Whilst the expression/activity of ornithine aminotransferase and ornithine transcarbamylase have not been specifically investigated in experimental and human studies of depressive or bipolar disorder, a recent study has reported that individuals with bipolar disorder had significantly higher plasma concentrations of spermine, a member of the polyamine family that is the end product of a series of enzymatic reactions that include ornithine decarboxylase, compared to healthy controls (879 ± 1021 vs. 174 ± 83 mmol/mL, p = 0.015) [89]. Here, OTC is linked to bipolar disorder.