S-adenosylhomocysteine, an amino acid derivative in metabolic pathways, is an intermediate in the synthesis of adenosine and cysteine.

In vitro: Previous study found that cystathionine β-synthase (CBS) deficient yeast could transport significantly more homocysteine into the media than wild-type yeast and had elevated homocysteine and S-adenosylhomocysteine (SAH). Moreover, it was found that SAH at 25 μM inhibited the growth of CBS deficient yeast, whereas had no effect on wild-type yeast. In addition, the measurement results of the intracellular S-adenosylmethionine (SAM) and SAH indicated that CBS deficient yeast had reduced SAM/SAH ratios when compared withy wild-type. Meanwhile, the growth inhibition by SAH in CBS deficient yeast could be reversed by addition of SAM to the media, suggesting that the ratio but not absolute level was critical for cell growth [1].

In vivo: The tissue distribution of SAH was found to be similar in both rat sexes, and was only slightly changed with age. The specific activity of SAH hydrolase exceeded that of methionine adenosyltransferase greatly, and the S-adenosylmethionine concentration was higher than that of S-adenosylhomocysteine in all examined tissues. However, the hepatic S-adenosylmethionine/SAH ratio was dependent on food supply and on the age of the animal. IP administration of methionine led to a profound but transient increase in the liver concentrations of S-adenosylmethionine and SAH [2].

Clinical trial: Up to now, S-adenosylhomocysteine is not applied for clinical usage.

References:
[1] Christopher SA,Melnyk S,James SJ,Kruger WD.? S-adenosylhomocysteine, but not homocysteine, is toxic to yeast lacking cystathionine beta-synthase. Mol Genet Metab.2002 Apr;75(4):335-43.
[2] Eloranta TO.? Tissue distribution of S-adenosylmethionine and S-adenosylhomocysteine in the rat. Effect of age, sex and methionine administration on the metabolism of S-adenosylmethionine, S-adenosylhomocysteine and polyamines. Biochem J.1977 Sep 15;166(3):521-9.