Sulfur is naturally found in foods in the amino acids cysteine and methionine and as sulfate in water.
In the body sulfur compounds play an important metabolic role in sulfur-iron clusters, which are found in the respiratory chain or bound to heme as heme thiolate, a cofactor for vitamin D activation and cortisol metabolism.
Cysteine can produce hydrogen sulfide (H2S, contains one sulfur and two hydrogen atoms) when it is broken down to pyruvate. Cysteine is first converted to 3-mercaptopyruvate, which releases hydrogen sulfide in this reaction:
3-mercaptopyruvate + reduced thioredoxin -> pyruvate + hydrogen sulfide + oxidized thioredoxin (1)
Hydrogen sulfide can stimulate the mitochondrial energy metabolism, because it contains two hydrogen atoms and can transfer these to coenzyme Q10, forming sulfide and ubiquinol (QH2). This increases the function of the respiratory chain (2).
Hydrogen sulfide and 3-mercaptopyruvate have been shown to stimulate energy production and raise the activity of the respiratory chain in low concentrations, while they had an inhibitory effect in high concentrations (3).
Hydrogen sulfide also has beneficial effects on blood vessels by promoting their growth and relaxation (4).
Thioredoxin helps form hydrogen sulfide from 3-mercaptopyruvate. Thioredoxin is a small protein that contains two cysteine-groups. The cysteine residues can donate the hydrogen of their sulfur-hydrogen (/-SH)-group and form a disulfide bond (S-S) and are involved in reactions like that. The disulfide bond in oxidized thioredoxin can be regenerated back to reduced thioredoxin by thioredoxin reductase. This enzyme needs FAD and NADPH as cofactors.
PQQ might play a role for thioredoxin, PQQ-deficiency has been shown to affect the transcription of thioredoxin-related genes negatively in animal trials (5). The positive effect of PQQ on thioredoxin action is not quite sure though, as it also inhibits thioredoxin regeneration by thioredoxin reductase and increases thioredoxin reductase-dependent recycling of quinones (6). Potential quinones could be coenzyme Q10 or tocopherol quinone, a special form of vitamin E. PQQ can have adverse effects on the metabolism by being able to act as an antioxidant and as a prooxidant and I will look into its effects further.