Maes M, Goossens F, Scharpé S, Meltzer HY, D'Hondt P, Cosyns P. Lower serum prolyl endopeptidase enzyme activity in major depression: further evidence that peptidases play a role in the pathophysiology of depression. Biol Psychiatry. 1994 Apr 15 ; 35(8):545-52. Pubmed Abstract