INHIBITION OF CYTOSOLIC HUMAN FOREBRAIN CHOLINE-ACETYLTRANSFERASE ACTIVITY BY PHOSPHO-L-SERINE - A PHOSPHOMONOESTER THAT ACCUMULATES DURING EARLY STAGES OF ALZHEIMERS-DISEASE

There is no satisfactory explanation for the cholinergic deficit characteristic of Alzheimer's disease. We have performed a series of experiments which demonstrate that (a) an inhibitor of cytosolic human brain choline acetyltransferase is present in the cytosol of Alzheimer brain tissue, (b) human brain cytosolic choline acetyltransferase activity is inhibited by phospho-L-serine in a competitive manner. Cytosol was prepared from human forebrain or amygdala and the K(m) for choline and acetyl CoA of the choline acetyltransferase were 750 muM and 12.5 muM, respectively. Phospho-L-serine was found to be a competitive inhibitor of this enzyme with respect to choline but not with respect to acetyl CoA with a Ki of 750 muM for the human forebrain and 3 mM for human amygdala. These concentrations of phospho-L-serine are present in brain tissue at early stages of Alzheimer's disease. Several other phosphomonoesters and phosphodiesters that are increased in Alzheimer's disease were either less inhibitory or without effect. The addition of heat denatured and non-heat denatured cytosol from Alzheimers forebrain inhibited the choline acetyltransferase activity present in control human brain cytosol. The inhibitory activity of the Alzheimers cytosol was retained in TCA deproteinized samples and removed by dialysis or by alkaline phosphatase treatment. Dialysis of the cytosol increased the choline acetyltransferase activity of 5 of 8 Alzheimer's disease samples from 21 to 118% with p values of <0.025 or <0.001, respectively. These observations provide evidence that an endogenous non-proteinaceous, dialyzable, phosphomonoester, present in Alzheimers brain inhibits the choline acetyltransferase of both control and Alzheimers brain. Therefore, an alteration in membrane phospholipid metabolism could secondarily contribute to the hypocholinergic state observed in Alzheimer's disease.