Transport ATPase: thimerosal inhibits the Na+ +K+-dependent ATPase activity without diminishing the Na+-dependent ATPase activity

Protective effects of 2,3-dimercaptopropane-1-sulfonate on mercuric chloride-induced acute inhibition of enzymes from rat duodenal mucosa and kidney cortex

Acute inhibitory effects of mercuric chloride on the activities of several enzymes from rat duodenal mucosa and kidney cortex and the protection provided by 2,3-dimercaptopropane-1-sulfonate (DMPS) were examined. Activities of carbonic anhydrase in homogenate, brush border and cytosol and of Mg(2+)-dependent, HCO3-stimulated ATPase in homogenate and brush border of duodenal mucosa and kidney cortex and activities of kidney microsomal Mg(2+)-dependent, Na(+)-K(+)-stimulated ATPase were all decreased following administration of HgCl2 (1-3 mg Hg/kg body weight s.c. once daily for 3 days). Decreases occurred generally in a dose-dependent manner and went back to near normal or normal levels by the combined administration of 20 and 30 mg DMPS/kg per day for 3 days. The concentration of serum urea nitrogen was increased about 8 times by the administration of 2 mg Hg/kg and restored to normal by the concomitant administration of 30 mg DMPS/kg. After the administration of 2 mg Hg/kg per day for 3 days, about half of the renal cortical proximal tubuli were necrotic. Administration of 30 mg DMPS/kg restored these changes to almost normal. The results suggest that DMPS is useful in mitigating acute Hg poisoning and that part of the protective effect of DMPS on enzyme damage induced by Hg poisoning may be due to its chelating action.

Mechanism of methylmercury cytotoxicity

Although a large number of epidemiological, clinical, and pathological studies on methylmercury intoxication have been published, these investigations have not been able to elucidate the detailed mechanisms by which the metal alkyl causes a wide variety of biological dysfunctions. Thus, the cultured cells which are free from the influence of whole body complexities, such as absorption, distribution, metabolism, etc., which complicate the interpretation of in vivo experimental results, attract the attention of many scientists who are interested in clarifying the mode of toxic action of methylmercury. The aim of this article is to review the recent studies on the toxicity of methylmercury at the cellular level and to outline the mechanisms which have been proposed to be responsible for cell injuries.

Uncoupling of ATP binding to Na+,K+-ATPase from its stimulation of ouabain binding: studies of the inhibition of Na+,K+-ATPase by a monoclonal antibody

The effects of a monoclonal antibody, prepared against the purified lamb kidney Na+,K+-ATPase, on the enzyme's Na+,K+-dependent ATPase activity were analyzed. This antibody, designated M10-P5-C11, is directed against the catalytic subunit of the "native" holoenzyme. It inhibits greater than 90% of the ATPase activity and acts as a noncompetitive or mixed inhibitor with respect to the ATP, Na+, and K+ dependence of enzyme activity. It inhibits the Na+- and Mg2+ATP-dependent phosphoenzyme intermediate formation. In contrast, it has no effect on K+-dependent p-nitrophenylphosphatase (pNPPase) activity, the interconversion of the phosphoenzyme intermediates, and ADP-sensitive or K+-dependent dephosphorylation. It does not alter ATP binding to the enzyme nor the covalent labeling of the enzyme at the presumed ATP site by fluorescein 5'-isothiocyanate (FITC), but it prevents the ATP-induced stimulation in the rate of cardiac glycoside [3H]ouabain binding to the Na+,K+-ATPase. M10-P5-C11 binding appears to inhibit enzyme function by blocking the transfer of the gamma-phosphoryl of ATP to the phosphorylation site after ATP binding to the enzyme has occurred. In the presence of Mg2+ATP, it also prevents the ATP-induced transmembrane conformational change that enhances cardiac glycoside binding. This uncoupling of ATP binding from its stimulation of ouabain binding and enzyme phosphorylation demonstrates the existence of an enzyme-Mg2+ATP transitional intermediate preceding the formation of the Na+-dependent ADP-sensitive phosphoenzyme intermediate. These results are also consistent with a model of the Na+,K+-ATPase active site being composed of two distinct but interacting regions, the ATP binding site and the phosphorylation site.

Modified cation activation of the (Na+K)-ATPase following treatment with thimerosal

Treatment of the Na,K-ATPase enzyme, isolated from canine renal outer medulla, with thimerosal (ethylmercurithiosalicylate) resulted in significant inhibition of the overall Na,K-ATPase with only slight, if any, inhibition of the Na-ATPase and ATP:ADP exchange activities. The K-stimulated PNPPase activity was stimulated [see G. R. Henderson and A. Askari (1977) Arch. Biochem. Biophys. 182, 221-226]. Examination of the Na dependence of the ATPase and ATP:ADP exchange activities revealed an Na-independent, ouabain-sensitive activity that was inhibited by Na in the range 0-10 mM. At greater than or equal to 10 mM concentration of Na the treated and modified enzymes showed similar activities. The apparent affinity of the modified enzyme for ATP in the presence of 100 mM Na was the same as that of the untreated enzyme (0.2-0.3 microM). In the absence of Na, the modified enzyme hydrolyzed ATP with a relatively low affinity (about 120 microM). The enhancement of p-nitrophenylphosphatase (PNPPase) activity measured in the presence of K ions was due to the appearance of K-independent, ouabain-sensitive PNPP activity. The modification was without major affect on the apparent affinity of the enzyme for K ions in the PNPPase activity. Treatment of the thimerosal-modified enzyme with dithiothreitol removed (or greatly reduced) the cation-independent, ouabain-sensitive activities and the Na,K-ATPase activity returned. Modification of a set of enzyme -SH groups in the Na,K-ATPase enzyme made it able to hydrolyze ATP in the absence of Na ions and PNPP in the absence of K ions. The -SH groups modified by thimerosal are evidently critical to the major dephosphoenzyme conformational changes but are not involved in the major transport conformational change between phosphoenzymes.

The protective effects of thiol-containing compounds on mercuric chloride-induced acute inhibition of enzymes from mouse kidney

The inhibitory effect of mercuric chloride on mouse kidney enzymes and the treatment of this inhibition with some thiol-containing compounds, e.g., dithiothreitol (DTT), dimercaptosuccinic acid (DMS) and D-penicillamine (Pen) was examined. To produce a significant inhibition of renal enzymes in vivo, it was necessary to administer 5 mg Hg/kg/day, s.c., once daily for 3 doses, and to sacrifice the animals 1 day after the last injection. With this Hg dose, microsomal Mg2+-Na+-K+-ATPase, mitochondrial Mg2+-HCO-3-ATPase and supernatant carbonic anhydrase activities decreased to about 30%, 70% and 60% of normal values, respectively. Combined administration of DTT (5-20 mg/kg, i.p.) and DMS (5-20 mg/kg, i.p.) with HgCl2 restored the enzyme activities to near normal or normal levels in parallel to the administered doses. The effect of Pen was slightly less than that of the above 2 compounds. This may be due to the number of thiol radicals, as Pen is a monothiol and the other 2 compounds are dithiol compounds. Since the toxicity of DMS is very low compared with DTT, DMS may be more suitable for the treatment of mercury poisoning.

Kinetic studies on the (Na+ + K+)-dependent ATPase evidence for coexisting sites for Na+, K+ and Mg2+

Na+-ATPase activity of a dog kidney (Na+ + K+)-ATPase enzyme preparation was inhibited by a high concentration of NaCl (100 mM) in the presence of 30 microM ATP and 50 microM MgCl2, but stimulated by 100 mM NaCl in the presence of 30 microM ATP and 3 mM MgCl2. The K0.5 for the effect of MgCl2 was near 0.5 mM. Treatment of the enzyme with the organic mercurial thimerosal had little effect on Na+ -ATPase activity with 10 mM NaCl but lessened inhibition by 100 mM NaCl in the presence of 50 microM MgCl2. Similar thimerosal treatment reduced (Na+ + K+)-ATPase activity by half but did not appreciably affect the K0.5 for activation by either Na+ or K+, although it reduced inhibition by high Na+ concentrations. These data are interpreted in terms of two classes of extracellularly-available low-affinity sites for Na+: Na+-discharge sites at which Na+-binding can drive E2-P back to E1-P, thereby inhibiting Na+-ATPase activity, and sites activating E2-P hydrolysis and thereby stimulating Na+-ATPase activity, corresponding to the K+-acceptance sites. Since these two classes of sites cannot be identical, the data favor co-existing Na+-discharge and K+-acceptance sites. Mg2+ may stimulate Na+-ATPase activity by favoring E2-P over E1-P, through occupying intracellular sites distinct from the phosphorylation site or Na+-acceptance sites, perhaps at a coexisting low-affinity substrate site. Among other effects, thimerosal treatment appears to stimulate the Na+-ATPase reaction and lessen Na+-inhibition of the (Na+ + K+)-ATPase reaction by increasing the efficacy of Na+ in activating E2-P hydrolysis.

Rat brain (Na+-K+)ATPase: modulation of its ouabain-sensitive K+-PNPPase activity by thimerosal

1. The (Na+ + K+) ATPase activity of a rat brain synaptic membrane preparation was inhibited by 10(-5) M thimerosal. 2. The ouabain inhibitable K+-PNPPase activity of thimerosal treated membranes was compared with that of untreated membranes with respect to sensitivity to temperature, ouabain, K+ and ATP. 3. All those kinetic characteristics were substantially altered by treatment with thimerosal.

Evidence for the existence of a monovalent cation-stimulated, magnesium-dependent adenosine triphosphatase activity in the isolated plasma membranes of amoebas of the slime mold dictyostelium discoideum

Evidence for a magnesium-dependent ATPase activity that can be stimulated by Na+ and K+, or equally by Na+ or K+ alone, has been found in the plasma membranes isolated from amoebas of the slime mold Dictyostelium discoideum when the membranes are isolated from cultures grown up to the stationary phase. This ATPase activity is scarcely inhibitable by ouabain or phlorizin, but is very sensitive to low concentrations of azide or thimerosal. When the plasma membranes are isolated from amoebas growing in logarithmic phase, this monovalent cation-stimulated Mg2+-dependent activity is barely detectable.

Specific modification of gastric K+-stimulated ATPase activity by thimerosal

Treatment of hog gastric microsomes with the sulfhydryl reagent, thimerosal (ethylmercurithiosalicylate), produced differential effects on the K+-ATPase and the K+-stimulated p-nitrophenylphosphatase activities. For example, exposure to 2 mM thimerosal for 3 min severely reduced the activity of K+-stimulated ATPase, while K+-p-nitrophenylphosphatase activity was enhanced 2- to 3-fold. Higher concentration of thimerosal, or longer incubation times, also led to inhibition of K+-p-nitrophenylphosphatase. The activated state of p-nitrophenylphosphatase could be sustained by a 20-fold, or greater, dilution of treated membranes, and could be reversed by reduction of membrane SH groups by exogenous thiols. Significant activation of K+-p-nitrophenylphosphatase was not produced by p-chloromercuribenzene sulfonate, p-chloromercuribenzoate or mersalyl; however, ethyl mercuric chloride had qualitatively similar activity effects as thimerosal. Kinetics of K+-p-nitrophenylphosphatase for thimerosal-treated membranes were altered as follows: V increased; Km for p-nitrophenylphosphate unchanged for Ka for K+ increased. ATP, which is a potent inhibitor of K+-p-nitrophenylphosphatase activity in native membranes (KI approximately 200 microM). These data suggest that there are multiple SH groups which differentially influence the gastric K+-stimulated ATPase activity. Defined treatments with thimerosal are interpreted as an uncoupling of the K+-stimulated phosphatase component of the enzyme (for which p-nitrophenylphosphatase is a presumed model reaction). Such differential modifications can be usefully applied to the study of partial reactions of the enzyme and their specific role in the related H+-transport reaction.