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Microtubule-associated protein T is abnormally hyperphosphorylated and aggregated in affected neurons of Alzheimer disease brain . This hyperphosphorylated -r can be dephosphorylated at some of the abnormal phosphorylated sites by purified protein phosphatase-1, 2A, and 213 in vitro . In the present study, we have developed an assay to measure protein phosphatase activity toward -r-1 sites (Ser' 9'/Ser 212) using the hyperphosphorylated T isolated from Alzheimer disease brain as substrate . Using this assay, we have identified that in normal brain, protein phosphatase-2A and 213 and, to a lesser extent, 1 are involved in the dephosphorylation of r. The Km values of dephosphorylation of the hyperphosphorylated T by protein phosphatase-2A and 2B are similar. The T phosphatase activity is decreased by -30% in brain of Alzheimer disease patients compared with those of age-matched controls . These findings suggest that a defect of protein phosphatase could be the cause of the abnormal hyperphosphorylation of r in Alzheimer disease . Key Words : Microtubule-associated protein T-Protein phosphatase-Protein dephosphorylation-Alzheimer disease .

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