Table 5 The significance of inflammation enzymes in the aetiology of depression

MnSOD

First line of defence against damage caused as a result of excessive production of the superoxide anion radical by the mitochondrion [9]. Experiments of recent years indicate that MnSOD protects cells of the CA1 region of the hippocampus against apoptosis [33]. Additionally, MnSOD also protects cells of the CA3 region of the hippocampus against generation of free radicals during excessive glutamatergic activity [34].

MPO

Protein of an immuno-inflammatory response and prooxidative enzyme, which may be a marker of inflammation as well as prooxidative processes in the patients with depression [35]. MPO is a heme enzyme whose expression is observed in both cells of the brain and immune cells, which indicates its crucial role in the regulation of inflammatory processes and the formation of oxidative stress [36]. MPO activation leads to the production of hypochlorous acid and other toxic oxidising agents [37]. According to Vaccarino et al. [23], a raised level of MPO may be considered a biomarker of inflammation in the course of rDD.

COX-2

COX-2 inhibition restricts the formation of reactive oxygen species [38]. COX-2 also affects an increase of activity in the hypothalamus – pituitary gland – adrenal glands axis, and an increase of the level of pro-inflammatory cytokines [39].

iNOS

Plays a decisive role not only in numerous biological processes, but also in the regulation of cognitive and emotional functions, which may be significant in the aetiology of anxiety and depressive disorders (most of all through participation in neuromodulation, neurotransmission and synaptic plasticity) [40,41]. Nitrous oxide – being one of free radicals – takes part in the regulation of oxidative stress [12]. NO plays a neuroprotective role in physiological conditions. Nevertheless, if produced in excess, or if cells are under the influence of oxidative stress, the action of nitrous oxide becomes harmful; then, it undergoes oxidation and reduction processes producing toxic compounds referred to as reactive nitrogen species (RNS), which cause cell damage. Both NO and RNS play a role in the pathogenesis and development of many neurodegenerative diseases [42].