MPO and major depressive disorder: The main objective of the study was to examine whether recurrent major depression (rDD) is accompanied by more profound inflammatory disturbances than the first episode of depression (ED-I), i.e. expression of mRNA and protein levels of four inflammatory enzymes, i.e. manganese superoxide dismutase (MnSOD, SOD-2), myeloperoxidase (MPO), inducible nitric oxide synthase (iNOS, NOS-2), and cyclooxygenase-2 (COX-2).