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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:

enter image description here

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  • $\begingroup$ Is English your first language? The statement "It converts accumulated homocysteine to the anti-oxidant glutathione during oxidative stress" has no qualification such as "only" that would restrict the conversion to oxidative stress. It does not preclude that this is the general pathway for synthesis of glutathione for use in any situation where its reducing power is required. And you will find that almost certainly to be the case if you research the pathways of glutathione synthesis, e.g. here. $\endgroup$ – David Mar 13 at 11:20
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    $\begingroup$ I'm voting to close this question as off-topic because it is based on a misunderstanding of a sentence in a paper and thus has no general value for SE Biology. $\endgroup$ – David Mar 13 at 11:22
  • $\begingroup$ I find the language in the paper you quote rather 'loose': "GTP serves as the substrate for the GTP-cyclohydrolase I enzyme (GTPCH) to produce tetrahydrobiopterin (BH4)' [GTPCH does not produce BH4 but converts a purine to a pteridine which is then converted to BH4 in a number of steps]. "which is the co- factor for enzymes producing dopamine, serotonin and NO" [but what about Phe hydroxylase: the conversion of Phe to Tyr is surely relevant.] In addition, glutathione is gamma-ECG and homocysteine is only the source of C (Cys) ? In all enz other than NO syn, BH4 acts as reducing agent. $\endgroup$ – user1136 Mar 13 at 13:40

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