Indoleamine 2,3-dioxygenase is a heme enzyme that catalyzes the oxidative degradation of L-Trp and other indoleamines. We have used resonance Raman spectroscopy to characterize the heme environment of purified recombinant human indoleamine 2,3-dioxygenase (hIDO). In the absence of L-Trp, the spectrum of the Fe

No abstract

L-Tryptophan is the least abundant essential amino acid in humans. Indoleamine 2,3-dioxgyenase (IDO) is a cytosolic heme protein which, together with the hepatic enzyme tryptophan 2,3-dioxygenase, catalyzes the first and rate-limiting step in the major pathway of tryptophan metabolism, the kynurenine pathway. The physiological role of IDO is not fully understood but is of great interest, because IDO is widely distributed in human tissues, can be up-regulated via cytokines such as interferon-␥, and can thereby modulate the levels of tryptophan, which is vital for cell growth. To identify which amino acid residues are important in substrate or heme binding in IDO, site-directed mutagenesis of conserved residues in the IDO gene was undertaken. Because it had been proposed that a histidine residue might be the proximal heme ligand in IDO, mutation to alanine of the three highly conserved histidines Indoleamine 2,3-dioxygenase (EC 1.13.11.17) is a cytosolic monomeric hemoprotein that catalyzes the first step in tryptophan catabolism by the kynurenine pathway (1). In humans, the kynurenine pathway catabolizes over 90% of tryptophan (2, 3), the first step of which is the oxidative cleavage of the tryptophan 2-3 double bond resulting in the production of NЈ-formyl-kynurenine, which undergoes further conversions yielding a number of metabolites, some of which are neurotoxic. One primary role of IDO 1 induction, which is up-regulated under a number of pathological conditions such as viral (4 -7), bacterial (1,8), and protozoan infections (9), appears to be suppression of the growth of pathogens by the removal of the essential amino acid tryptophan. Deprivation of tryptophan following IDO induction has been related to reductions in tumor growth (10 -12) and also appears to be a mechanism by which an allogenic fetus is prevented from being rejected (13).Increased levels of the kynurenine pathway metabolites quinolinic acid and 3-hydroxykynurenine have been observed in a number of neurological disorders. Quinolinic acid, which is an N-methyl-D-aspartate agonist, may be involved in the pathogenesis of AIDS dementia complex (14 -16), cerebral malaria, and Huntington's disease (17,18). 3-Hydroxykynurenine readily oxidizes in air with the formation of H 2 O 2 , and the resulting oxidation products bind proteins (19 -21). With the induction of IDO being implicated in several disease states, its suppression may be of potential pharmacological importance. To date there are no potent therapeutic inhibitors of IDO; however, an understanding of the structure of IDO will aid in the development of such compounds.Heme proteins, such as IDO, are the most extensively studied of all metalloproteins. All heme proteins carry iron coordinated to protoporphyrin IX. The four coordination sites provided by the porphyrin ring are not sufficient to satisfy the coordination requirements of the iron. Normally, groups from the heme-binding protein occupy the remaining coordination sites. The common coordinating functional groups are the imidazole nitr...

Increased kynurenine pathway metabolism has been implicated in the etiology of AIDS dementia complex (ADC).

Increased kynurenine pathway metabolism has been implicated in the aetiology of the AIDS dementia complex (ADC). The rate limiting enzyme for this pathway is indoleamine 2,3-dioxygenase (IDO). We tested the efficacy of different strains of HIV-1 (HIV1-BaL, HIV1-JRFL and HIV1-631) to induce IDO in cultured human monocyte-derived macrophages (MDM). A significant increase in both IDO protein and kynurenine synthesis was observed after 48 h in MDM infected with the brain derived HIV-1 isolates, laboratory adapted (LA) HIV1-JRFL, and primary isolate HIV1-631. In contrast, almost no kynurenine production or IDO protein was evident in MDM infected with the high replicating macrophage tropic LA strain, HIV1-BaL. The induction of IDO and kynurenine synthesis by HIV1-JRFL and HIV1-631 declined to baseline levels by day-8 post-infection. Together, these results indicate that only selected strains of HIV-1 are capable of inducing IDO synthesis and subsequent oxidative tryptophan catabolism in MDM.