Comparative studies of the indoleamine dioxygenase-like myoglobin from the abalone Sulculus diversicolor

Inhibition of indoleamine 2,3 dioxygenase activity by H2O2

Indoleamine 2,3-dioxygenase is the first and rate limiting enzyme of the kynurenine pathway of tryptophan metabolism, has potent effects on cell proliferation and mediates antimicrobial, antitumorogenic, and immunosuppressive effects. As a potent cytotoxic effector, the mechanisms of indoleamine 2,3-dioxygenase inhibition deserve greater attention. The work presented here represents the first systematic study exploring the mechanisms by which low levels of hydrogen peroxide (10-100 microM) inhibit indoleamine 2,3-dioxygenase in vitro. Following brief peroxide exposure both enzyme inhibition and structural changes were observed. Loss of catalysis was accompanied by oxidation of several cysteine residues to sulfinic and sulfonic acids, observed by electrospray and MALDI mass spectrometry. Enzyme activity could in part be preserved in the presence of sulfhydryl containing compounds, particularly DTT and methionine. However, these structural alterations did not prevent substrate (l-tryptophan) binding. Some enzyme activity could be recovered in the presence of thioredoxin, indicating that the inhibitory effect of H(2)O(2) is at least partially reversible in vitro. We present evidence that cysteine oxidation represents one mechanism of indoleamine 2,3-dioxygenase inhibition.

Indoleamine 2,3-dioxygenase in cancer: targeting pathological immune tolerance with small-molecule inhibitors

Indoleamine 2,3-dioxygenase (IDO) is an interferon (IFN)-gamma-inducible, extrahepatic enzyme that catalyses the initial and rate-limiting step in the degradation of the essential amino acid tryptophan. Elevated tryptophan catabolism mediated by IDO is associated with a wide variety of human cancers and has historically been thought to be a tumoricidal consequence of IFN-gamma exposure. Evidence of a physiological requirement for IDO activity in protecting the allogeneic fetus from rejection by the maternal immune system has stimulated a radical shift in thinking about the role of IDO in cancer. Evidence now suggests that tumours can exploit IDO-mediated peripheral tolerance to promote immune escape. This review summarises key studies that implicate IDO as an important mediator of peripheral immune tolerance as well as the development of a promising new anticancer modality that incorporates the use of IDO inhibitors. The second part focuses on the current state of development of IDO inhibitory compounds as potential pharmaceutical agents.

Do molluscs possess indoleamine 2,3-dioxygenase?

Indoleamine 2,3-dioxygenase (IDO)-like myoglobin (Mb) was discovered in 1989 in the buccal mass of the abalone Sulculus diversicolor, and it has since been isolated from several archaegastropods. The amino acid sequences and genomic structures of IDO-like Mbs show significant homology with those of mammalian IDOs, suggesting that they have evolved from a common ancestral gene. However, details of the evolutionary relationships between them remain unknown. Here, we isolated a novel multicopy gene from Sulculus named molluscan IDO-like protein (MIP). The amino acid sequences of MIPs show the highest homology (about 60% identity) with Sulculus IDO-like Mb, and their exon/intron structures are also highly homologous. However, MIPs are mainly expressed in the gut whereas IDO-like Mb was found only in the buccal mass, suggesting that MIPs are not simply isoforms of IDO-like Mb. A bacterial expression study showed that MIP is a heme-binding protein, and that His335 is the proximal ligand of heme. Although we could not detect IDO activity using a recombinant glutathione S-transferase (GST)-MIP fusion protein in the present study, MIP should have some function other than that of an oxygen carrier like myoglobin, and it might in fact be molluscan IDO.

Comparison of the sequences of Turbo and Sulculus indoleamine dioxygenase-like myoglobin genes

Some archaeogastropodic molluscs, including Sulculus and Turbo, contain an unusual approximately 40 kDa myoglobin in their buccal masses. This myoglobin can bind oxygen reversibly, but has a lower oxygen affinity than vertebrate and invertebrate myoglobins. Amino acid sequences clearly show that Sulculus and Turbo myoglobins evolved not from the globin gene but from the gene for indoleamine dioxygenase (IDO), a tryptophan-degrading enzyme. The Turbo myoglobin gene has been determined to consist of 14 exons and 13 introns. Compared with the known Sulculus IDO-like myoglobin gene, all splice junctions except two are conserved exactly between the two genes. The exon/intron organization of these myoglobin genes is also highly homologous with human IDO (ten exon/nine intron structure); splice junctions of six introns were exactly conserved among the three genes, suggesting that these introns have been conserved for at least 600 million years. To look for putative IDO genes in Turbo or Sulculus, we re-examined the genomic DNA fragments amplified by PCR in full detail, and found intron 2 in two distinct Sulculus fragments (A and B). Fragment A with a 576 bp intron corresponded exactly to the myoglobin gene of Sulculus. On the other hand, fragment B, containing a 239 bp intron, differed significantly from fragment A in nucleotide and translated amino acid sequences. Detailed sequence comparison suggests that fragment B may be derived from a putative IDO gene of Sulculus.

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.

Amino acid sequence, spectral, oxygen-binding, and autoxidation properties of indoleamine dioxygenase-like myoglobin from the gastropod mollusc Turbo cornutus

Myoglobin was isolated from the radular muscle of the archaeogastropod mollusc Turbo cornutus (Turbinidae). This myoglobin is a monomer carrying one protoheme group; the molecular mass was estimated by SDS-PAGE to be about 40 kDa, 2.5 times larger than that of usual myoglobin. The cDNA-derived amino acid sequence of 375 residues was determined, of which 327 residues were identified directly by chemical sequencing of internal peptides. The amino acid sequence of Turbo myoglobin showed no significant homology with any other usual 16-kDa globins, but showed 36% identity with the myoglobin from Sulculus diversicolor (Haliotiidae) and 27% identity with human indoleamine 2,3-dioxygenase, a tryptophan-degrading enzyme containing heme. Thus, the Turbo myoglobin can be counted among the myoglobins which evolved from the same ancestor as that of indoleamine 2,3-dioxygenase. The absorbance ratio of gamma to CT maximum (gamma/CT) of Turbo metmyoglobin was 17.8, indicating that this myoglobin probably possesses a histidine residue near the sixth coordination position of heme iron. The Turbo myoglobin binds oxygen reversibly. Its oxygen equilibrium properties are similar to those of Sulculus myoglobin, giving P50 = 3.5 mm Hg at pH 7.4 and 20 degrees C. The pH dependence of autoxidation of Turbo oxymyoglobin was quite different from that of mammalian myoglobin, suggesting a unique protein folding around the heme cavity of Turbo myoglobin. A kinetic analysis of autoxidation indicates that the amino acid residue with pKa = 5.4 is involved in the reaction. The autoxidation reaction was enhanced markedly at pH 7.6, but not at pH 5.5 and 6.3 in the presence of tryptophan. We suggest that a noncatalytic binding site for tryptophan, in which several dissociation groups with pKa > or = 7.6 are involved, remains in Turbo myoglobin as a relic of molecular evolution.

A myoglobin evolved from indoleamine 2,3-dioxygenase, a tryptophan-degrading enzyme

The distribution, isolation, spectral and oxygen-binding properties, stability of ferrous state (autoxidation), amino acid sequence and gene structure of indoleamine 2,3-dioxygenase (IDO)-like myoglobins are summarized, and their evolution is discussed. Although it has long been thought that all hemoglobins and myoglobins have evolved from a common ancestral gene encoding a 14-16 kDa polypeptide, the discovery of IDO-like myoglobin from several gastropod molluscs clearly indicates that there was an alternative pathway for myoglobin evolution.

The cDNA-derived amino acid sequence of indoleamine dioxygenase like-myoglobin from the gastropod mollusc Omphalius pfeifferi

Myoglobin was isolated from the radular muscle of the archaegastropod mollusc Omphalius pfeifferi (Trochidae). The molecular mass was estimated by SDS-PAGE to be about 40 kDa, 2.5 times larger than that of usual myoglobin. The cDNA for Omphalius myoglobin was amplified by polymerase chain reaction, and the cDNA-derived amino acid sequence of 375 residues was determined, of which 73 residues were identified directly by the chemical sequencing of internal peptides. The amino acid sequence of Omphalius myoglobin showed no significant homology with any other usual 16-kDa globins, but showed 84% and 36% identities with indoleamine dioxygenase-like myoglobins from Battilus (Turbinidae) and Sulculus (Haliotiidae), respectively. It also shows significant homology (26% identity) with human indoleamine 2,3-dioxygenase, a tryptophan-degrading enzyme containing heme. The distribution of indoleamine dioxygenase-like myoglobins suggests that they must have arisen exclusively along the specified lineage including the three families Haliotiidae, Turbinidae, and Trochidae of Archaegastropoda in molluscan evolution.