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Abstract: Effect of neonatal hypothyroidism on kinetic properties of Ca2+-ATPase from rat brain synaptic plasma membranes and microsomes were examined. Neonatal hypothyroidism resulted in significant decrease in the enzyme activity in both the membrane systems. The synaptic membranes in control group displayed presence of one kinetic component whereas a low affinity component became evident in the hypothyroid group. In the microsomes, both control as well as hypothyroid groups showed presence of two kinetic components with the latter group showing two-fold increase in the K m. The Ca2+ binding characteristics were generally unaltered in the enzyme from both the membrane systems. Our results suggest that impairment in the Ca2+-ATPase activity together with altered kinetic properties could be one of the underlying biochemical mechanisms leading to CNS dysfunction as a consequence of thyroid hormone deprivation during critical stages of brain development. [Copyright &y& Elsevier]