Kinetic properties of human liver tryptophan pyrrolase

Human liver tryptophan pyrrolase (TPO) activity exhibited substrate level regulation. TPO showed biphasic activity to tryptophan when low ascorbate was used as an activator. The high affinity form (Km for tryptophan: 0.05 mM) was promoted by low ascorbate and low tryptophan. The low affinity form (Km for tryptophan: 0.4 mM) was induced by high concentrations of tryptophan or ascorbate. Both high and low affinity forms showed the same affinity to oxygen. The high affinity form was also induced by pyrroloquinoline quinone, but this effect was decreased by catalase, suggesting the participation of H2O2.