From the paper titled "The basis for folinic acid treatment in neuro-psychiatric disorders" by Ramaekers et al., 2016:
On the left side of the figure, the purine metabolite GTP serves as the substrate for the GTP-cyclohydrolase I enzyme (GTPCH) to produce tetrahydrobiopterin (BH4), which is the co- factor for enzymes producing dopamine, serotonin and NO. At the upper right of the figure the trans-sulfuration pathway is shown. It converts accumulated homocysteine to the anti-oxidant glutathione during oxidative stress.
Is this so? Is it true that this pathway is only active when the cell is under oxidative stress conditions?
Or maybe the authors meant to say that the pathway converts homocysteine to glutathione, which is later used to counteract oxidative stress?
Here's the figure from the paper: