Effects of lead on pH and temperature-dependent substrate-activation kinetics of ATPase system and its protection by thiol compounds in rat brain.
Lead (Pb) inhibited the activities of Na(+)-K+ ATPase (IC50 = 2.0 x 10(-6) M), K(+)-Para-Nitrophenyl phosphatase (PNPPase) (IC50 = 3.5 x 10(-6) M) and [3H]-ouabain binding (IC50 = 4.0 x 10(-5) M) in rat brain P2 fraction. A variable temperature or pH significantly elevated the inhibition of Na(+)-K+ ATPase by Pb in buffered acidic, neutral and alkaline pH ranges. Noncompetitive inhibition with respect to activation of Na(+)-K+ ATPase by ATP was indicated by a variation in Vmax values with no significant changes in Km values at any temperature studied. In the presence of Pb, for Na(+)-K+ ATPase at pH 6.5 and 8.5, Vmax was decreased with an increase in Km values suggesting a mixed type of inhibition. Sulfhydryl agents such as dithiothreitol (DTT) and cysteine (Cyst), but not glutathione (GSH) offered varied levels of protection against Pb-inhibition of Na(+)-K+ ATPase at pH 7.5 and 8.5. The present data suggest that inhibition of Na(+)-K+ ATPase by Pb is both temperature and pH-dependent. These results also indicate that Pb inhibited Na(+)-K+ ATPase by interfering with phosphorylation of enzyme molecule and dephosphorylation of the enzyme-phosphoryl complex and exerted an effect similar to that of SH-blocking agents.
Rajanna
B
B
Department of Natural Science, Selma University, AL 36701.
Chetty
C S
CS
Stewart
T C
TC
Rajanna
S
S
D009597:4-Nitrophenylphosphatase / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001921:Brain / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism; D004789:Enzyme Activation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007854:Lead / Q000633:toxicity*; D008297:Male; D010042:Ouabain / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013379:Substrate Specificity; D013438:Sulfhydryl Compounds / Q000494:pharmacology*; D013696:Temperature
D013438:Sulfhydryl Compounds; D007854:Lead; D010042:Ouabain; D000255:Adenosine Triphosphate; D009597:4-Nitrophenylphosphatase; D000254:Sodium-Potassium-Exchanging ATPase