Indoleamine 2,3-dioxygenase. Purification and some properties.

Area of Science:

• Biochemistry
• Enzymology

Background:

• Indoleamine 2,3-dioxygenase (IDO) is an enzyme involved in tryptophan metabolism.
• Understanding the biochemical properties of IDO is crucial for its biological roles.

Purpose of the Study:

• To purify and characterize indoleamine 2,3-dioxygenase (IDO) from rabbit small intestine.
• To elucidate the enzyme's molecular weight, prosthetic group, and catalytic activities.

Main Methods:

• Purification using polyacrylamide gel electrophoresis and analytical ultracentrifugation.
• Characterization of molecular weight, amino acid composition, and carbohydrate content.
• Analysis of prosthetic group (heme) and metal content (copper).
• Electron paramagnetic resonance (EPR) spectroscopy of the enzyme's nitric oxide complex.
• Assays for catalytic activity on various tryptophan derivatives and peroxidase/catalase activity.

Main Results:

• Purified IDO is a monomeric protein (41,000 Da) with significant hydrophobic amino acid content and 5% carbohydrate by weight.
• The enzyme contains protoheme IX as a prosthetic group (0.8 mol/mol enzyme) with minimal copper content.
• EPR spectra suggest a nitrogen atom, likely from an imidazole group, as the fifth heme ligand.
• The purified enzyme efficiently catalyzes the oxygenative ring cleavage of D-/L-tryptophan, D-/L-5-hydroxytryptophan, tryptamine, and serotonin.
• IDO demonstrated peroxidase activity but lacked catalase activity.

Conclusions:

• Rabbit small intestine IDO is a heme-containing glycoprotein with specific substrate specificities.
• The enzyme's structure and catalytic mechanism involve heme coordination and oxygenative ring cleavage.
• IDO's peroxidase activity suggests potential roles beyond tryptophan metabolism.