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Here is the quote: "Because BH4 is not present in saturating concentrations under basal conditions, it is crucial in regulating TH activity." This is from Fundamental Neuroscience, page 121.

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    $\begingroup$ Have you tried a dictionary to understand what "saturating" means and what "basal" means? And it says "saturating concentrations" in the quote, not "conditions" like in your title. It's fine to ask about scientific terminology but you should show some attempt to understand the meaning in English in your question first. $\endgroup$ – Bryan Krause Jan 12 at 16:57
  • $\begingroup$ Sorry about the title--I messed up there. I think I get what basal conditions means, but I don't understand what they refer to in "saturating concentrations". Are they saying that because BH4 is not present in the original saturating concentrations, when the body is no longer in the basal condition, it has to regulate TH because it has built up during the basal condition? $\endgroup$ – Vidha Yadav Ganji Jan 12 at 17:25
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I will rewrite the sentence another way that preserves what a believe is the intended meaning:

There is little enough BH4 under normal conditions such that levels of BH4 are important for TH activity.

BH4 is an enzyme cofactor; in this context they're talking about it used with tyrosine hydroxylase (TH) to make a precursor (L-DOPA) to dopamine.

If there was so much BH4 available at basal (normal) conditions that adding more doesn't speed up TH activity, we can say it's saturated. In that context, BH4 changes don't really regulate TH activity (unless they are huge changes that bring it well below the 'saturated' level).

The sentence you quoted is saying this isn't the case. BH4 isn't present at saturated levels, so you can influence TH activity (and therefore synthesis of L-DOPA) by influencing BH4 concentration.

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