Concentrations reported for indolic microbial metabolites of tryptophan in human and rodent brain, cerebrospinal fluid, plasma, saliva and feces were compiled and discussed. A systematic review of the literature was accomplished by key word searches of Pubmed, Google Scholar and the Human Metabolome Data Base (HMDB), and by searching bibliographies of identified publications including prior reviews. The review was prompted by the increasing appreciation of the physiological importance of the indolic compounds in human health and disease. The compounds included were indoleacetic acid (IAA), indole propionic acid (IPA), indoleacrylic acid (IACR), indolelactic acid (ILA) indolepyruvic acid (IPY), indoleacetaldehyde (IAALD), indolealdehyde (IALD), tryptamine (TAM), indole (IND) and skatole (SKT). The undertaking aimed to vet and compare existing reports, to resolve apparent discrepancies, to draw biological inferences from the consideration of multiple analytes across sample types, to survey the analytical methodologies used, and to point out areas in need of greater attention.
Keywords: Brain; Cerebrospinal fluid; Indolic metabolites; Plasma; Saliva; Tryptophan.
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