D-Homocysteine-d4 is labelled D-Homocysteine (H591955), a stereoisomer of L-Homocysteine (H591950) which is the hydrolysis product of adenosine (A280400), a purine nucleoside. Metabolic rate of L-Homocysteine is converted back to L-Methionine( M260440) via tetrahydrofolate.