I don't have the answer you're looking for, but it's possible that
glutathione could be very prevalent in the body and yet produced
sparingly. It may be a type of substance that does not break down very
easily. As an antioxidant, it may take-on free radicals to neutralize
them, but then just as quickly pass them off, and become ready to take
on another one. I have no real basis for claiming that is the case with
glutathione, but I know that most of the body's enzymes work in exactly
that fashion, thus they do not need to be produced in large quantities.