Increased activity of oligo-2'',5''-adenylate synthetase in Down''s syndrome and epilepsy.

The level of oligo-2',5'-adenylate synthetase activity is a good marker for the response of cells to interferon. This enzyme can polymerize ATP to form oligonucleotides in the presence of double-stranded RNA, i.e. polyinosinate-cytidylate in vitro. The activity of this enzyme in peripheral blood mononuclear leucocytes was significantly increased in Down's syndrome (P less than 0.01) and epilepsy (P less than 0.01) compared with that in healthy controls, but the increase of activity was not significant in multi-infarct dementia (MID) (P greater than 0.05). Although the patients with Down's syndrome showed higher levels of this enzyme activity than the controls, interferon activity was never detected in the circulation. In addition, the serum of patients with Down's syndrome lacked the capacity to induce this enzyme in NC-37 and FL cells, and furthermore it was shown that the inhibitor of interferon activity was not found in the serum of patients. This discrepancy in Down's syndrome may be the result of the hypersensitivity of cells to interferon. On the contrary, interferon activity (32 IU/ml) was detectable in the circulation of one patient with epilepsy, and the serum of this patient had the capacity to induce this enzyme in NC-37 and FL cells.