Tryptophan metabolism in relation to amino acid alterations during typhoid fever

With the onset of fever in volunteers exposed to virulent Salmonella typhii as part of a vaccine evaluation study, urinary excretion of kynurenine, acetyl-kynurenine and o-amino-hippurate significantly increased by N-methylnicotinamide decreased. Serum tryptophan concentration at that time was 116% of control in contrast to total serum amino acid values at 83% of control. A 3 gram oral tryptophan load given during illness further increased serum tryptophan concentration to 186% of control and enhanced excretion of the aforementioned catabolites. In addition, urinary anthranilate glucuronide, kynurenate and xanthurenate rose significantly; 3-hydroxykynurenine increased only slightly and N-methyl-nicotinamide was unaltered. Thus, though a tryptophan load aids in detection of alterations in tryptophan catabolism via the kynurenine pathway as a whole, it also distorts the pattern of metabolites detected, seemingly by causing more kynurenine to be shunted to the mitochondria to be acted upon by kynurenine 3-hydroxylase and kynurenine aminotransferase. In unloaded patients the increased urinary excretion of tryptophan metabolites during typhoid fever merely reflect the enhanced flux of amino acids from muscle to liver, akin to that noted in other infections, in excess of that needed for the synthesis of serum proteins or for other metabolic pathways.