Demonstration of endogenous inhibitors of monoamine oxidase in dog cerebrospinal fluid.

Cerebrospinal fluid (CSF) from dogs competitively inhibited A-form MAO, but was non-competitive with B-form MAO. Heat treatment of CSF (90 degrees C, 20 min) had no effect on the inhibition. Digestion with trypsin and chymotrypsin reduced the MAO inhibitory activity. After ultrafiltration of the CSF through a membrane to remove substances of greater than 5,000 M.W., significant inhibitory activity persisted. These results suggest that CSF contains endogenous substances that act like MAO inhibitor to inhibit A and B-form MAO, and these substances are peptides of less than 5,000 M.W.