Microbial profile of the vitreous aspirates in culture proven exogenous endophthalmitis: A 10-year retrospective study

PURPOSE

To describe the microbiological profile and clinical outcome in the eyes with culture-proven exogenous endophthalmitis.

METHODS

A retrospective analysis of 495 eyes diagnosed as exogenous endophthalmitis was performed over a period of 10 years. In all, aseptically collected aqueous and vitreous aspirates were cultured for bacteria and fungus using standard microbiological techniques. Gram-stain and KOH preparation of the specimens were also performed. The antibiotic susceptibility testing for bacterial isolates was performed by Kirby-Bauer disk diffusion method. The treatment was modified according to the antibiotic sensitivity profile. The final clinical ocular condition was divided into improved, stable or deteriorated.

RESULTS

Of 148 culture-proven endophthalmitis eyes, 137 (92.57%) were referred from elsewhere, and 11 (7.43%) belonged to our institute. Aetiologically, 76 (51.35%) eyes were post-cataract surgery, 61 (41.22%) were post-traumatic, 5 (3.38%) eyes post-intravitreal anti-vascular endothelial growth factor injection, 5 associated with corneal diseases and 1 bleb-related endophthalmitis. In 31 (20.95%) eyes, primary intravitreal antibiotics were given outside. The cultures revealed monomicrobial growth in 92.57% (n = 137) and polymicrobial growth in 7.43% (n = 11). Among the bacteria (n = 121, 81.76%), Pseudomonas species dominated overall (n = 32, 27.11%) and post-operative (n = 26, 38.23%) endophthalmitis group. Staphylococcus epidermidis (n = 14, 28%) was prominent in post-traumatic endophthalmitis group. Ninety-two percent (n = 108 isolates) of bacteria were sensitive to vancomycin. In 78 (52.7%) eyes, the clinical ocular condition improved or remained stable while deteriorated in 51 (34.46%).

CONCLUSION

A bacterial predominance was observed among causative organisms of exogenous endophthalmitis with Pseudomonas species being the most common. The appropriate surgical intervention improved or stabilised the visual acuity in nearly 50% eyes.

ArsP: a methylarsenite efflux permease

Trivalent organoarsenic compounds are far more toxic than either pentavalent organoarsenicals or inorganic arsenite. Many microbes methylate inorganic arsenite (As(III)) to more toxic and carcinogenic methylarsenite (MAs(III)). Additionally, monosodium methylarsenate (MSMA or MAs(V)) has been used widely as an herbicide and is reduced by microbial communities to MAs(III). Roxarsone (3-nitro-4-hydroxybenzenearsonic acid) is a pentavalent aromatic arsenical that is used as antimicrobial growth promoter for poultry and swine, and its active form is the trivalent species Rox(III). A bacterial permease, ArsP, from Campylobacter jejuni, was recently shown to confer resistance to roxarsone. In this study, C. jejuni arsP was expressed in Escherichia coli and shown to confer resistance to MAs(III) and Rox(III) but not to inorganic As(III) or pentavalent organoarsenicals. Cells of E. coli expressing arsP did not accumulate trivalent organoarsenicals. Everted membrane vesicles from those cells accumulated MAs(III) > Rox(III) with energy supplied by NADH oxidation, reflecting efflux from cells. The vesicles did not transport As(III), MAs(V) or pentavalent roxarsone. Mutation or modification of the two conserved cysteine residues resulted in loss of transport activity, suggesting that they play a role in ArsP function. Thus, ArsP is the first identified efflux system specific for trivalent organoarsenicals.

ArsH is an organoarsenical oxidase that confers resistance to trivalent forms of the herbicide monosodium methylarsenate and the poultry growth promoter roxarsone

Environmental organoarsenicals are produced by microorganisms and are introduced anthropogenically as herbicides and antimicrobial growth promoters for poultry and swine. Nearly every prokaryote has an ars (arsenic resistance) operon, and some have an arsH gene encoding an atypical flavodoxin. The role of ArsH in arsenic resistance has been unclear. Here we demonstrate that ArsH is an organoarsenical oxidase that detoxifies trivalent methylated and aromatic arsenicals by oxidation to pentavalent species. Escherichia coli, which does not have an arsH gene, is very sensitive to the trivalent forms of the herbicide monosodium methylarsenate [MSMA or MAs(V)] and antimicrobial growth promoter roxarsone [Rox(V)], as well as to phenylarsenite [PhAs(III), also called phenylarsine oxide or PAO]. Pseudomonas putida has two chromosomally encoded arsH genes and is highly resistant to the trivalent forms of these organoarsenicals. A derivative of P. putida with both arsH genes deleted is sensitive to MAs(III), PhAs(III) or Rox(III). P. putida arsH expressed in E. coli conferred resistance to each trivalent organoarsenical. Cells expressing PpArsH oxidized the trivalent organoarsenicals. PpArsH was purified, and the enzyme in vitro similarly oxidized the trivalent organoarsenicals. These results suggest that ArsH catalyzes a novel biotransformation that confers resistance to environmental methylated and aromatic arsenicals.

Species-specific antimonial sensitivity in Leishmania is driven by post-transcriptional regulation of AQP1

Leishmania is a digenetic protozoan parasite causing leishmaniasis in humans. The different clinical forms of leishmaniasis are caused by more than twenty species of Leishmania that are transmitted by nearly thirty species of phlebotomine sand flies. Pentavalent antimonials (such as Pentostam or Glucantime) are the first line drugs for treating leishmaniasis. Recent studies suggest that pentavalent antimony (Sb(V)) acts as a pro-drug, which is converted to the more active trivalent form (Sb(III)). However, sensitivity to trivalent antimony varies among different Leishmania species. In general, Leishmania species causing cutaneous leishmaniasis (CL) are more sensitive to Sb(III) than the species responsible for visceral leishmaniasis (VL). Leishmania aquaglyceroporin (AQP1) facilitates the adventitious passage of antimonite down a concentration gradient. In this study, we show that Leishmania species causing CL accumulate more antimonite, and therefore exhibit higher sensitivity to antimonials, than the species responsible for VL. This species-specific differential sensitivity to antimonite is directly proportional to the expression levels of AQP1 mRNA. We show that the stability of AQP1 mRNA in different Leishmania species is regulated by their respective 3’-untranslated regions. The differential regulation of AQP1 mRNA explains the distinct antimonial sensitivity of each species.

The degree of response to antimonial drugs varies widely between species and even among strains of the same species of the protozoan parasite Leishmania. However, the molecular mechanism(s) is unknown. In this study, we show that Leishmania aquaglyceroporin AQP1 drives this species-specific antimonial resistance. Aquaglyceroporins are channel proteins that facilitate the passage of small uncharged molecules, such as glycerol and water, across the biological membranes. AQP1 helps the parasite cope with the osmotic challenges it faces during its life cycle. Additionally, AQP1 is an adventitious facilitator of antimonite, the active form of pentavalent antimonial drugs. We show that AQP1 expression level is species-specific, and less AQP1 in visceral species compared to the cutaneous species results in increased resistance to antimonials. We also demonstrate that the 3’-untranslated regions (3’-UTR) of the AQP1 mRNA is a major determining factor of species-specific regulation of AQP1. Along with water homeostasis, aquaglyceroporins are also involved in directed cell migration. The variable levels of AQP1 in different Leishmania species may enable them to find their appropriate niches in vertebrate hosts and cope with the species-specific osmotic challenges during their life cycles.

Mutations in the ArsA ATPase that restore interaction with the ArsD metallochaperone

The ArsA ATPase is the catalytic subunit of the ArsAB As(III) efflux pump. It receives trivalent As(III) from the intracellular metallochaperone ArsD. The interaction of ArsA and ArsD allows for resistance to As(III) at environmental concentrations. A quadruple mutant in the arsD gene encoding a K2A/K37A/K62A/K104A ArsD is unable to interact with ArsA. An error-prone mutagenesis approach was used to generate random mutations in the arsA gene that restored interaction with the quadruple arsD mutant in yeast two-hybrid assays. A number of arsA genes with multiple mutations were isolated. These were analyzed in more detail by separation into single arsA mutants. Three such mutants encoding Q56R, F120I and D137V ArsA were able to restore interaction with the quadruple ArsD mutant in yeast two-hybrid assays. Each of the three single ArsA mutants also interacted with wild type ArsD. Only the Q56R ArsA derivative exhibited significant metalloid-stimulated ATPase activity in vitro. Purified Q56R ArsA was stimulated by wild type ArsD and to a lesser degree by the quadruple ArsD derivative. The F120I and D137V ArsAs did not show metalloid-stimulated ATPase activity. Structural models generated by in silico docking suggest that an electrostatic interface favors reversible interaction between ArsA and ArsD. We predict that mutations in ArsA propagate changes in hydrogen bonding and salt bridges to the ArsA-ArsD interface that affect their interactions.

Aquaglyceroporins: generalized metalloid channels

BACKGROUND

Aquaporins (AQPs), members of a superfamily of transmembrane channel proteins, are ubiquitous in all domains of life. They fall into a number of branches that can be functionally categorized into two major sub-groups: i) orthodox aquaporins, which are water-specific channels, and ii) aquaglyceroporins, which allow the transport of water, non-polar solutes, such as urea or glycerol, the reactive oxygen species hydrogen peroxide, and gases such as ammonia, carbon dioxide and nitric oxide and, as described in this review, metalloids.

SCOPE OF REVIEW

This review summarizes the key findings that AQP channels conduct bidirectional movement of metalloids into and out of cells.

MAJOR CONCLUSIONS

As(OH)3 and Sb(OH)3 behave as inorganic molecular mimics of glycerol, a property that allows their passage through AQP channels. Plant AQPs also allow the passage of boron and silicon as their hydroxyacids, boric acid (B(OH)3) and orthosilicic acid (Si(OH)4), respectively. Genetic analysis suggests that germanic acid (GeO2) is also a substrate. While As(III), Sb(III) and Ge(IV) are toxic metalloids, borate (B(III)) and silicate (Si(IV)) are essential elements in higher plants.

GENERAL SIGNIFICANCE

The uptake of environmental metalloids by aquaporins provides an understanding of (i) how toxic elements such as arsenic enter the food chain; (ii) the delivery of arsenic and antimony containing drugs in the treatment of certain forms of leukemia and chemotherapy of diseases caused by pathogenic protozoa; and (iii) the possibility that food plants such as rice could be made safer by genetically modifying them to exclude arsenic while still accumulating boron and silicon. This article is part of a Special Issue entitled Aquaporins.

Modulation of Leishmania major aquaglyceroporin activity by a mitogen-activated protein kinase

Leishmania major aquaglyceroporin (LmjAQP1) adventitiously facilitates the uptake of antimonite [Sb(III)], an active form of Pentostam® or Glucantime®, which are the first line of defence against all forms of leishmaniasis. The present paper shows that LmjAQP1 activity is modulated by the mitogen-activated protein kinase, LmjMPK2. Leishmania parasites coexpressing LmjAQP1 and LmjMPK2 show increased Sb(III) uptake and increased Sb(III) sensitivity. When subjected to a hypo-osmotic stress, these cells show faster volume recovery than cells expressing LmjAQP1 alone. LmjAQP1 is phosphorylated in vivo at Thr-197 and this phosphorylation requires LmjMPK2 activity. Lys-42 of LmjMPK2 is critical for its kinase activity. Cells expressing altered T197A LmjAQP1 or K42A LmjMPK2 showed decreased Sb(III) influx and a slower volume recovery than cells expressing wild-type proteins. Phosphorylation of LmjAQP1 led to a decrease in its turnover rate affecting LmjAQP1 activity. Although LmjAQP1 is localized to the flagellum of promastigotes, upon phosphorylation, it is relocalized to the entire surface of the parasite. Leishmania mexicana promastigotes with an MPK2 deletion showed reduced Sb(III) uptake and slower volume recovery than wild-type cells. This is the first report where a parasite aquaglyceroporin activity is post-translationally modulated by a mitogen-activated protein kinase.

The role of alanine 163 in solute permeability of Leishmania major aquaglyceroporin LmAQP1

Leishmania major aquaglyceroporin LmAQP1 allows adventitious passage of antimonite, an activated form of the drug Pentostam, which is used as the first line treatment for leishmaniasis. The extracellular C-loop of an aquaglyceroporin confers substrate specificity. Alteration of Glu125 to serine in the Plasmodium falciparum aquaglyceroporin PfAQP has been shown to selectively affect water but not glycerol permeability. The C-loop of LmAQP1 is twelve residues longer than PfAQP, and Ala163 is at an equivalent position as Glu125 of PfAQP. The role of Ala163 in LmAQP1 solute permeability was investigated. Alteration of Ala163 to serine or threonine did not significantly affect conduction of solutes. However, alteration to aspartate, glutamate, and glutamine blocked passage of water, glycerol, and other organic solutes. While LmAQP1 is a mercurial insensitive water channel, mutation of the adjacent threonine (Thr164) to cysteine led to inhibition of water passage by Hg(2+). This inhibition could be reversed upon addition of β-mercaptoethanol. These data suggest that, unlike Glu125 (PfAQP), Ala163 is not involved in stabilization of the C-loop and selective solute permeability. Ala163 is located near the pore mouth of the channel, and replacement of Ala163 by bulkier residue sterically hinders the passage of solutes. Alteration of Ala163 to serine or threonine affected metalloid uptake in the order, wild-type>A163S>A163T. Metalloid conduction was near completely blocked when Ala163 was mutagenized to aspartate, glutamate, or glutamine. Mutations such as A163S and A163T that reduced the permeability to antimonite, without a significant loss in water or solute conductivity raises the possibility that, subtle changes in the side chain of the amino acid residue in position 163 of LmAQP1 may play a role in drug resistance.

Biochemical characterization of a novel ArsA ATPase complex from Alkaliphilus metalliredigens QYMF

The two putative ars operons in Alkaliphilus metalliredigens QYMF are distinctive in that the arsA gene is split in halves, amarsA1 and amarsA2, and, acr3 but not an arsB gene coexists with arsA. Heterologous expression of one of the A. metalliredigensars operons (ars1) conferred arsenite but not antimonite resistance to DeltaarsEscherichia coli. Only the co-expressed AmArsA1 and AmArsA2 displayed arsenite or antimonite stimulated ATPase activity. The results show that AmArsA1-AmArsA2 interaction is needed to form the functional ArsA ATPase. This novel AmArsA1-AmArsA2 complex may provide insight in how it participates with Acr3 in arsenite detoxification.

Adventitious arsenate reductase activity of the catalytic domain of the human Cdc25B and Cdc25C phosphatases

A number of eukaryotic enzymes that function as arsenate reductases are homologues of the catalytic domain of the human Cdc25 phosphatase. For example, the Leishmania major enzyme LmACR2 is both a phosphatase and an arsenate reductase, and its structure bears similarity to the structure of the catalytic domain of human Cdc25 phosphatase. These reductases contain an active site C-X(5)-R signature motif, where C is the catalytic cysteine, the five X residues form a phosphate binding loop, and R is a highly conserved arginine, which is also present in human Cdc25 phosphatases. We therefore investigated the possibility that the three human Cdc25 isoforms might have adventitious arsenate reductase activity. The sequences for the catalytic domains of Cdc25A, -B, and -C were cloned individually into a prokaryotic expression vector, and their gene products were purified from a bacterial host using nickel affinity chromatography. While each of the three Cdc25 catalytic domains exhibited phosphatase activity, arsenate reductase activity was observed only with Cdc25B and -C. These two enzymes reduced inorganic arsenate but not methylated pentavalent arsenicals. Alteration of either the cysteine and arginine residues of the Cys-X(5)-Arg motif led to the loss of both reductase and phosphatase activities. Our observations suggest that Cdc25B and -C may adventitiously reduce arsenate to the more toxic arsenite and may also provide a framework for identifying other human protein tyrosine phosphatases containing the active site Cys-X(5)-Arg loop that might moonlight as arsenate reductases.

Role of signature lysines in the deviant walker a motifs of the ArsA ATPase

The ArsA ATPase belongs to the P-loop GTPase subgroup within the GTPase superfamily of proteins. Members of this subgroup have a deviant Walker A motif which contains a signature lysine that is predicted to make intermonomer contact with the bound nucleotides and to play a role in ATP hydrolysis. ArsA has two signature lysines located at positions 16 and 335. The role of Lys16 in the A1 half and Lys335 in the A2 half was investigated by altering the lysines individually to alanine, arginine, leucine, methionine, glutamate, and glutamine by site-directed mutagenesis. While Lys16 mutants show similar resistance phenotypes as the wild type, the Lys335 mutants are sensitive to higher concentrations of arsenite. K16Q ArsA shows 70% of wild-type ATPase activity while K335Q ArsA is inactive. ArsA is activated by binding of Sb(III), and both wild-type and mutant ArsAs bind Sb(III) with a 1:1 stoichiometry. Although each ArsA binds nucleotide, the binding affinity decreases in the order wild type > K16Q > K335Q. The results of limited trypsin digestion analysis indicate that both wild type and K16Q adopt a similar conformation during activated catalysis, whereas K335Q adopts a conformation that is resistant to trypsin cleavage. These biochemical data along with structural modeling suggest that, although Lys16 is not critical for ATPase activity, Lys335 is involved in intersubunit interaction and activation of ATPase activity in both halves of the protein. Taken together, the results indicate that Lys16 and Lys335, located in the A1 and A2 halves of the protein, have different roles in ArsA catalysis, consistent with our proposal that the nucleotide binding domains in these two halves are functionally nonequivalent.

Properties of arsenite efflux permeases (Acr3) from Alkaliphilus metalliredigens and Corynebacterium glutamicum

Members of the Acr3 family of arsenite permeases confer resistance to trivalent arsenic by extrusion from cells, with members in every phylogenetic domain. In this study bacterial Acr3 homologues from Alkaliphilus metalliredigens and Corynebacterium glutamicum were cloned and expressed in Escherichia coli. Modification of a single cysteine residue that is conserved in all analyzed Acr3 homologues resulted in loss of transport activity, indicating that it plays a role in Acr3 function. The results of treatment with thiol reagents suggested that the conserved cysteine is located in a hydrophobic region of the permease. A scanning cysteine accessibility method was used to show that Acr3 has 10 transmembrane segments, and the conserved cysteine would be predicted to be in the fourth transmembrane segment.

Aquaglyceroporins: ancient channels for metalloids

The identification of aquaglyceroporins as uptake channels for arsenic and antimony shows how these toxic elements can enter the food chain, and suggests that food plants could be genetically modified to exclude arsenic while still accumulating boron and silicon.

Alteration in glycerol and metalloid permeability by a single mutation in the extracellular C-loop of Leishmania major aquaglyceroporin LmAQP1

The Leishmania major aquaglyceroporin, LmAQP1, is responsible for the transport of antimonite [Sb(III)], an activated form of Pentostam or Glucantime. Downregulation of LmAQP1 provides resistance to trivalent antimony compounds and increased expression of LmAQP1 in drug-resistant parasites can reverse the resistance. Besides metalloid transport, LmAQP1 is also permeable to water, glycerol, methylglyoxal, dihydroxyacetone and sugar alcohols. LmAQP1 also plays a physiological role in volume regulation and osmotaxis. In this study, we examined the role of extracellular C-loop glutamates (Glu143, Glu145 and Glu152) in LmAQP1 activity. Alteration of both Glu143 and Glu145 to alanines did not affect either the biochemical or physiological properties of the protein, suggesting that neither residue is critical for LmAQP1 activity. Alteration of Glu152 to alanine, aspartate and glutamine affected metalloid transport in the order, wild-type > E152Q > E152D > E152A. In fact, axenic amastigotes expressing E152A LmAQP1 accumulated negligible levels of either arsenite [As(III)] or Sb(III). Alteration of Glu152 significantly affected volume regulation and osmotaxis, suggesting that Glu152 is critical for the physiological activity of the parasite. More importantly, alteration of Glu152 to alanine did not affect glycerol permeability. Although the metalloids, As(III) and Sb(III), are believed to be transported through aquaglyceroporin channels as they behave as inorganic molecular mimic of glycerol, this is the first report where metalloid and glycerol transport can be dissected by a single mutation at the extracellular pore entry of LmAQP1 channel.

Role of conserved aspartates in the ArsA ATPase

The ArsA ATPase is the catalytic subunit of the arsenite-translocating ArsAB pump that is responsible for resistance to arsenicals and antimonials in Escherichia coli. ATPase activity is activated by either arsenite or antimonite. ArsA is composed of two homologous halves A1 and A2, each containing a nucleotide binding domain, and a single metalloid binding or activation domain is located at the interface of the two halves of the protein. The metalloid binding domain is connected to the two nucleotide binding domains through two DTAPTGH sequences, one in A1 and the other in A2. The DTAPTGH sequences are proposed to be involved in information communication between the metal and catalytic sites. The roles of Asp142 in A1 D 142TAPTGH sequence, and Asp447 in A2 D 447TAPTGH sequence was investigated after altering the aspartates individually to alanine, asparagine, and glutamate by site-directed mutagenesis. Asp142 mutants were sensitive to As(III) to varying degrees, whereas the Asp447 mutants showed the same resistance phenotype as the wild type. Each altered protein exhibited varying levels of both basal and metalloid-stimulated activity, indicating that neither Asp142 nor Asp447 is essential for catalysis. Biochemical characterization of the altered proteins imply that Asp142 is involved in Mg (2+) binding and also plays a role in signal transduction between the catalytic and activation domains. In contrast, Asp447 is not nearly as critical for Mg (2+) binding as Asp142 but appears to be in communication between the metal and catalytic sites. Taken together, the results indicate that Asp142 and Asp447, located on the A1 and A2 halves of the protein, have different roles in ArsA catalysis, consistent with our proposal that these two halves are functionally nonequivalent.

Characterization of the metalloactivation domain of an arsenite/antimonite resistance pump

The ArsAB extrusion pump encoded by the ars operon of Escherichia coli plasmid R773 confers resistance to the toxic trivalent metalloids arsenite [As(III)] and antimonite [Sb(III)]. The ArsA ATPase, the catalytic subunit of the pump, has two homologous halves, A1 and A2. At the interface of these two halves are two nucleotide-binding domains and a metalloid-binding domain. Cys-113 and Cys-422 have been shown to form a high-affinity metalloid binding site. The crystal structure of ArsA shows two other bound metalloid atoms, one liganded to Cys-172 and His-453, and the other liganded to His-148 and Ser-420. The contribution of those putative metalloid sites was examined. There was little effect of mutagenesis of residues His-148 and Ser-420 on metalloid binding. However, a C172A ArsA mutant and C172A/H453A double mutant exhibited significantly decreased affinity for Sb(III). These results suggest first that there is only a single high-affinity metalloid binding site in ArsA, and second that Cys-172 controls the affinity of this site for metalloid and hence the efficiency of metalloactivation of the ArsAB efflux pump.

Biochemical characterization of Leishmania major aquaglyceroporin LmAQP1: possible role in volume regulation and osmotaxis

The Leishmania major aquaglyceroporin, LmAQP1, is responsible for the transport of trivalent metalloids, arsenite and antimonite. We have earlier shown that downregulation of LmAQP1 provides resistance to trivalent antimony compounds whereas increased expression of LmAQP1 in drug-resistant parasites can reverse the resistance. In this paper we describe the biochemical characterization of LmAQP1. Expression of LmAQP1 in Xenopus oocytes rendered them permeable to water, glycerol, methylglyoxal, dihydroxyacetone and sugar alcohols. The transport property of LmAQP1 was severely affected when a critical Arg230, located inside the pore of the channel, was altered to either alanine or lysine. Immunofluorescence and immuno-electron microscopy revealed LmAQP1 to be localized to the flagellum of Leishmania promastigotes and in the flagellar pocket membrane and contractile vacuole/spongiome complex of amastigotes. This is the first report of an aquaglyceroporin being localized to the flagellum of any microbe. Leishmania promastigotes and amastigotes expressing LmAQP1 could regulate their volume in response to hypoosmotic stress. Additionally, Leishmania promastigotes overexpressing LmAQP1 were found to migrate faster towards an osmotic gradient. These results taken together suggest that Leishmania LmAQP1 has multiple physiological roles, being involved in solute transport, volume regulation and osmotaxis.

Crystal structure of the flavoprotein ArsH from Sinorhizobium meliloti

Purified ArsH from Sinorhizobium meliloti exhibits NADPH:FMN-dependent reduction of molecular O2 to hydrogen peroxide and catalyzes reduction of azo dyes. The structure of ArsH was determined at 1.8A resolution. ArsH crystallizes with eight molecules in the asymmetric unit forming two tetramers. Each monomer has a core domain with a central five-stranded parallel beta-sheet and two monomers interact to form a classical flavodoxin-like dimer. The N- and C-terminal extensions of ArsH are involved in interactions between subunits and tetramer formation. The structure may provide insight in how ArsH participates in arsenic detoxification.

Bifunctional role of the leishmanial antimonate reductase LmACR2 as a protein tyrosine phosphatase

LmACR2 is the first identified antimonate reductase responsible for the reduction of pentavalent antimony in pentostam to the active trivalent form of the drug in Leishmania. LmACR2 is a homologue of the yeast arsenate reductase Acr2p and Cdc25 phosphatases and has the HC[X]5R phosphatase motif. Purified LmACR2 exhibited phosphatase activity in vitro and was able to dephosphorylate a phosphotyrosine residue from a synthetic peptide. This phosphatase activity was inhibited by classical inhibitors such as orthovanadate. LmACR2-catalyzed phosphatase activity was inhibited by either antimonate or arsenate. Site-directed mutagenesis experiments showed that the H74C[X]5R81 motif was involved in catalysis. This is the first report of a metalloid reductase with a bifunctional role in protein tyrosine phosphatase activity. Leishmania is never exposed to metalloids during its life cycle. It is therefore unlikely that it would evolve an enzyme exclusively for drug activation. We propose that the physiological function of LmACR2 is to dephosphorylate phosphotyrosine residues in leishmanial proteins.

Targeted disruption of the mouseAsna1gene results in embryonic lethality

The bacterial ArsA ATPase is the catalytic component of an oxyanion pump that is responsible for resistance to arsenicals and antimonials. Homologues of the bacterial ArsA ATPase are widespread in nature. We had earlier identified the mouse homologue (Asna1) that exhibits 27% identity to the bacterial ArsA ATPase. To identify the physiological role of the protein, heterozygous Asna1 knockout mice (Asna1+/-) were generated by homologous recombination. The Asna1+/- mice displayed similar phenotype as the wild-type mice. However, early embryonic lethality was observed in homozygous Asna1 knockout embryos, between E3.5 (E=embryonic day) and E8.5 stage. These findings indicate that Asna1 plays a crucial role during early embryonic development.

Bifunctional role of the leishmanial antimonate reductase LmACR2 as a protein tyrosine phosphatase

LmACR2 is the first identified antimonate reductase responsible for the reduction of pentavalent antimony in pentostam to the active trivalent form of the drug in Leishmania. LmACR2 is a homologue of the yeast arsenate reductase Acr2p and Cdc25 phosphatases and has the HC[X]5R phosphatase motif. Purified LmACR2 exhibited phosphatase activity in vitro and was able to dephosphorylate a phosphotyrosine residue from a synthetic peptide. This phosphatase activity was inhibited by classical inhibitors such as orthovanadate. LmACR2-catalyzed phosphatase activity was inhibited by either antimonate or arsenate. Site-directed mutagenesis experiments showed that the H74C[X]5R81 motif was involved in catalysis. This is the first report of a metalloid reductase with a bifunctional role in protein tyrosine phosphatase activity. Leishmania is never exposed to metalloids during its life cycle. It is therefore unlikely that it would evolve an enzyme exclusively for drug activation. We propose that the physiological function of LmACR2 is to dephosphorylate phosphotyrosine residues in leishmanial proteins.

Cys-113 and Cys-422 form a high affinity metalloid binding site in the ArsA ATPase

The arsRDABC operon of Escherichia coli plasmid R773 encodes the ArsAB extrusion pump for the trivalent metalloids As(III) and Sb(III). ArsA, the catalytic subunit has two homologous halves, A1 and A2. Each half has a consensus signal transduction domain that physically connects the nucleotide-binding domain to the metalloid-binding domain. The relation between metalloid binding by ArsA and transport through ArsB is unclear. In this study, direct metalloid binding to ArsA was examined. The results show that ArsA binds a single Sb(III) with high affinity only in the presence of Mg(2+)-nucleotide. Mutation of the codons for Cys-113 and Cys-422 eliminated Sb(III) binding to purified ArsA. C113A/C422A ArsA has basal ATPase activity similar to that of the wild type but lacks metalloid-stimulated activity. Accumulation of metalloid was assayed in intact cells, where reduced uptake results from active extrusion by the ArsAB pump. Cells expressing the arsA(C113A/C422A)B genes had an intermediate level of metalloid resistance and accumulation between those expressing only arsB alone and those expressing wild type arsAB genes. The results indicate that, whereas metalloid stimulation of ArsA activity enhances the ability of the pump to reduce the intracellular concentration of metalloid, high affinity binding of metalloid by ArsA is not obligatory for transport or resistance. Yet, in mixed populations of cells bearing either arsAB or arsA(C113A/C422A)B growing in subtoxic concentrations of arsenite, cells bearing wild type arsAB replaced cells with mutant arsA(C113A/C422A)B in less than 1 week, showing that the metalloid binding site confers an evolutionary advantage.

Novel pathway for arsenic detoxification in the legume symbiont Sinorhizobium meliloti

We report a novel pathway for arsenic detoxification in the legume symbiont Sinorhizobium meliloti. Although a majority of ars operons consist of three genes, arsR (transcriptional regulator), arsB [As(OH)3/H+ antiporter], and arsC (arsenate reductase), the S. meliloti ars operon includes an aquaglyceroporin (aqpS) in place of arsB. The presence of AqpS in an arsenic resistance operon is interesting, since aquaglyceroporin channels have previously been shown to adventitiously facilitate uptake of arsenite into cells, rendering them sensitive to arsenite. To understand the role of aqpS in arsenic resistance, S. meliloti aqpS and arsC were disrupted individually. Disruption of aqpS resulted in increased tolerance to arsenite but not arsenate, while cells with an arsC disruption showed selective sensitivity to arsenate. The results of transport experiments in intact cells suggest that AqpS is the only protein of the S. meliloti ars operon that facilitates transport of arsenite. Coexpression of S. meliloti aqpS and arsC in a strain of E. coli lacking the ars operon complemented arsenate but not arsenite sensitivity. These results imply that, when S. meliloti is exposed to environmental arsenate, arsenate enters the cell through phosphate transport systems and is reduced to arsenite by ArsC. Internally generated arsenite flows out of the cell by downhill movement through AqpS. Thus, AqpS confers arsenate resistance together with ArsC-catalyzed reduction. This is the first report of an aquaglyceroporin with a physiological function in arsenic resistance.

Nonequivalence of the nucleotide binding domains of the ArsA ATPase

The arsRDABC operon of Escherichia coli plasmid R773 encodes the ArsAB pump that catalyzes extrusion of the metalloids As(III) and Sb(III), conferring metalloid resistance. The catalytic subunit, ArsA, is an ATPase with two homologous halves, A1 and A2, connected by a short linker. Each half contains a nucleotide binding domain. The overall rate of ATP hydrolysis is slow in the absence of metalloid and is accelerated by metalloid binding. The results of photolabeling of ArsA with the ATP analogue 8-azidoadenosine 5'-[alpha-(32)P]-triphosphate at 4 degrees C indicate that metalloid stimulation correlates with a >10-fold increase in affinity for nucleotide. To investigate the relative contributions of the two nucleotide binding domains to catalysis, a thrombin site was introduced in the linker. This allowed discrimination between incorporation of labeled nucleotides into the two halves of ArsA. The results indicate that both the A1 and A2 nucleotide binding domains bind and hydrolyze trinucleotide, even in the absence of metalloid. Sb(III) increases the affinity of the A1 nucleotide binding domain to a greater extent than the A2 nucleotide binding domain. The ATP analogue labeled with (32)P at the gamma position was used to measure hydrolysis of trinucleotide at 37 degrees C. Under these catalytic conditions, both nucleotide binding domains hydrolyze ATP, but hydrolysis in A1 is stimulated to a greater degree by Sb(III) than A2. These results suggest that the two homologous halves of the ArsA may be functionally nonequivalent.

Drug uptake and pharmacological modulation of drug sensitivity in leukemia by AQP9

Leukemia is the most common childhood cancer. Trisenox, the active ingredient of which is trivalent arsenic, is the first line of treatment for acute promyelocytic leukemia. Since drug action usually requires uptake of the drug, it is of importance to determine the transport system responsible for Trisenox uptake. Recently, human aquaglyceroporin 9 (AQP9) has been shown to transport As(III) in Xenopus oocytes. In this study we report to show that AQP9 expression modulates the drug sensitivity of leukemic cells. AQP9 was transfected into the chronic myelogenous leukemia cell line K562. The transfectants became hypersensitive to Trisenox and Sb(III). The promyelocytic leukemia cell line HL60 treated with vitamin D showed higher expression of AQP9 and hypersensitivity to Trisenox and Sb(III). This sensitivity was due to higher rates of uptake of the trivalent metalloids by the cell lines overexpressing AQP9. Trisenox hypersensitivity results from increased expression of AQP9 drug uptake system. The possibility of using pharmacological agents to increase expression of AQP9 gene delivers the promise of new therapies for the treatment of leukemia. Thus, drug hypersensitivity can be correlated with increased expression of the drug uptake system. This is the first demonstration that AQP9 can modulate drug sensitivity in cancer.

Drug uptake and modulation of drug resistance in Leishmania by an aquaglyceroporin

Leishmaniasis is a protozoan parasitic disease that affects 12 million people worldwide. The first line choice for the treatment of this disease is antimonial drugs. In the endemic regions, resistance to this class of drugs is a major impediment to treatment. Microbes often become resistant to drugs by mutation or down-regulation of uptake systems, but the uptake system for the antimonial drugs in Leishmania is unknown. In other organisms, aquaglyceroporins have been shown to facilitate uptake of trivalent metalloids. In this study, we report the identification and characterization of aquaglyceroporins from Leishmania major (LmAQP1) and Leishmania tarentolae (LtAQP1), respectively. These Leishmania proteins have the conserved signature motifs of aquaglyceroporins. Transfection of LmAQP1 into three species of Leishmania, L. tarentolae, Leishmania infantum, and L. major, produced hypersensitivity to both As(III) and Sb(III) in all three strains. Increased production of LmAQP1 was detected by immunoblotting. Drug-resistant parasites with various mutations leading to resistance mechanisms became hypersensitive to both metalloids after expression of LmAQP1. Increased rates of uptake of As(III) or Sb(III) correlated with metalloid sensitivity of the wild type and drug-resistant transfectants. Transfection of LmAQP1 in a Pentostam-resistant field isolate also sensitized the parasite in the macrophage-associated amastigote form. One allele of LmAQP1 was disrupted in L. major, and the resulting cells became 10-fold more resistant to Sb(III). This is the first report of the uptake of a metalloid drug by an aquaglyceroporin in Leishmania, suggesting a strategy to reverse resistance in the field.

UDPgalactose 4-epimerase from Saccharomyces cerevisiae. A bifunctional enzyme with aldose 1-epimerase activity

UDPgalactose 4-epimerase (epimerase) catalyzes the reversible conversion between UDPgalactose and UDPglucose and is an important enzyme of the galactose metabolic pathway. The Saccharomyces cerevisiae epimerase encoded by the GAL10 gene is about twice the size of either the bacterial or human protein. Sequence analysis indicates that the yeast epimerase has an N-terminal domain (residues 1-377) that shows significant similarity with Escherichia coli and human UDPgalactose 4-epimerase, and a C-terminal domain (residues 378-699), which shows extensive identity to either the bacterial or human aldose 1-epimerase (mutarotase). The S. cerevisiae epimerase was purified to > 95% homogeneity by sequential chromatography on DEAE-Sephacel and Resource-Q columns. Purified epimerase preparations showed mutarotase activity and could convert either alpha-d-glucose or alpha-d-galactose to their beta-anomers. Induction of cells with galactose led to simultaneous enhancement of both epimerase and mutarotase activities. Size exclusion chromatography experiments confirmed that the mutarotase activity is an intrinsic property of the yeast epimerase and not due to a copurifying endogenous mutarotase. When the purified protein was treated with 5'-UMP and l-arabinose, epimerase activity was completely lost but the mutarotase activity remained unaffected. These results demonstrate that the S. cerevisiae UDPgalactose 4-epimerase is a bifunctional enzyme with aldose 1-epimerase activity. The active sites for these two enzymatic activities are located in different regions of the epimerase holoenzyme.

The Saccharomyces cerevisiae Arr4p is involved in metal and heat tolerance

Homologues of the bacterial ArsA ATPase are found in nearly every organism. While the enzyme is involved in arsenic detoxification in bacteria, the roles of eukaryotic homologues have not been identified. This article reports the function of the Saccharomyces cerevisiae homologue encoded by ARR4 gene (YDL100c ORF). Disruption of ARR4 was not lethal, but the disrupted strain displayed increased sensitivity to As3+, As5+, Co2+, Cr3+, Cu2+ or VO4(3-) salts and temperature. A plasmid-encoded copy of a wild-type ARR4 gene could complement the heat- or metal-related stress responses. Mutation of a codon within the consensus sequence for the nucleotide-binding site resulted in loss of complementation of the disrupted strain and produced a dominant negative phenotype in a wild type strain. Wild type and mutant Arr4p were purified from Escherichia coli. The wild type protein exhibited a low level of ATPase activity, and the mutant was inactive. The purified ATPase eluted as a dimer of 80-kDa species. A fusion of ARR4 and the GFP (green fluorescent protein) gene was constructed. The gene fusion was able to complement stress-related phenotype of the ARR4 disruption. Under non-stress conditions, GFP fluorescence was found diffusely in the cytosol. Under stress conditions GFP was localized in a few punctate bodies resembling late endosomes. It is proposed that under heat or metal stress, the soluble ATPase becomes membrane-associated, perhaps through interaction with a partner protein, and that this complex is involved in stress tolerance.

Structure-function analysis of the ArsA ATPase: contribution of histidine residues

The ArsA ATPase is the catalytic subunit of the ArsAB oxyanion pump in Escherichia coli that is responsible for extruding arsenite or antimonite from inside the cell, thereby conferring resistance. Either antimonite or arsenite stimulates ArsA ATPase activity. In this study, the role of histidine residues in ArsA activity was investigated. Treatment of ArsA with diethyl pyrocarbonate (DEPC) resulted in complete loss of catalytic activity. The inactivation could be reversed upon subsequent incubation with hydroxylamine, suggesting specific modification of histidine residues. ATP and oxyanions afforded significant protection against DEPC inactivation, indicating that the histidines are located at the active site. ArsA has 13 histidine residues located at position 138, 148, 219, 327, 359, 368, 388, 397, 453, 465, 477, 520, and 558. Each histidine was individually altered to alanine by site-directed mutagenesis. Cells expressing the altered ArsA proteins were resistant to both arsenite and antimonite. The results indicate that no single histidine residue plays a direct role in catalysis, and the inhibition by DEPC may be caused by steric hindrance from the carbethoxy group.

Genomic organization and chromosomal localization of the Asna1 gene, a mouse homologue of a bacterial arsenic-translocating ATPase gene

The plasmid encoded ArsA ATPase in Escherichia coli is the catalytic component of an oxyanion pump that is responsible for resistance to arsenicals and antimonials. Arsenite or antimonite allosterically activates the ArsA ATPase activity. In this paper, we report the cloning and characterization of the mouse homologue (Asna1) of the bacterial arsA gene. The Asna1 gene encodes an open reading frame of 348 amino acids and exhibits 27% identity to the bacterial ArsA protein and 99% similarity to its human counterpart (hASNA-1). The Asna1 mRNA is a approximately 1.3 kb transcript and is present at high levels in kidney and testis, moderate levels in brain, liver, lung and skin, and low levels in heart, small intestine, spleen, stomach, and thymus. A negligible amount of Asna1 transcript is detected in skeletal muscle. We have also characterized the genomic structure of the Asna1 gene. The gene spans over 7 kb and consists of seven exons and six introns. All splice sites conform to the GT-AG rule, except for the splice donor site of intron 4 that is GC instead of GT. Fluorescence in situ hybridization indicates that the Asna1 gene is localized in the C3-D1 region of mouse chromosome 8.

Structure-function relationships in an anion-translocating ATPase

The ArsAB ATPase is an efflux pump located in the inner membrane of Escherichia coli. This transport ATPase confers resistance to arsenite and antimonite by their extrusion from the cells. The pump is composed of two subunits, the catalytic ArsA subunit and the membrane subunit ArsB. The complex is similar in many ways to ATP-binding cassette ('ABC') transporters, which typically have two groups of six transmembrane-spanning helical segments and two nucleotide-binding domains (NBDs). The 45 kDa ArsB protein has 12 transmembrane-spanning segments. ArsB contains the substrate translocation pathway and is capable of functioning as an anion uniporter. The 63 kDa ArsA protein is a substrate-activated ATPase. It has two homologous halves, A1 and A2, which are clearly the result of an ancestral gene duplication and fusion. Each half has a consensus NBD. The mechanism of allosteric activation of the ArsA ATPase has been elucidated by a combination of molecular genetics and biochemical, structural and kinetic analyses. Conformational changes produced by binding of substrates, activator and/or products could be revealed by stopped-flow fluorescence measurements with single-tryptophan derivatives of ArsA. The results demonstrate that the rate-limiting step in the overall reaction is a slow isomerization between two conformations of the enzyme. Allosteric activation increases the rate of this isomerization such that product release becomes rate-limiting, thus accelerating catalysis. ABC transporters, which exhibit similar substrate activation of ATPase activity, can undergo similar conformational changes to overcome a rate-limiting step. Thus the ArsAB pump is a useful model for elucidating mechanistic aspects of the ABC superfamily of transport ATPases.

Role of conserved histidine residues in metalloactivation of the ArsA ATPase

The ArsA ATPase is the catalytic subunit of a pump that is responsible for resistance to arsenicals and antimonials in Escherichia coli. Arsenite or antimonite allosterically activates the ArsA ATPase activity. ArsA homologues from eubacteria, archaea and eukarya have a signature sequence (DTAPTGHT) that includes a conserved histidine. The ArsA ATPase has two such conserved motifs, one in the NH2-terminal (Al) half and the other in the COOH-terminal (A2) half of the protein. These sequences have been proposed to be signal transduction domains that transmit the information of metal occupancy at the allosteric to the catalytic site to activate ATP hydrolysis. The role of the conserved residues His148 and His453, which reside in the A1 and A2 signal transduction domains respectively, was investigated by mutagenesis to create H148A, H453A or H148A/H453A ArsAs. Each altered protein exhibited a decrease in the Vmax of metalloid-activated ATP hydrolysis, in the order wild type ArsA>H148A>H453A>H148A/H453A. These results suggest that the histidine residues play a role in transmission of the signal between the catalytic and allosteric sites.

Mechanism of the ArsA ATPase

The ArsAB ATPase confers metalloid resistance in Escherichia coli by pumping toxic anions out of the cells. This transport ATPase shares structural and perhaps mechanism features with ABC transporters. The ArsAB pump is composed of a membrane subunit that has two groups of six transmembrane segments, and the catalytic subunit, the ArsA ATPase. As is the case with many ABC transporters, ArsA has an internal repeat, each with an ATP binding domain, and is allosterically activated by substrates of the pump. The mechanism of allosteric activation of the ArsA ATPase has been elucidated at the molecular level. Binding of the activator produces a conformational change that forms a tight interface of the nucleotide binding domains. In the rate-limiting step in the overall reaction, the enzyme undergoes a slow conformational change. The allosteric activator accelerates catalysis by increasing the velocity of this rate-limiting step. We postulate that similar conformational changes may be rate-limiting in the mechanism of ABC transporters.

Capsulorhexis in intumescent cataract

This technique of anterior capsulorhexis is for use in eyes with high intralenticular pressure and absence of red reflex as encountered in intumescent cataract. The initial steps of fashioning the anterior capsular flap and lens decompression are done under the microscope's high magnification and noncoaxial oblique illumination. Intralenticular pressure is controlled by filling the anterior chamber with a viscoelastic agent. Once the lens is decompressed, the capsulorhexis is completed with a capsule forceps and an endoilluminator is used as an oblique source of illumination. This technique allows controlled capsulorhexis in eyes with intumescent cataract.

UDP-galactose 4-epimerase from Kluyveromyces fragilis: reconstitution of holoenzyme structure after dissociation with parachloromercuribenzoate

UDP-galactose 4-epimerase from yeast Kluyveromyces fragilis (Kluyveromyces marxianus var. marxianus) is a homodimer of molecular mass 75 kDa/subunit and has one mol NAD firmly bound/dimer. The pathway for the assembly of the holoenzyme structure has been studied after dissociating the native epimerase with p-chloromercuribenzoate into inactive mercurated monomers. The process of dissociation was not associated with unfolding of the molecules. Reconstitution of the functional holoenzyme was done by reduction with dithiothreitol and addition of extra NAD. The reaction was thus followed to monitor maturation of the enzyme from the folded monomeric state. The reconstituted enzyme was similar to the native enzyme in terms of a number of physiochemical properties such as secondary, tertiary and quarternary structures, Km for the substrate UDP-galactose, reductive inhibition, interaction with the fluorophore 1-anilino 8-naphthalene sulphonic acid (ANS), etc. Reconstitution under low ionic strength buffer (I = 0.011) shows that the presence of NAD is essential for the formation of a dimeric structure. However, dimeric apoenzyme could also be stabilized under high ionic strength buffer (I = 0.1). Reactivation was strongly dependent on pH, being most effective at pH 8.1. Kinetic evidence suggested that, at low ionic strength, assembly of NAD over dimeric apoenzyme is the rate-limiting step in expressing catalytic activity. This process has a low energy of activation of 27.2 kJ/mol.

Spatial proximity of Cys113, Cys172, and Cys422 in the metalloactivation domain of the ArsA ATPase

ArsA ATPase activity is allosterically activated by salts of the semimetal arsenic or antimony. Activation is associated with the presence of three cysteine residues in ArsA: Cys113, Cys172, and Cys422. To determine the distance between cysteine residues, wild type ArsA and ArsA proteins with cysteine to serine substitutions were treated with the bifunctional alkylating agent dibromobimane, which reacts with thiol pairs within 3-6 A of each other to form a fluorescent adduct. ArsA proteins in which single cysteine residues were altered by site-directed mutagenesis still formed fluorescent adducts. Proteins in which two of the three cysteine residues were substituted did not form fluorescent adducts. These results demonstrate that Cys113, Cys172, and Cys422 are in close proximity of each other. We propose a model in which As(III) or Sb(III) interacts with these three cysteines in a trigonal pyramidal geometry, forming a novel soft metal-thiol cage.

Role of cysteinyl residues in metalloactivation of the oxyanion-translocating ArsA ATPase

The ArsA protein, the catalytic subunit of the oxyanion-translocating ATPase responsible for resistance to arsenicals and antimonials in Escherichia coli, is activated by arsenite or antimonite. Activation is associated with dimerization of the ArsA protein. Enzymatic activity was rapidly but reversibly inhibited by the sulfhydryl reagent methyl methanethiosulfonate, suggesting that at least one cysteinyl residue is required for catalytic activity. Each of the four cysteinyl residues in the ArsA protein, Cys26, Cys113, Cys172, and Cys422, were individually changed to seryl residues. The C26S protein had normal properties. Cells expressing the other three mutations lost resistance to arsenite and antimonite. The C113S, C172S, and C422S enzymes each had relatively normal Km values for ATP but reduced affinity for antimonite and arsenite. The Vmax of the activated enzymes ranged from very low for the C113S and C422S enzymes to near normal for the C172S enzyme. These results suggest a mechanism of activation by formation of a tricoordinate complex between Sb(III) or As(III) and the cysteine thiolates 113, 172, and 422.

Mechanisms of metalloregulation of an anion-translocating ATPase

The ars (arsenical resistance) operon cloned from R-factor R773 has five genes that encode two repressor proteins, ArsR and ArsD, and three structural proteins, ArsA, ArsB, and ArsC. The ArsA and ArsB proteins form a membrane-bound pump that functions as an oxyanion-translocating ATPase. The substrates of the pump are the oxyanions arsenite or antimonite. The ArsC protein is an arsenate reductase that reduces arsenate to arsenite, which is subsequently pumped out of the cell. This review deals with the mechanism of transcriptional regulation by the ArsR repressor and allosteric regulation of the ArsA protein, the catalytic subunit of the pump. The chemical nature of the inducer plays an important role in regulation. In solution arsenite or antimonite exist as oxyanions and reacts with the cysteines in proteins. In both transcriptional regulation by the ArsR repressor and allosteric regulation of the ArsA ATPase, the ability of As(III) and Sb(III) to interact with the cysteines of the proteins, involves their action as effector.

Distinct functional roles of two active site thiols in UDPglucose 4-epimerase from Kluyveromyces fragilis

UDPglucose 4-epimerase from Kluyveromyces fragilis was earlier shown to have two conformationally vicinal thiols at the active site. Upon treatment with diamide, these thiols form a disulfide linkage across the subunits that results in coordinated loss of catalytic activity and coenzyme fluorescence (Ray, M., and Bhaduri, A. (1980) J. Biol. Chem. 255, 10777-10786). Employing a number of thiol-specific reagents, we now suggest discriminatory and nonidentical roles for these two thiols. Kinetic and statistical analysis of 5,5'-dithiobis-(2-nitrobenzoic acid) and N-ethylmaleimide modification reaction of epimerase show that only one thiol is essential for activity. Consecutive modification experiments clearly show that the same active thiol is modified in both cases. However, significant differences are observed when the reactivity of these reagents is monitored in terms of coenzyme fluorescence. Treatment with N-ethylmaleimide leads to a form of inactive enzyme that fully retains its fluorescent properties whereas modification with 5,5'-dithiobis-(2-nitrobenzoic acid), on the other hand, results in the loss of both activity and fluorescence. The closely spaced nonessential second thiol, which is not modified by N-ethylmaleimide is therefore involved in generating and maintaining the coenzyme fluorescence. Modification studies with a series of spin-labeled maleimide shows that only 3-(maleimidomethyl)proxyl causes partial quenching of coenzyme fluorescence. This suggests that the active thiol is situated at a distance of 4.5 A approximately from the coenzyme fluorophore.