S-Adenosyl-L-homocysteine (SAH), a potent methyltransferase inhibitor and a substrate of the s-adenosylhomocysteine hydrolase, is an amino acid derivative and an intermediator or modulator of several metabolic pathways, including the activated methyl cycle and cysteine biosynthesis. It was shown, that the plasma SAH might be a novel biomarker for the early clinical identification of cardiovascular disease. In addition, the elevated SAH in Alzheimer's brain was related to markers of disease progression and cognitive impairment.

1,25-dihydroxycalciferol (1,25-dihydroxyvitamin D2 or ercalcitriol) is a metabolite of vitamin D. It exerts antirachitic activity in animals. 1,25-dihydroxycalciferol and its analogs have antineoplastic actions in vitro.

L-alloisoleucine (2S, 3R), a diastereomer of L-isoleucine (2S, 3S), is a normal constituent of human plasma. It was shown, that the plasma L-alloisoleucine above the cutoff value of 5 micromol/L is the most specific and most sensitive diagnostic marker for all forms of maple syrup urine disease (MSUD). The precise mechanism of L-alloisoleucine formation is unclear, but existed suggestions, that R-3-methyl-2-oxopentanoate is an immediate and inevitable byproduct of L-isoleucine transamination and that alloisoleucine is primarily formed via transamination of 3-methyl-2-oxopenanoate in vivo.