Table 1.

Glycated, oxidized, and nitrated amino acid metabolites.

Metabolite class	Urinary metabolite	Comment
Glycation		Early-stage glycation adduct [25]. Formed from glucose nonenzymatically and reporting on exposure to increased glucose concentration. Repaired intracellularly by fructosamine 3-phosphokinase [14]. Free adduct absorbed after digestion of food proteins [69].
		A major quantitative arginine-derived AGE formed from methylglyoxal. Linked to increased fasting and postprandial glucose exposure, insulin resistance, and cardiovascular disease [9, 10, 47, 70]. Free adduct absorbed after digestion of food proteins [9].
		A major quantitative lysine-derived AGE—particularly in food. Formed by the oxidative degradation of FL from and other sources. Free adduct absorbed after digestion of food proteins [71].
		Major quantitative crosslink formed in protein glycation [17].
		Low level pentose sugar-derived glycation crosslink and intense fluorophore. Considered to reflect pentose phosphate pathway activity [72].
		Glucose-derived AGE formed at high temperatures of culinary processing; originating only from food [23, 24].
Oxidation		Oxidative crosslink formed spontaneously in oxidative stress and enzymatically by dual oxidase (DUOX) [16, 25].
		Major “protein carbonyl” formed by the oxidative deguanidylation of arginine and oxidative ring-opening of proline [73].
Nitration		Major proteolysis product of proteins endogenously nitrated by peroxynitrite and nitryl chloride [25, 74].
Transglutaminase-linked crosslink		Major protein crosslink formed enzymatically.
Branched-chain amino acids (BCAAs)		Essential amino acids previously linked to the development of T2DM and CKD [21, 22].

Molecular structures showing ionisation under physiological conditions.