Homocysteine, an amino acid synthesized intracellularly by removal of the N-methyl group from the essential amino acid methionine. High plasma level of homocysteine is called hyperhomocysteinemia is a clinical biomarker for increased risk of cardiovascular disease, thromboembolic diseases, and myocardial infarction. It was shown, that hyperhomocysteinemia could be an independent risk factor for dementia and Alzheimer's disease. The falling of homocysteine concentrations in response to increasing B-group-vitamin status, have the hope that mental decline, or Alzheimer's disease, could be prevented by dietary modification or food fortification. Besides, homocysteine can behave as an anti-oxidant agent by increasing the antioxidant capacity of the tumor and endothelial cells.

Androsterone, a neurosteroid, acts as a positive allosteric modulator of GABA(A) receptors, that can cross into the brain and could have effects on brain function. It was discovered, that the association of beta subunits with alpha subunits GABA(A) receptor affects the sensitivity of glycine receptors to androsterone. In spite of that, androsteron is considered as an inactive metabolite of testosterone. In addition, was studied that androsterone possessed anticonvulsant properties. Although of low potency, the androsterone was present in high abundance and was able to represent endogenous modulator of seizure susceptibility.

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.

Neopterin is a byproduct of the tetrahydrobiopterin (BH4) biosynthetic pathway, which requires Mg2+, Zn2+, and NADPH as cofactors. Tetrahydrobiopterin is an obligatory cofactor for phenylalanine, tyrosine, tryptophan hydroxylases and alkylglycerol monooxygenase, and for all isoforms of nitric oxide synthase (NOS). BH4 is synthesized by multiple metabolic routes, namely the de novo, salvage and recycling pathways. The de novo via generates BH4 from guanosine triphosphate (GTP) by the concert action of guanosine triphosphate Cyclohydrolase I (GTPCH), 6-pyruvoyl Tetrahydropterin synthase (PTPS) and sepiapterin reductase. GTPCH catalyzes the conversion of GTP to 7,8-dihydroneopterin triphosphate. Then, Alkaline Phosphatases removes the phosphates to generate, 8-dihydroneopterin, which is further converted to Neopterin by non-enzymatic oxidation. Neopterin is a recognized biomarker for immune system activation. IFN-g, which is released from activated Th1 cells during the initiation of the immunological cellular response, is one of the main stimuli for neopterin formation. The source of neopterin in the central nervous system (CNS) is not well understood. The evidence available in the literature has suggested that neopterin crosses the blood-brain barrier, therefore the CSF levels may reflect the serum or plasma neopterin concentrations. Cell culture studies strongly suggest that neopterin is not an inert compound, but a cytoprotective molecule synthesized and secreted by nerve cells as a response to damage or inflammation.