@DebInAustralia asked me to forward this here (thanks Deb!), Life Extension's response in relation to the concerns that PQQ depletes glutathione:
Thank you for your recent correspondence.
We are sorry that you are dealing with severe tinnitus and hyperacusis, and would be happy to provide information on PQQ to ease your concerns.
PQQ (pyrroloquinoline quinone) is an antioxidant in plants, and can be found, in small amounts, in foods like green peppers, papaya, and kiwi. As you know, one of the most interesting things about PQQ is that it supports the growth of new mitochondria. Additionally, PQQ protects the mitochondrial DNA from damage, enhances the production of ATP in the body, protects the nerves from damage, and quenches free radicals.
Mitochondria are organelles that reside in our cells. They make an energy compound called ATP (adenosine tri-phosphate) which fuels the metabolic reactions in our bodies. Whenever there is an increase in energy production, as is the case when a person exercises or takes PQQ, you can get an increase in oxidative species (free radicals), as they are a byproduct of enhanced mitochondrial activity. The mitochondria produces free radicals, such as superoxide anion, hydrogen peroxide and peroxynitrite, please see the link below for the abstract to an article that discusses mitochondrial free radical production and cell signaling. If the mitochondria are dysfunctional, cells are dysfunctional and are more likely to die.
http://www.ncbi.nlm.nih.gov/pubmed/15051313
Glutathione, an essential nutrient for humans, is synthesized in the body from the amino acids L-cysteine, L-glutamic acid, and glycine. The sulfhydryl group (SH) of cysteine serves as a proton donor and is responsible for its biological activity. Cysteine is the rate-limiting factor in cellular glutathione biosynthesis, since this amino acid is relatively rare in foods. Glutathione works as an electron donor, donating electrons to certain proteins, and converting them into cysteines. This converts glutathione to the oxidized form, also known as glutathione disulfide (GSSG) or L-glutathione.
Glutamate, or glutamic acid, is mostly metabolized in the brain and is an essential neurotransmitter which is necessary for normal, healthy brain function. It is also one third of glutathione, and adequate amounts of glutamic acid are necessary for optimal glutathione levels. However, excessive glutamate stimulation of brain cells, or excitotoxicity, is a major factor in the development of long-term neurodegenerative disorders, stroke, and schizophrenia. Glutamate-induced excitotoxicity triggers undesired programmed cell death, or apoptosis. Fortunately, not only can PQQ help protect against the damaging effects of excitotoxicity, it can also help prevent it from occurring to begin with. For more information, please see the link below:
http://www.lifeextension.com/Magazine/2016/4/How-PQQ-Protects-the-Brain/Page-01
In conclusion, when PQQ quenches free radicals it becomes a reduced form of PQQ called PQQH2. It is recycled back to PQQ using glutathione. In order to maintain optimal levels of glutathione, we suggest taking antioxidants and supplements that support glutathione levels, such as vitamin C, N-Acetyl-L-Cysteine (NAC) and R-lipoic acid. Below, we have provided links to the aforementioned products.
http://www.lifeextension.com/Vitamins-Supplements/item01541/Glutathione-Cysteine-C
http://www.lifeextension.com/Vitamins-Supplements/item01208/Super-R-Lipoic-Acid
http://www.lifeextension.com/Vitamins-Supplements/item01534/N-Acetyl-L-Cysteine
http://www.lifeextension.com/Vitamins-Supplements/item01717/Fast-C-with-Dihydroquercetin