EFFECTS OF TRYPTOPHAN LOAD ON AMINO-ACID-METABOLISM IN TYPE-1 DIABETIC-PATIENTS

Tolerance to an oral tryptophan load (50 mg/kg body weight) was evalua ted in a group of 15 insulin-dependent diabetic patients of both sexes in poor metabolic control. Tryptophan was measured fluorometrically, and the plasma levels of the other physiological amino acids were dete rmined by HPLC. The ratio of the plasma concentration of each large ne utral amino acid (LNAA) to the sum of the others was calculated to ser ve as an index for the competitive transport of these amino acids into the brain. The results show that post-loading plasma tryptophan level s in diabetic patients increased less than in healthy controls, sugges ting enhanced liver catabolism of this amino acid (as reported for dia betic animals). Small changes were observed in the post-loading plasma concentrations of other amino acids. Therefore, the increment in the tryptophan/LNAA ratio in controls (basal, 0.12+/-0.01; 120 min after t he load, 0.89+/-0.04; 240 min, 0.51+/-0.03) was greatly attenuated in diabetic patients (basal, 0.11+/-0.01, NS; 120 min, 0.46+/-0.04, p < 0 .01; 240 min, 0.31+/-0.04, p < 0.01). Post-loading excursions in some other ratios were slightly larger in control than diabetic subjects. T hese differences, which may occur to a lesser extent after a protein-r ich meal, could modify the availability of precursor amino acids to th e brain for synthesis of neurotransmitters. Thus, as happens in certai n animal species, an impairment of the post-absorptive accumulation of tryptophan and serotonin in the brain may occur in diabetic patients as a result of altered metabolic disposal of tryptophan.