GLUL and Alzheimer disease: The ammonia hypothesis of AD was generated due to the following observations: increased ammonia levels measured in the plasma from AD patients [124, 125], decreased glutamine synthetase enzyme activity in AD astrocytes to scavenge ammonia [96, 126], increased adenosine deaminase activity in AD brain [127], and increased monoamine oxidase activity in AD brain [128, 129] (the latter two enzymes produce ammonia).