Endogenous generation of sulfur dioxide in rat tissues

While sulfur dioxide (SO₂) has been previously known for its toxicological effects, it is now known to be produced endogenously in mammals from sulfur-containing amino acid l-cysteine. l-cysteine is catalyzed by cysteine dioxygenase (CDO) to l-cysteinesulfinate, which converts to β-sulfinylpyruvate through transamination by aspartate aminotransferase (AAT), and finally spontaneously decomposes to pyruvate and SO₂. The present study explored endogenous SO₂ production, and AAT and CDO distribution in different rat tissue. SO₂ content was highest in stomach, followed by tissues in the right ventricle, left ventricle, cerebral gray matter, pancreas, lung, cerebral white matter, renal medulla, spleen, renal cortex and liver. AAT activity and AAT1 mRNA expression were highest in the left ventricle, while AAT1 protein expression was highest in the right ventricle. AAT2 and CDO mRNA expressions were both highest in liver tissue. AAT2 protein expression was highest in the renal medulla, but CDO protein expression was highest in liver tissue. In all tissues, AAT1 and AAT2 were mainly distributed in the cytoplasm rather than the nucleus. These observed differences among tissues endogenously generating SO₂ and associated enzymes are important in implicating the discovery of SO₂ as a novel endogenous signaling molecule.