Mechanism of inhibition of rat brain (Na ⁺‐K ⁺)‐stimulated adenosine triphosphatase reaction by cadmium and methyl mercury

Abstract

The mechanisms of inhibition of rat brain Na+-K+-ATPase by cadmium chloride (CdCl2) and methylmercuric chloride (CH3HgCl) were studied in vitro by assessing the effects of these heavy metals on this enzyme and associated component parameters. Both the heavy metals significantly inhibited the overall Na+-K+-ATPase in a concentration-dependent manner with an estimated median inhibitory concentration (IC-50) of 3.2 X 10(-5) M for CdCl2 and 6 X 10(-6) M for CH3HgCl. Protection of enzyme against heavy metal inhibition by 5 X 10(-5) M to 1 X 10(-4) M dithiothreitol (DTT) and glutathione (GSH) or cysteine (CST) indicates that both monothiols and dithiols have the same ability in regenerating sulfhydryl (-SH) groups or chelating the metals. Inhibition of K+-p-nitrophenyl phosphatase (K+-PNPPase), the component enzyme catalyzing the K+-dependent dephosphorylation in the overall Na+-K+-ATPase reaction by these heavy metals, indicates that the mechanism of inhibition involves binding to this phosphatase. Reversal of K+-PNPPase inhibition by DTT, GSH, and CST suggests sulfhydryl groups as binding sites. Binding of 3H-ouabain, a cardiac glycocide and inhibitor of both phosphorylation and dephosphorylation, to brain fraction was significantly decreased by CH3HgCl, and this inhibition was reversed by the three thiol compounds, suggesting presence of -SH group(s) in the ouabain receptor site. Cadmium chloride failed to inhibit the binding of this receptor, indicating that the mechanics of inhibition of ATPase by CH3HgCl and CdCl2 are different from each other. The results suggest that the critical conformational property of enzyme common to both kinase (E1) and phosphatase (E2) is susceptible to CH3HgCl whereas only phosphatase is sensitive to CdCl2.

Effect of Cadmium in Feed on Organs and Meat Colour of Growing Pigs

“…Substrates such as amino acids and glucose are cotransported down the Na + gradient into the cellular compartment (Na + -dependent transport); the higher Na + /K + -ATPase activity may be indicative of higher metabolite transport into hepatic cells. Heavy-metal binding to sulphydryl groups has often been implicated in both Na + /K + -ATPase inhibition and physiological effects (Rajanna et al, 1983;Ahammadsahib et al, 1987;Bertoni and Sprenkle, 1991). The present results show that 10 mg/kg cadmium leads to a decrease of Na + /K + -ATPase activity in liver (see Table III), which indicates changes in the metabolism of substrates and the distribution of ions and affects weight gain of pigs.…”

Application of metabolic control analysis to the study of toxic effects of copper in muscle glycolysis

“…Pollutants such as heavy metals easily enter living organisms and accumulate in several tissues [5–7]. It is well described that several heavy metals have a high affinity for the free electron pairs of the S‐H groups [5, 8]. Therefore, heavy metals irreversibly inhibit several enzymes of the metabolism at the same time, e.g.…”

Effects of Sequential Exposure to Multiple Concentrations of Methylmercury in the Rat Hippocampal Slice

“…Past experiments indicate that MeHg can interfere with Ca>-mediated synaptic mechanisms and cellular aerobic respiration (Atchison and Narahashi, 1982;Atchison, 1986Atchison, , 1987Atchison, , 1988McKay et al, 1986;Levesque and Atchison, 1988;Kauppinen et al, 1989;Komulainen and Bondy, 1987;Levesque et al, 1992;Ahammadsahib et al, 1987;Ally et al, 1984;Yee and Choi, 1992). Results from presynaptic recordings indicated that 50 M MeHg suppressed both presynaptic and postsynaptic excitability.…”

“…Evidence from studies of neuromuscular junction and cerebrocortical synaptosomes suggests that MeHg interferes with Ca>-mediated synaptic mechanisms via effects on Ca> stores within the nerve terminal ([Ca>] ) (Atchison and Narahashi, 1982;Atchison, 1986Atchison, , 1987Atchison, , 1988McKay et al, 1986;Levesque and Atchison, 1988;Kauppinen et al, 1989;Komulainen and Bondy, 1987;Levesque et al, 1992). Other experiments suggest that MeHg interferes with cellular aerobic respiration (Ahammadsahib et al, 1987;Ally et al, 1984;Yee and Choi, 1992).…”