The 26 remaining compounds decreased in dementia patients (P < 0.05) (Fig. 1 B–E). They consisted of four subgroups (B to E), having distinct characteristics. Group B compounds include ET and five other trimethyl-ammonium compounds. To our knowledge, except for ET (17), these are all not previously reported as dementia markers, probably because they are enriched in RBCs and scarcely studied in connection with dementia. ET is an antioxidant, a thiourea derivative of trimethyl-histidine. Two other ET-related, but less abundant, compounds, S-methyl-ET and trimethyl-histidine (hercynine), also declined strikingly in blood of dementia patients.
Group C compounds also decreased in dementia patients. These included ATP, NADP+ (oxidoreductive coenzyme), GSSG (redox compound), pantothenate (vitamin B5), S-adenosyl-methionine (SAM; methyl donor) (21), and gluconate (zinc carrier) (22). They are related to energy, redox reactions, methylation, and metal ions. Group C compounds were all enriched in RBCs, and four of the six are not previously reported as dementia markers. Two of them (SAM and GSSG) were previously shown to be AD-related (21, 23).
Trimethyl-tryptophan (hypaphorine), trimethyl-phenylalanine, glycerophosphocholine, dodecanoyl-carnitine (24), and trimethyl-tyrosine, all of which contain trimethyl-ammonium ions, also declined. The extent of reduction for trimethyl-tryptophan (0.10) and trimethyl-tyrosine (0.08) was striking. These reductions may be due to instability or reduced synthesis, or to reduced import in dementia patients. Of the nine compounds that contain a trimethyl-ammonium moiety, six of them that contain ET are enriched in RBCs and classified as group B compounds (Table 1).
Twelve group D metabolites (Table 1) are enriched in blood plasma and seven of them were previously reported to be dementia or AD markers. They include standard amino acids, glutamine (19, 25), phenylalanine (19, 26), tyrosine (19), histidine (19, 25), methionine, and tryptophan (regular amino acids) (18, 19), a pyrimidine nucleoside, uridine (27), and organic acids, 2-hydroxybutyrate (lipid-degradation product) and keto(iso)leucine (keto acid). Caffeine is a known dementia marker (28). Dimethyl-xanthine is a metabolite of caffeine. These greatly declined in dementia and are highly correlated with and isolated from other metabolites (see below) so they are designated as group E. Consistency of group D plasma metabolites as dementia markers but not group B and C RBC metabolites validated the method of searching dementia markers that we employed in the present study. The great majority of metabolites enriched in RBCs were not identified in the previous studies.
Nine trimethylated ammonium compounds were diminished in dementia patients.
Of nine trimethylated compounds that decreased in dementia, six are enriched in RBCs (SI Appendix, Fig. S2). Three of them (betaine, glycerophosphocholine, and dodecanoyl-carnitine) are present in plasma and are synthesized in the human body, whereas the other six, containing an aromatic moiety, are derived from food (29, 30). Most strikingly, six of these nine compounds are highly abundant (H, H-M, or H-L) in plasma and RBCs in healthy subjects, and are highly correlated so that their behavior may be highly coordinated. Hence, the sharp declines of these amphipathic compounds (possessing both hydrophilic and lipophilic properties and forming the basis of lipid polymorphism) in blood of dementia patients may strongly affect the physicochemical properties of neuronal systems.
Seven metabolites increased in dementia.
Interestingly, the seven metabolites of group A, comprising three nucleosides and four amino acid derivatives, increased in dementia (Fig. 1A). None was highly abundant and none was enriched in RBCs, and their increase in dementia occurred in plasma. Indoxyl-sulfate, kynurenine, and quinolinic acid (18) are involved in tryptophan metabolism and possibly act as excitatory toxins in the brain (31, 32), while N6-acetyl-lysine is implicated in histone and nonhistone protein modification (33). Pseudouridine, adenosine (19), and dimethyl-guanosine are degradation products of RNAs present in urine and are thought to be oxidized (34, 35). Increases of these metabolites in dementia are of great interest, as some are reportedly toxic in the central nervous system (CNS) and may lead to impairment of the brain (36⇓–38).