Metabolism of S-adenosyl-L-homocysteine in vivo by the rat.

After intravenous injection of S-adenosyl-L-homocysteine3H (homocysteine-labeled) into rats, less than 15% of the radioisotope was incorporated into protein methionine or excreted in the urine as oc-ketobutyrate. The remaining radioisotope was found associated with an unknown keto acid excreted in the urine. Intravenous injection of Sadenosyl-L-homocysteine-35S or S-adenosyl-3H-L-homocysteine (adenosine-labeled) revealed that both the sulfur atom and purine moiety remained associated with this compound. These findings suggest that S-adenosylhomocysteine undergoes direct deamination. This compound was isolated by ion exchange chromatography and precipitated with phosphotungstic acid. After decomposition of the phosphotungstate complex with organic solvents, crystallization was effected from aqueous ethanol. This compound has an absorption maximum at 262 mp, en = 12,300, is ninhydrin-negative, 2,4-dinitrophenylhydrazine-positive, reduces platinic iodide, and reacts with orcinol. Further analytical data including elemental analysis and infrared spectral analysis indicate the chemical structure of this compound to be S-adenosyl-y-thio-cr-ketobutyrate.