Chellu S. Chetty

Effects of cadmium and mercury on Na+-K+ ATPase and uptake of 3H-dopamine in rat brain synaptosomes

Effects in vivo of cadmium (Cd), mercury (Hg) and methylmercury (CH3Hg) on Na(+)-K+ ATPase and uptake of 3H-dopamine (DA) in rat brain synaptosomes were studied. These heavy metals significantly inhibited the Na(+)-K+ ATPase activity in a dose-dependent manner. Similarly, inhibition of DA uptake by synaptosomes was also observed in rats treated with these metals. Intraperitoneal route of metal administration was found to be more effective than per os treatment. Mercuric compounds compared to Cd elicited a higher inhibition of Na(+)-K+ ATPase and DA uptake in rat brain synaptosomes.

Effects of chronic treatment with cadmium on ATPases, uptake of catecholamines, and lipid peroxidation in rat brain synaptosomes

The chronic effects of cadmium on specific activities of oligomycin-sensitive (OS) and -insensitive Mg2+ ATPase, Na(+)-K+ ATPase, lipid peroxidation, and uptake of catecholamines in brain synaptosomes of rats treated daily for 2 or 4 months were studied. Cadmium significantly decreased the specific activities of OS-Mg2+ ATPase, Na(+)-K+ ATPase, and uptake of [3H]-dopamine (3H-DA) and [3H]norepinephrine (3H-NE) and increased the lipid peroxidation.

Effect of mercuric chloride on the kinetics of cationic and substrate activation of the rat brain microsomal ATPase system

Mercuric chloride (HgCl2), a neurotoxic compound, inhibited the adenosine triphosphatase (ATPase) system in a concentration-dependent manner. Hydrolysis of ATP was linear with time with or without HgCl2 in the reaction mixtures. Higher inhibition of (Na(+)-K+)ATPase activity by HgCl2 was observed in alkaline (8.0 to 9.0) pH and at lower temperatures (17 to 32 degrees). Activation energy values were increased slightly in the presence of HgCl2. Activation of (Na(+)-K+)ATPase by ATP in the presence of HgCl2 showed a decrease in Vmax from 15.29 to 5.0 mumol of inorganic phosphate (Pi)/mg protein/hr with no change in Km. Similarly, activation of K(+)-stimulated p-nitrophenyl phosphatase (K(+)-PNPPase) in the presence of HgCl2 showed a decrease in Vmax from 3.26 to 1.35 mumols of p-nitrophenol (PNP)/mg protein/hr with no change in Km. K(+)-activation kinetic studies indicated that HgCl2 decreased Vmax from 14.01 to 4.30 mumols Pi/mg protein/hr in the case of (Na(+)-K+)ATPase and from 3.45 to 2.40 mumols PNP/mg protein/hr in the case of K(+)-PNPPase with no changes in Km. Na(+)-activation of (Na(+)-K+)ATPase in the presence of HgCl2 showed a decrease in Vmax from 11.06 to 3.23 mumols Pi/mg protein/hr and an increase in Km from 1.06 to 2.08 mM. Preincubation of microsomes with sulfhydryl (SH) agents dithiothreitol, cysteine and glutathione protected HgCl2-inhibition of (Na(+)-K+)ATPase. The data suggest that HgCl2 inhibited (Na(+)-K+)ATPase by interfering with the dephosphorylation of the enzyme-phosphoryl complex.

The effects of cadmium in vitro on adenosine triphosphatase system and protection by thiol reagents in rat brain microsomes

Cadmium (Cd) inhibited the activities of Na+-K+ ATPase (IC50=5.0×10−5 M), K+-p-nitrophenyl phosphatase (PNPPase) (IC50=4.0×10−5 M) and 3H-ouabain binding (IC50=7.5×10−5 M) in rat brain microsomes. Monothiols (cysteine but not glutathione and D-pencillamine) and dithiols (dimercaprol, dimercaptosuccinic acid and dithiothreitol) offered varied levels of protection against Cd-inhibition of Na+-K+ ATPase. Protection of Na+-K+ ATPase by these sulfhydryl (SH) agents was higher at 7.5 as compared to 8.5 pH. The present data suggest that Cd-inhibited Na+-K+ ATPase, by interfering with phosphorylation of enzyme molecule and dephosphorylation of the enzyme-phosphoryl complex and exerts a similar effect to that of SH-blocking agents.

Inhibition of rat brain microsomal Na+/K(+)-ATPase and ouabain binding by mercuric chloride.

This study concerned the effects of mercuric chloride on Na+/K(+)-ATPase and [3H]ouabain binding in rat brain microsomes in vitro. The data showed that HgCl2 inhibited Na+/K(+)-ATPase effectively at micromolar concentrations. The degree of inhibition was decreased with increases in enzyme concentration and incubation time. Variations in the ionic strength of Na+ and K+ did not alter the percent inhibition of Na+/K(+)-ATPase activity by HgCl2. Repeated washings partially restored enzyme activity. The binding of [3H]ouabain to microsomal membranes was inhibited by HgCl2 in a concentration-dependent manner. Cumulative inhibition studies with HgCl2 and ouabain indicated that these inhibitors did not act concurrently and independently on Na+/K(+)-ATPase.

Effects of lead on kinetics of 3H-dopamine uptake by rat brain synaptosomes☆

The effects of lead on 3H-dopamine (3H-DA) uptake in rat brain synaptosomes were studied. Pb inhibited 3H-DA uptake in vitro and in vivo in a concentration-dependent manner. Altered pH versus 3H-DA uptake demonstrated comparable inhibition in buffered acidic, neutral, and alkaline pH ranges, and higher inhibition was observed in neutral pH. Kinetic studies of ATP activation of 3H-DA uptake indicated competitive inhibition by Pb. The inhibition of 3H-DA uptake followed the Na+ concentrations. The results indicate that the Pb inhibition of 3H-DA uptake is pH, ATP, and Na+ dependent.

In vitro interaction of heavy metals with ouabain receptors in rat brain microsomes

This study investigates the influence of heavy metals on ouabain-binding in presence of thiol (sulfhydryl) compounds. The data on in vitro effects of mercury (Hg), lead (Pb) and cadmium (Cd) showed significant inhibition of 3H-ouabain binding to microsomal membrane in a concentration-dependent manner. Maximum inhibition of 3H-ouabain binding was observed at 1 microM for Hg and 100 microM each for Pb and Cd. Preincubation with monothiol (L-cysteine or glutathione) or dithiol (dithiothreitol) protected inhibition of 3H-ouabain binding to the membranes by Hg or Pb. Dithiol but not monothiols partially protected Cd-inhibition. The present data confirm that the heavy metals interact with ouabain receptors in a manner similar to SH-blocking agents and protection of metal-inhibited 3H-ouabain binding by thiol compounds is metal specific.

Ionic imbalances in the CNS of neonate rat during induced benthiocarb stress

The effect of benthiocarb, an organocarbamate herbicide, on the ionic profiles of the developing central nervous system of rat has been studied. The ionic profiles altered by benthiocarb toxicity indicate possible perturbations in the electric activity of neurons, in oxygen consumption, and in the ATPase system, as well as disruption in the movement of ions across ionic pumps and impairment of synaptic transmission.

Effects in vitro of mercury on rat brain Mg++-ATPase

Mercuric chloride (Hg) in micromolar concentrations inhibited Mg(++)-dependent ATPase activity in rat brain microsomes. Inhibition was higher in oligomycin-sensitive (O.S.) than oligomycin-insensitive (O.I.) Mg(++)-ATPase. Hydrolysis of ATP with 15 and 50 micrograms of microsomal protein for 45 min without and with (2.10(-7M) Hg showed linear rates for 15-20 min. Altered pH vs activity demonstrated comparable inhibitions by Hg in buffered (neutral greater than acidic greater than basic) pH ranges. Inhibition of enzyme activity by Hg was found to be greater at 37 degrees C than at lower temperatures suggesting positive correlation trend. An uncompetitive inhibition with respect to the activation of Mg(++)-ATPase, O.S. Mg(++)-ATPase and O.I. Mg++ ATPase by ATP was indicated by a decrease in apparent Vmax and Km. Mg(++)-activation kinetic studies indicated that Hg causes uncompetitive inhibition of Mg(++)-ATPase and O.I. Mg(++)-ATPase and mixed inhibition of O.S. Mg(++)-ATPase. Inhibition was partially restored by repeated washings. These results indicate that the inhibition of microsomal Mg(++)-ATPase by Hg was pH, temperature, enzyme and Mg++ concentration dependent. Additionally, the data also suggest that O.S. compared to O.I. Mg(++)-ATPase is more sensitive to Hg toxicity.

Substrate-affinity relationship of succinate dehydrogenase in brain of albino rat during dieldrin toxicity

Substrate kinetics of succinate dehydrogenase (EC 1.3.99.1) were studied in the brain of control and dieldrin-treated albino rats. Dieldrin inhibited the enzyme activity to various degrees and also altered the Michaelis-Menten constant (Km) of the enzyme. Activation energy values (delta E) were increased, suggesting decreased maximal velocity Vmax, and the high Km obtained for the enzyme also revealed decreased enzyme-substrate affinity and masking of active sites by dieldrin.