Figure 1.

Endogenous generation of sulfur dioxide. Firstly, a sulfur-containing amino acid is metabolized to L-cysteine that is then oxidated into L-cysteinesulfinate under the role of CDO. L-Cysteinesulfinate, an analogue of L-asparaginic acid, can be transaminated into β-sulfinylpyruvate under the role of AAT, which is spontaneously decomposed into pyruvate and SO₂. In vivo, SO₂ can produce HSO₃ ⁻/SO₃ ²⁻ (molar ratio of 1 : 3) after combining with water, which can be oxidated into SO₄ ²⁻ by sulfite oxidase and then excreted through the kidneys. On the other hand, L-cysteinesulfinate can be also decarboxylated into CO₂ and hypotaurine under the role of CSD. A large majority of hypotaurine can be further oxidized into taurine. SO₂: sulfur dioxide; CDO: cysteine dioxygenase; CSD: cysteinesulfinate decarboxylase; AAT: aspartate aminotransferase.