Molecular studies define the primary structure of alpha(1)-antichymotrypsin (ACT) protease inhibitor in Alzheimer's disease brains - Comparison of ACT in hippocampus and liver

An alpha(1)-antichymotrypsin-like serpin has been implicated in Alzheimer's disease (AD) based on immunochemical detection of alpha(1)-antichymotrypsi n (ACT) in amyloid plaques from the hippocampus of AD brains. The presence of neuroendocrine isoforms of ACTs and reported variations in human liver A CT cDNA sequences raise the question of the molecular identity of ACT in br ain. In this study, direct reverse transcription-polymerase chain reaction and cDNA sequencing indicate that the hippocampus ACT possesses the reactiv e site loop that is characteristic of serpins, with Leu as the predicted P1 residue interacting with putative chymotrypsin-like target proteases, The deduced primary sequence of the human hippocampus ACT possesses more than 9 0% homology with reported primary sequences br the human liver ACT. Moreove r, identical ACT primary sequences deduced from the cDNAs were demonstrated in the hippocampus of control and AD brains. Northern blots showed that AC T mRNA expression in hippocampus was 900 times lower than that in liver. Al so, hippocampus and liver ACT proteins demonstrated differential sensitivit ies to deglycosylation. Overall, reverse transcription polymerase chain rea ction combined with cDNA and primary sequence analyses have defined the mol ecular identity of human hippocampus ACT in control and AD brains. The dete rmined reactive site loop domain of hippocampus ACT will allow prediction o f potential target proteases inhibited by ACT in AD.