A microsomal iron-porphyrin activator of rat liver tryptophan pyrrolase.

Tryptophan pyrrolase, the enzyme that catalyzes the oxidation of tryptophan to formylkynurenine, has been localized in the 11,000 X g supernatant fraction of mammalian liver homogenates (l), which contains microsomes and cell sap. In the course of the present investigations (Z), it was observed that the isolated microsomal fraction of rat liver is devoid of this enzyme. However, addition of this microsomal fraction was found to stimulate the tryptophan pyrrolase activity of cell sap of rat liver homogenates. Studies on the nature of this activation suggest that the activating principle of microsomes is ferriprotoporphyrin, which may be serving as the prosthetic group of tryptophan pyrrolase.