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.

Glycodeoxycholic acid (GDCA), a bile acid, is formed in the liver from chenodeoxycholate and glycine. GDCA was one of twenty-five plasma metabolites, which were significantly different among asthma and healthy controls. Besides, GDCA was elevated in patients with acetaminophen-induced acute liver failure (AALF) and was significantly increased in non-surviving AALF patients compared with survivors. Thus GDCA level could serve as a prognostic biomarker for AALF patients.

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.