Tryptophan and indolic
tryptophan metabolites were measured in the serum of normal subjects and
chronic renal failure patients. The
tryptophan free/total ratio was 0.07 in the normal serum, but the other
tryptophan metabolite values were below detectable levels. The free/total ratio was 0.66 in patients with
chronic renal failure, and both increase of free
tryptophan and decrease of total
tryptophan were observed. At the same time, 5-hydroxyindoleacetic
acid,
indole acetic acid, 5-hydroxytryptophol, and
N-acetyltryptophan also increased in the serum of
chronic renal failure patients, existing not only in free form but in
protein-bound form. In the serum samples, following
hemodialysis the indolic
tryptophan metabolites showed a tendency to decrease, and the
tryptophan free/total ratio of 0.42 a approached the normal value. When indolic
tryptophan metabolites were added to normal serum in vitro, the free
tryptophan level rose, and competition for binding to
albumin between
tryptophan and indolic
tryptophan metabolites was observed. These findings suggested that the cause of the decrease in
protein-bound
tryptophan in uremic serum may be an accumulation of indolic
tryptophan metabolites which complete for binding to
albumin in
uremia.