Effects of Lead on pH and Temperature-Dependent Substrate-Activation Kinetics of ATPase System and its Protection by Thiol Compounds in Rat Brain

摘要

Lead (Pb) inhibited the activities of Na^+-K^+ ATPase (IC_(50)=2.0×10^(-6) M), K^+-Para-Nitrophenyl phosphatase (PNPPase) (IC_(50)=3.5×10(-6) M) and [~3H]-ouabain binding (IC_(50)= 4.0×10^(-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 V_(max) 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, V_(max) 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 pHdependent. These