselective. Although the hepatotoxic effects of protoporphyrin may be more generalized, the present data underscore protoporphyrin's toxic interaction with liver surface membranes.

5'-Nucleotidase↗
PubMed1986

Halobacterial adenosine triphosphatases and the adenosine triphosphatase from Halobacterium saccharovorum.

Membranes prepared from various members of the genus Halobacterium contained a Triton X-100 activated adenosine triphosphatase. The enzyme from Halobacterium saccharovorum was unstable in solutions of low ionic strength (< 3 M NaCl) and maximally active in the presence of 3.5 M NaCl. A variety of nucleotide triphosphates was hydrolyzed. MgADP, the product of ATP hydrolysis, was not hydrolyzed and was a competitive inhibitor with respect to MgATP. The enzyme from H. saccharovorum was composed of at least 2 and possibly 4 subunits. The 83-kDa and 60-kDa subunits represented about 90% of total protein. The 60-kDa subunit reacted with dicyclohexylcarbodiimide (DCCD) when inhibition was carried out in an acidic medium. The significance of the two minor components (28 kDa and 12 kDa is not established. The enzyme from H. saccharovorum, which differs from previously described halobacterial ATPases, possesses properties of an F1F0 as well as an E1E2 ATPase.

Adenosine Triphosphatases↗
PubMed1983

Inhibition of Na,K-dependent adenosine triphosphatase by adenosine in intact pigeon erythrocytes.

In intact pigeon erythrocytes, adenosine is a potent inhibitor of Na,K-dependent adenosine triphosphatase. In purified cell-membrane preparations, adenosine is only a weak competitive inhibitor of Na,K-ATPase, with respect to ATP. This indicates that adenosine must not be a direct inhibitor of the sodium pump in intact red cells per se; instead, adenosine exerts its inhibitory effect via endogenous cell factors.

Adenosine↗
PubMed1981

Transient state in the phosphorylation of sodium- and potassium- transport adenosine triphosphatase by adenosine triphosphate.

The rate of phosphorylation of sodium and potassium ion-transport adenosine triphosphatase by 10 microM [gamma-32P]ATP was much slower with Ca2+ than with Mg2+ (0.13-10 mM) in the presence of 16 to 960 mM Na+ at 0 degrees C and pH 7.4. In the presence of a fixed concentration of Mg2+ or Ca2+, the rate became slower with increasing Na+ concentration. When the Na+ concentration was fixed, the rate became slower with decreasing divalent cation concentration. Sodium ions appear to antagonize the divalent cation in the phosphorylation to slow its rate. In the presence of 1 mM Ca2+ and 126 or 270 mM Na+, the rate was slow enough to permit the manual addition of a chasing solution at various times before the phosphorylation reached the steady state. Therefore, we studied the time-dependent change of the sensitivity to ADP or to K+ of the phosphoenzyme by a chase with unlabeled ATP containing ADP or K+ during the time range from the transient to the steady state of the phosphorylation. The ADP sensitivity decreased and the K+ sensitivity increased with the progress of the phosphorylation. With 270 mM Na+, the phosphoenzyme found at 1 s, when its amount was 5.5% of the maximum level, was virtually completely sensitive to ADP. Under these conditions, it was concluded that the form of the phosphoenzyme initially produced from the enzyme.ATP complex has ADP sensitivity and that the phosphoenzyme acquires K+ sensitivity later. The initially produced ADP-sensitive phosphoenzyme partially lost its normal instability and sensitivity upon adding a chelating agent, probably because of dissociation of a divalent cation from the phosphoenzyme.

Adenosine Triphosphate↗
PubMed1984

Comparison of effects of inhibitors on adenosine triphosphatase and adenosine diphosphatase activities in rat-liver mitochondria.

Adenosine diphosphatase (ADPase) activity and ATPase activity were assayed in rat liver mitochondria and outer mitochondrial membrane preparations with [beta-32P]ADP and [gamma-32P]ATP as substrates. Inhibition studies were performed with the mitochondrial ATPase inhibitor oligomycin and the adenine nucleotide transport inhibitor, carboxyatractyloside. Kinetic studies were also performed with the nucleotide thiophosphate analogs adenosine 5'-O-thiophosphate, adenosine 5'-O-(2-thiodiphosphate) and adenosine 5'-O-(3-thiotriphosphate) which can act as inhibitors of phosphohydrolases. It is concluded that part of the apparent ADPase activity of intact mitochondria is mediated via ATPase, presumably in conjunction with adenylate kinase. In addition the outer mitochondrial membrane appears to show a distinct ADPase not attributable to contamination by inner membrane ATPase.

Adenosine Diphosphate↗