Concentrations Of Large Neutral Amino Acids Research Articles

The treatment of obesity by carbohydrate deprivation suppresses plasma tryptophan and its ratio to other large neutral amino acids.

We measured plasma concentrations of tryptophan (Trp) and the other large neutral amino acids (LNAA) in 6 control and 7 obese subjects before and after they consumed a low-carbohydrate "protein-sparing modified fast" (PSMF) diet; LNAA levels in control subjects were also assessed after supplemental oral Trp. Consumption of the PSMF diet by non-obese subjects, or obesity per se, caused major reductions in the ratio of the plasma Trp concentration to the summed plasma concentrations of the other LNAA (i.e., the "plasma Trp ratio"), and may thus have diminished brain serotonin synthesis. Administration of even 2 g of supplemental Trp did not elevate the plasma Trp ratio beyond the normal range observed previously in subjects consuming carbohydrate-containing meals.

Effect of various oral glucose doses on plasma neutral amino acid levels

Six healthy, nonobese, fasting subjects each received, on different days 0, 6, 12.5, 25, or 50 g of glucose (Glucola) in a total volume of 100 ml. Blood was taken at intervals and assayed for plasma levels of the branched-chain amino acids (valine, isoleucine and leucine); the other major large neutral amino acids (LNAA) (methionine, phenylalanine, tyrosine and tryptophan); and, in some cases, insulin and glucose. Insulin levels were significantly elevated 30 min after consumption of 12.5, 25, or 50 g of glucose, and were higher after the 50 g dose than after 12.5 g. Changes in plasma glucose concentrations were small and did not correlate with glucose dose. Mean percent reductions of LNAA tended to exhibit dose-dependence, most clearly observed after 120 min. In some subjects as little as 6 g of glucose transiently decreased LNAA concentrations. Branched-chain amino acids were most sensitive, decreasing by 35%–41% after 50 g of glucose. Plasma tryptophan concentrations fell only by 23%, hence the ratio of plasma tryptophan to other plasma LNAA (which affects brain serotonin synthesis) increased significantly.

Developmental changes in brain indoles, serum tryptophan and other serum neutral amino acids in the rat

The rates at which brain neurons synthesize and release serotonin depend in part on brain tryptophan concentrations; these, in turn, vary directly with serum (or plasma) tryptophan, and inversely with the serum concentrations of other large neutral amino acids (LNAA). Concentrations of serum tryptophan, LNAA and brain indoles were examined in samples drawn at noontime from rats aged 0–59 days. Developmental changes in serum tryptophan largely paralleled those in the tryptophan/LNAA ratio, and brain tryptophan concentrations. Brain serotonin and 5-hydroxyindole acetic acid (5-HIAA) levels also increased postnatally; the changes in 5-HIAA tended to parallel those in brain tryptophan while those in serotonin did not.