Human Indoleamine 2,3-Dioxygenase 1 Is an Efficient Mammalian Nitrite Reductase

Abstract

The heme enzyme indoleamine 2,3-dioxygenase-1 (IDO1) catalyzes the first reaction of l-tryptophan oxidation along the kynurenine pathway. IDO1 is a central immunoregulatory enzyme with important implications for inflammation, infectious disease, autoimmune disorders, and cancer. Here we demonstrate that IDO1 is a mammalian nitrite reductase capable of chemically reducing nitrite to nitric oxide (NO) under hypoxia. Ultraviolet–visible absorption and resonance Raman spectroscopy showed that incubation of dithionite-reduced, ferrous–IDO1 protein (FeII–IDO1) with nitrite under anaerobic conditions resulted in the time-dependent formation of an FeII–nitrosyl IDO1 species, which was inhibited by substrate l-tryptophan, dependent on the concentration of nitrite or IDO1, and independent of the concentration of the reductant, dithionite. The bimolecular rate constant for IDO1 nitrite reductase activity was determined as 5.4 M–1 s–1 (pH 7.4, 23 °C), which was comparable to that measured for myoglobin (3.6 M–1 s–1; pH 7.4, 23 °C), an efficient and biologically important mammalian heme-based nitrite reductase. IDO1 nitrite reductase activity was pH-dependent but differed with myoglobin in that it showed a reduced proton dependency at pH >7. Electron paramagnetic resonance studies measuring NO production showed that the conventional IDO1 dioxygenase reducing cofactors, ascorbate and methylene blue, enhanced IDO1’s nitrite reductase activity and the time- and IDO1 concentration-dependent release of NO in a manner inhibited by l-tryptophan or the IDO inhibitor 1-methyl-l-tryptophan. These data identify IDO1 as an efficient mammalian nitrite reductase that is capable of generating NO under anaerobic conditions. IDO1’s nitrite reductase activity may have important implications for the enzyme’s biological actions when expressed within hypoxic tissues.

The Noncanonical Pathway for In Vivo Nitric Oxide Generation: The Nitrate-Nitrite-Nitric Oxide Pathway

“…Similar to Mb and Hb, the heme in IDO1 is in a pentacoordinate state, which may account for the increased rates, as discussed above. A contrasting feature, in comparison with other heme-containing nitrite reductases, is that the nitrite reductase activity of IDO1 is enhanced in alkaline conditions (pH 8), which Lim and colleagues speculate may be due to the close proximity of histidine residues potentially capable of donating a proton (Lim et al, 2019). These initial observations have been made in recombinant human IDO1, and further studies are required to assess the in vivo relevance of these findings.…”

“…Indoleamine 2,3-Dioxygenase 1. Very recently, it has been demonstrated that indoleamine 2,3-dioxygenase 1 (IDO1), a cytosolic heme enzyme involved in the initial and rate-limiting step of L-tryptophan metabolism in the kynurenine pathway, is capable of reducing nitrite to •NO under anaerobic conditions (Lim et al, 2019). Similar to Mb and Hb, the heme in IDO1 is in a pentacoordinate state, which may account for the increased rates, as discussed above.…”

Melatonin, Its Metabolites and Their Interference with Reactive Nitrogen Compounds

“…Nitrite reductase activities have been described for several metalloproteins and metal-containing enzymes known for different functions. These comprise hemoglobin [ 37 , 47 ], myoglobin [ 48 ], cystathionine β-synthase [ 49 ] and indoleamine 2,3-dioxygenase 1 [ 48 ]. Even though the latter enzyme is known for its melatonin-oxidizing property, there is no reason to assume a functional cross-connection in this place.…”

A Complex Metabolic Network Confers Immunosuppressive Functions to Myeloid-Derived Suppressor Cells (MDSCs) within the Tumour Microenvironment

“…Therefore, a drug that blocks the signalling activity of IDO1 in combination with its catalytic site may improve IDO1-targeting immunotherapy. Recently, it was discovered that under anaerobic conditions IDO1 is an efficient nitrite reductase capable of generating NO [161]. This study provides new insights into the role of IDO1 as a novel enzyme source of NO in tumours.…”