In this study, we assayed the activities of three main thiamine metabolizing enzymes: thiamine diphosphatase (TDPase), thiamine monophosphatase (TMPase), and thiamine pyrophosphokinase (TPK) in AD patients and control subjects with matched age, gender, and educational background, to elucidate the underlying mechanism for the reduction of TDP level in AD. Here, ACP3 is linked to Alzheimer disease.