Tryptophan loading test is done to detect deficiency of:
TPP
PLP
Biotin
Folate
Tryptophan loading test detects xanthurenic acid in urine which indicates deficiency of Vitamin B6 i.e. PLP (Pyridoxal phosphate). In the absence of Vitamin B6 the excess of 3-OH-Kynurenine is conveed to Xanthurenic acid after given a load of tryptophan (see Fig below) Figure: Tryptophan catabolism pathway Note:- Histidine load test detects FIGLU (Form-Imino Glutamate) in urine which indicates deficiency of Vitamin B9 Deficiency Common metabolite Specific metabolite Vitamin B6 Homocysteine Xanthurenic acid Vitamin B9 Homocysteine FIGLU Vitamin B12 Homocysteine L-methyl malonic acid
Tryptophan metabolism detail