Simply put, MAO enzymes are there to neutralise a host of poisons normally present in our diet that would otherwise make us unwell or even kill us. Tyramine is one of the substances commonly found in our diets that is neutralised by MAO. DMT is another (this may be stretching it a bit..) "poison" that is neutralised by MAO enzymes.
Other than protection, normal turnover of other molecules is also another function of MAOs, like the breakdown of monoamines. The latter is a normal part of physiology; for every system in the body where there an active substance doing something, there is a mechanism to quench its action. Degradation of the active substance is one of such quenching strategies and MAO enzymes are involved in the degradation of active monoamines like serotonin and dopamine.
As to what happens to degraded enzymes, that varies. Most are excreted, but some can be used to make other stuff. The latter is unlike as terminally oxidised molecules such as the end-products of MAO enzymes, are (enzymatically and chemically speaking) energetically unfavourable to recycle.
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