Lysosomal storage of oligosaccharide and glycosphingolipid in imino sugar treated cells

Sandhoff and Tay-Sachs disease are autosomal recessive GM2 gangliosidoses where a deficiency of lysosomal β-hexosaminidase results in storage of glycoconjugates. Imino sugar (2-acetamido-1,4-imino-1,2,4-trideoxy-L-arabinitol) inhibition of β-hexosaminidase in murine RAW264.7 macrophage-like cells led to lysosomal storage of glycoconjugates that were characterised structurally using fluorescence labelling of the free or glycolipid-derived oligosaccharides followed by HPLC and mass spectrometry. Stored glycoconjugates were confirmed as containing non-reducing GlcNAc or GalNAc residues resulting from the incomplete degradation of N-linked glycoprotein oligosaccharide and glycolipids, respectively. When substrate reduction therapeutics N-butyl-deoxynojirimycin (NB-DNJ) or N-butyldeoxygalactonojirimycin (NB-DGJ) were applied to the storage phenotype cells, an increase in glucosylated and galactosylated oligosaccharide species was observed due to endoplasmic reticulum α-glucosidases and lysosomal β-galactosidase inhibition, respectively. Hexosaminidase inhibition triggered a tightly regulated cytokine-mediated inflammatory response that was normalised using imino sugars NB-DNJ and NB-DGJ, which restored the GM2 ganglioside storage burden but failed to reduce the levels of GA2 glycolipid or glycoprotein-derived N-linked oligosaccharides. Using a chemically induced gangliosidosis phenotype that can be modulated with substrate lowering drugs, the critical role of GM2 ganglioside in the progression of inflammatory disease is also demonstrated.     

Reversible change in the fibroblast lysosomal enzyme dipeptidyl aminopeptidase-1 (cathepsin c) related to the commercial source of fetal bovine serum in the culture medium

Summary The commercial source of fetal bovine serum used to supplement the growth medium of human skin fibroblasts alters the activity of the lysosomal enzyme dipeptidyl aminopeptidase-1 (DAP-1). Cells grown with one serum were found to have a threefold higher level of DAP-1 than those grown with serum from another source (P<0.001). The effect on DAP-1 activity was specific inasmuch as no differences were found in the activities of a variety of other lysosomal and nonlysosomal hydrolases: DAP-II, DAP-III, DAP-IV, β-glucosidase, β-glucuronidase, andN-acetyl-β-galactosaminidase. The effect is reversible and is observed over a wide range of cell population doublings. Cell growth kinetics were not significantly different with the different sera. This work was supported in part by grants from the National Institutes of Health, Bethesda, MD (NS 16287).     

β-Glucuronidase and hexosaminidase are marker enzymes for different compartments of the endo-lysosomal system in mussel digestive cells

In environmental toxicology, the most commonly used techniques used to visualise lysosomes in order to determine their responses to pollutants (LSC test: lysosomal structural changes test; LMS test: lysosomal membrane stability test) are based on the histochemical application of lysosomal marker enzymes. In mussel digestive cells, the marker enzymes used are β-glucuronidase (β-Gus) and hexosaminidase (Hex). The present work has been aimed at determining the distribution of these lysosomal marker enzymes in the various compartments of the endo-lysosomal system (ELS) of mussel digestive cells and at exploring whether intercellular transfer of lysosomal enzymes occurs between digestive and basophilic cells. Immunogold cytochemistry has allowed us to conclude that β-Gus is present in every compartment of the digestive cell ELS, whereas Hex is not so widely distributed. Moreover, Hex is intimately linked to the lysosomal membrane, whereas β-Gus appears to be not necessarily membrane-bound. Therefore, two populations of heterolysosomes with different enzyme load and membrane stability have been distinguished in the digestive cell. In addition, heterolysosomes of different electron density have been commonly observed merging together by contact; we suggest that some might act as storage granules for lysosomal enzymes. On the other hand, β-Gus seems to be released to the digestive alveolar lumen in secretory lysosomes produced by basophilic cells and endocytosed by digestive cells. Regarding the implications of the present study on the interpretation of lysosomal biomarkers, we conclude that β-Gus, but not Hex, histochemistry provides an appropriate marker for the LSC test and that, although both lysosomal marker enzymes can be employed in the LMS test, different values would be obtained depending on the marker enzyme employed. This study was funded by the University of the Basque Country through a grant to Consolidated Research Groups. U.I. is a recipient of a pre-doctoral fellowship from the Basque Government.     

Production of N,N′-diacetylchitobiose from chitin using temperature-sensitive chitinolytic enzyme preparations of Aeromonas sp. GJ-18

Summary N,N′-diacetylchitobiose was produced from chitin as a major hydrolytic product by controlling the ratio of β-N-acetylglucosaminidase to N,N′-diacetylchitobiohydrolase activities in the crude enzyme preparation of Aeromonas sp. GJ-18. When the enzyme preparation was preincubated at 50 °C, β-N-acetylglucosaminidase was nearly inactivated, while the N,N′-diacetylchitobiohydrolase was still active. Thus, the composition of chitin oligosaccharides depended on the preincubation temperature of the crude enzyme preparations. Typically, after 7 days of incubation with the substrate chitin, 78.9 and 56.6% of N,N′-diacetylchitobiose yields were obtained from swollen α-chitin and powdered β-chitin, respectively, with enzyme preparations that had been pretreated at 50 °C for 60 min.     

Purification and partial characterization of β-galactosidase from Tritrichomonas foetus

The work presented in this paper describes the purification and properties of a β-galactosidase from the protozoan Tritrichomonas foetus. An inexpensive and straightforward method for extraction of the enzyme involving ammonium sulphate precipitation, ion exchange and affinity chromatography resulted in a high level of purification. After purification β-N-acetylglucosaminidase was the only enzyme present as a contaminant at a significant level. The β-galactosidase isolated had a pH optimum of 5.8. The K_m determined at pH 5.8 was found to be 2.2 mM. Interesting results were obtained when studies were carried out to determine the effect of various metal ions on enzyme activity. Of the metal ions used in this study only manganese ions were found to activate the enzyme. This seems to be a characteristic of trichomonad enzymes, as N-acetyl-β-glucosaminidase, a-galactosidase and N-acetyl-a-galactosaminidase are also activated by manganese ions. The strongest inhibition was recorded with lead and to a lesser extent by zinc. The result with lead is not unexpected as the heavy metal is known to cause irreversible inhibition by binding to the amino-acid backbone of the enzyme. The result with zinc is interesting as high levels of zinc are present and trichomonads are known to be apathogenic in semen. The purified β-galactosidase was found to have the capacity to hydrolyse lactose (Gal β1-4 Glc), lacto-N-biose 1 (Gal β1-3 GlcNAc) and N-acetyllactosamine (Gal β1-4 GlcNAc). When the enzyme was applied to a non-denaturing polyacrylamide gel a single band was observed when stained with Coomassie brilliant blue. This band coincided with that obtained when the gel was stained with p-nitrophenyl β-galactopyranoside. When the same gel was incubated with p-nitrophenyl N-acetyl β-glucopyranoside a band was detected which did not coincide with that of β-galactosidase. Since the β-N-acetylglucosaminidase enzyme does not move to the same position on a non-denaturing gel as the β-galactosidase, we will use this technique to isolate the latter enzyme and determine the N-terminal sequence as a prelude to cloning and further study of the gene. This revised version was published online in November 2006 with corrections to the Cover Date.     

Therapeutic Effects of Stem Cells and Substrate Reduction in Juvenile Sandhoff Mice

Sandhoff Disease (SD) involves the CNS accumulation of ganglioside GM2 and asialo-GM2 (GA2) due to inherited defects in the β-subunit gene of β-hexosaminidase A and B (Hexb gene). Substrate reduction therapy, utilizing imino sugar N-butyldeoxygalactonojirimycin (NB-DGJ), reduces ganglioside biosynthesis and levels of stored GM2 in SD mice. Intracranial transplantation of Neural Stem Cells (NSCs) can provide enzymatic cross correction, to help reduce ganglioside storage and extend life. Here we tested the effect of NSCs and NB-DGJ, alone and together, on brain β-hexosaminidase activity, GM2, and GA2 content in juvenile SD mice. The SD mice received either cerebral NSC transplantation at post-natal day 0 (p-0), intraperitoneal injection of NB-DGJ (500 mg/kg/day) from p-9 to p-15, or received dual treatments. The brains were analyzed at p-15. β-galactosidase staining confirmed engraftment of lacZ-expressing NSCs in the cerebral cortex. Compared to untreated and sham-treated SD controls, NSC treatment alone provided a slight increase in Hex activity and significantly decreased GA2 content. However, NSCs had no effect on GM2 content when analyzed at p-15. NB-DGJ alone had no effect on Hex activity, but significantly reduced GM2 and GA2 content. Hex activity was slightly elevated in the NSC + drug-treated mice. GM2 and GA2 content in the dual treated mice were similar to that of the NB-DGJ treated mice. These data indicate that NB-DGJ alone was more effective in targeting storage in juvenile SD mice than were NSCs alone. No additive or synergistic effect between NSC and drug was found in these juvenile SD mice.     

Knock‐down of HEXA and HEXB genes correlate with the absence of the immunostimulatory function of HSC‐derived dendritic cells

In an attempt to investigate whether the genetic defect in the HEXA and HEXB genes (which causes the absence of the lysosomal β‐N‐acetyl‐hexosaminidase), are related to the wide inflammation in GM2 gangliosidoses (Tay‐Sachs and Sandhoff disease), we have chosen the dendritic cells (DCs) as a study model. Using the RNA interference approach, we generated an in vitro model of HEXs knock‐down immunogenic DCs (i‐DCs) from CD34⁺‐haemopoietic stem cells (CD34⁺‐HSCs), thus mimicking the Tay‐Sachs (HEXA?/?) and Sandhoff (HEXB?/?) cells. We showed that the absence of β‐N‐acetyl‐hexosaminidase activity does not alter the differentiation of i‐DCs from HSCs, but it is critical for the activation of CD4⁺T cells because knock‐down of HEXA or HEXB gene causes a loss of function of i‐DCs. Notably, the silencing of the HEXA gene had a stronger immune inhibitory effect, thereby indicating a major involvement of β‐N‐acetyl‐hexosaminidase A isoenzyme within this mechanism. Copyright © 2011 John Wiley & Sons, Ltd.     

Development of a functional bioassay for arylsulfatase B using the natural substrates of the enzyme

A functional bioassay has been developed for measuring the intracellular activity of recombinant human arylsulfatase B (rhASB) on its natural glycosaminoglycan (GAG) substrates, dermatan sulfate (DS), and chondroitin sulfate (CS) when the enzyme is taken up into cultured ASB-deficient human fibroblasts (GM00519). The enzyme ASB is a lysosomal exohydrolase, cleaving sulfate from the N-acetylgalactosamine-4-sulfate (GalNAc-4S) residue at the nonreducing terminal of GAG structures. ASB-deficient cells accumulate DS and CS, which may be partially hydrolyzed by other lysosomal hydrolases, with the reactions stopping if a GalNAc-4S residue is reached on the nonreducing end of the oligosaccharide. When rhASB is added to the culture medium, the enzyme is taken up and translocates to the lysosomes and the intracellular DS and CS are depleted, demonstrating that the uptake of rhASB is able to restore lysosomal function in an in vitro cell-based assay. The accumulation and depletion of DS and CS are measured by digesting the residual intracellular DS and CS content with chondroitin ABC lyase and monitoring a characteristic disaccharide digestion product by laser-induced fluorescence–capillary zone electrophoresis (LIF–CZE). In the proposed assay format, GM00519 cells are cultured 5 weeks postconfluence to accumulate DS/CS, followed by incubation with rhASB (1–20 pM) for 5 days, and the CS/DS depletion profiles are compared between samples. The assay measures depletion of DS/CS independently of their molecular size or processing state; in this approach, all DS- and CS-like substances accumulating in the absence of ASB activity are considered to be natural substrates of the enzyme.     

Syntheses of mucin-type <Emphasis Type="Italic">O</Emphasis>-glycopeptides and oligosaccharides using transglycosylation and reverse-hydrolysis activities of <Emphasis Type="Italic">Bifidobacterium</Emphasis> endo-α-<Emphasis Type="Italic">N</Emphasis>-acetylgalactosaminidase

Endo-α-N-acetylgalactosaminidase catalyzes the release of Galβ1-3GalNAc from the core 1-type O-glycan (Galβ1-3GalNAcα1-Ser/Thr) of mucin glycoproteins and synthetic p-nitrophenyl (pNP) α-linked substrates. Here, we report the enzymatic syntheses of core 1 disaccharide-containing glycopeptides using the transglycosylation activity of endo-α-N-acetylgalactosaminidase (EngBF) from Bifidobacterium longum. The enzyme directly transferred Galβ1-3GalNAc to serine or threonine residues of bioactive peptides such as PAMP-12, bradykinin, peptide-T and MUC1a when Galβ1-3GalNAcα1-pNP was used as a donor substrate. The enzyme was also found to catalyze the reverse-hydrolysis reaction. EngBF synthesized the core 1 disaccharide-containing oligosaccharides when the enzyme was incubated with either glucose or lactose and Galβ1-3GalNAc prepared from porcine gastric mucin using bifidobacterial cells expressing endo-α-N-acetylgalactosaminidase. Synthesized oligosaccharides are promising prebiotics for bifidobacteria.     

Heterogeneous Properties of Individual Molecules of β-Galactosidase from the Thermophilic Bacteria <Emphasis Type="Italic">Geobacillus stearothermophilus</Emphasis>

Single enzyme molecule assays were performed on β-galactosidase from the thermophilic bacteria Geobacillus stearothermophilus using a capillary electrophoresis-based continuous flow assay and the substrate DDAO-β-d-galactoside. The enzyme was found to be heterogeneous with respect to catalytic rate, electrophoretic mobility and activation energy of catalysis. Catalytic rate was also found to vary over time for individual molecules at elevated temperature. Comparison with β-galactosidase from the mesophilic bacteria Escherichia coli showed that the variation in activity over time was less pronounced and the average activation energy of catalysis was lower for the Geobacillus stearothermophilus enzyme. Attempts to measure the properties of individual β-galactosidase molecules from the thermophilic bacteria Thermus thermophilus and the cold-adapted bacteria Pseudoalteromas haloplanktis using this assay were unsuccessful.     

Purification and molecular characterization of exo-β-1,3-glucanases from the marine yeast Williopsis saturnus WC91-2

The extracellular β-1,3-glucanases in the supernatant of cell culture of the marine yeast Williopsis saturnus WC91-2 was purified to homogeneity with a 115-fold increase in specific β-1,3-glucanase activity as compared to that in the supernatant by ultrafiltration, gel filtration chromatography, and anion-exchange chromatography. According to the data from sodium dodecyl sulfate polyacrylamide gel electrophoresis, the molecular mass of the purified enzyme was estimated to be 47.5 kDa. The purified enzyme could convert laminarin into monosaccharides and disaccharides, but had no killer toxin activity. The optimal pH and temperature of the purified enzyme were 4.0 and 40°C, respectively. The enzyme was significantly stimulated by Li⁺, Ni²⁺, and Ba²⁺. The enzyme was inhibited by phenylmethylsulfonyl fluoride, iodoacetic acid, ethylenediamine tetraacetic acid, ethylene glycol bis(2-aminoethyl ether)-N,N,N′,N′-tetraacetic acid, and 1,10-phenanthroline. The K _m and V _max values of the purified enzyme for laminarin were 3.07 mg/ml and 4.02 mg/min ml, respectively. Both matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectroscopy and DNA sequencing identified a peptide YIEAQLDAFEKR which is the conserved motif of the β-1,3-glucanases from other yeasts.     

β-Carbon stereoselectivity of N-carbamoyl-d-α-amino acid amidohydrolase for α,β-diastereomeric amino acids

N-Carbamoyl-d-α-amino acid amidohydrolase (d-carbamoylase) was found to distinguish stereochemistry not only at the α-carbon but also at the β-carbon of N-carbamoyl-d-α-amino acids. The enzyme selectively acted on one of the four stereoisomers of N-carbamoyl-α,β-diastereomeric amino acids. This simultaneous recognition of two chiral centers by d-carbamoylase was useful for the fine stereoselective synthesis of α,β-diastereomeric amino acids such as threonine, isoleucine, 3,4-methylenedioxyphenylserine and β-methylphenylalanine. The stereoselectivity for the β-carbon was influenced by the pH of the reaction mixture and by the bulk of the substituent at the β-carbon. Received: 18 June 1999 / Received revision: 30 July 1999 / Accepted: 6 August 1999     

Histochemical study of a number of hydrolases,including a new acid phosphatase,in various tissues of men and mice

Summary Two acid phosphatases have been demonstrated histochemically in mouse ventral prostate, seminal vesicles, coagulating glands, and liver and in human prostate. The first is the lysosomal acid phosphatase demonstrable by the Gomori technique. The second differs from thisβ-glycerophosphatase in that it splits naphthol AS phosphates but notβ-glycerophosphate; it has a different histochemical pH optimum and it is not inhibited by MoO₄ or NaF. The enzyme does not represent the “tail” of alkaline phosphatase activity as it is not inhibited by inhibitors of alkaline phosphatase and it has a different localization in liver and in human prostate. The enzyme may be membrane-bound but a lysosomal localization has still to be confirmed.     

Quick and Simple Detection Technique to Assess the Binding of Antimicrotubule Agents to the Colchicine-Binding Site

Development of antimitotic binding to the colchicine-binding site for the treatment of cancer is rapidly expanding. Numerous antimicrotubule agents are prepared every year, and the determination of their binding affinity to tubulin requires the use of purified tubulins and radiolabeled ligands. Such a procedure is costly and time-consuming and therefore is limited to the most promising candidates. Here, we report a quick and inexpensive method that requires only usual laboratory resources to assess the binding of antimicrotubules to colchicine-binding site. The method is based on the ability of N,N'-ethylene-bis(iodoacetamide) (EBI) to crosslink in living cells the cysteine residues at position 239 and 354 of β-tubulin, residues which are involved in the colchicine-binding site. The β-tubulin adduct formed by EBI is easily detectable by Western blot as a second immunoreacting band of β-tubulin that migrates faster than β-tubulin. The occupancy of colchicine-binding site by pertinent antimitotics inhibits the formation of the EBI: β-tubulin adduct, resulting in an assay that allows the screening of new molecules targeting this binding site.     

β-Galactosidase-deficient mouse as an animal model for GM1-gangliosidosis

GM1-gangliosidosis is a progressive neurological disease in humans caused by deficiency of lysosomal acid β-galactosidase, which hydrolyses the terminal β-galactosidic residue from ganglioside GM1 and other glycoconjugates. In this study, we generated a mouse model for GM1-gangliosidosis by gene targeting in embryonic stem cells. The mouse homozygous for the disrupted β-galactosidase gene showed β-galactosidase deficiency, presented with progressive spastic diplegia, and died of emaciation at 7–10 months of age. Pathologically, PAS-positive intracytoplasmic storage was observed in neuronal cells of various areas in the brain. Biochemical analysis revealed a marked accumulation of ganglioside GM1 and asialo GM1 in brain tissue. This animal model will be useful for pathogenetic analysis and therapeutic trial of human GM1-gangliosidosis. This revised version was published online in November 2006 with corrections to the Cover Date.     

Single Molecule Assays Reveal Differences Between In Vitro and In Vivo Synthesized β-Galactosidase

Genomic DNA was extracted from wild-type Escherichia coli strains ATCC 35321 and 8677. The lac Z gene was amplified and used as a template for in vitro synthesis of β-galactosidase. In addition the enzyme was synthesized in vitro with a C-terminal His₆ tag. The enzyme expression was also induced in these strains using isopropyl-β-D-galactoside. Single enzyme molecule assays were performed using a capillary electrophoresis-based protocol on both the in vitro and in vivo synthesized enzyme. In vivo produced enzyme from strains 35321 and 8677 showed average combined turnover numbers for the 4 active sites of the individual enzyme molecules of 53,400 ± 18,400 (N = 139) and 34,300 ± 17,800 min⁻¹ (N = 181) respectively. Average combined turnover numbers of 35,800 ± 20,900 (N = 302) and 31,700 ± 17,700 min⁻¹ (N = 315) were obtained respectively for the in vitro synthesized enzyme from strain 35321 with the absence and presence of a C-terminal His₆ tag. For strain 8677, the average combined turnover numbers were 29,000 ± 17,900 (N = 288) and 25,200 ± 12,600 min⁻¹ (N = 240) respectively for the absence and presence of a C-terminal His₆ tag. The average combined turnover numbers of the enzyme from both strains synthesized in vivo and in vitro and with the presence and absence of a His₆ tag were found to differ significantly. This indicates that the in vivo and in vitro produced enzymes are not identical and the presence of a C-terminal His₆ tag alters the activity of β-galactosidase.     

Efficient enzymatic synthesis of 4-methylumbelliferyl N-acetyllactosaminide and 4-methylumbelliferyl sialyl N-acetyllactosaminides employing beta-D-galactosidase and sialyltransferases

4-Methylumbelliferyl N-acetyllactosaminide and 4-methylumbelliferyl sialyl N-acetyllactosaminides, which are used for the assay of sialytransferase, neuraminidase and fucosyltransferase, were synthesized, respectively, by the β-D-galactosidase from Bacillus circulans and by a recombinant rat α2,3-(N)-sialyltransferase or rat liver α2,6-(N)-sialyltransferase with CMP-N-acetylneuraminic acid as donor.     

Localization of lysosomal and digestive enzymes in cytoplasmic vacuoles in caerulein-pancreatitis

Summary Intracellular localization and enzymatic activities of lysosomal enzymes (cathepsin B,N-acetyl-β-glucosaminidase, and β-glucuronidase) were studied in control rats and after induction of caerulein pancreatitis. In control rats high enzymatic activities were found in the postnuclear 1000g fraction (purified zymogen granules). The corresponding subcellular fraction in pancreatitis animals additionally contained larger secretory vacuoles and autophagosomes and revealed a marked increase in lysosomal enzyme activities. Immunolabelling studies at the ultrastructural level for trypsinogen and cathepsin B demonstrated a colocalization of lysosomal and digestive enzymes in zymogen granules in healthy controls. After induction of pancreatitis immunolabelling still demonstrated a colocalisation of cathepsin B and trypsinogen in secretory granules and newly formed Golgi-derived secretory vacuoles. Concomitantly appearing autophagosomes were, however, only labelled for cathepsin B. It is concluded that segregation of lysosomal and digestive enzymes is incomplete in normal acinar cells resulting in a colocalization in zymogen granules. In pancreatitis colocalization in secretory granules is maintained, whereas only lysosomal enzymes were sufficiently transferred into autophagic vacuoles. No indication for impaired mechanisms of molecular sorting of lysosomal and digestive enzymes in caerulein-induced pancreatitis was found.     

A screening method for β-glucan hydrolase employing Trypan Blue-coupled β-glucan agar plate and β-glucan zymography

A new screening method for β-(1,3–1,6) glucan hydrolase was developed using a pure β-glucan from Aureobaisidum pullulans by zymography and an LB-agar plate. Paenibacillus sp. was screened as a producer a β-glucan hydrolase on the Trypan Blue-coupled β-glucan LB-agar plate and the activity of the enzyme was analyzed by SDS-β-glucan zymography. The β-glucan was not hydrolyzed by Bacillus spp. strains, which exhibit cellulolytic activity on CMC zymography. The gene, obtaining by shotgun cloning and encoding the β-glucan hydrolase of Paenibacillus sp. was sequenced.     

Purification and characterization of a <Emphasis Type="Italic">N</Emphasis>-acetylglucosaminidase produced by <Emphasis Type="Italic">Talaromyces emersonii</Emphasis> during growth on algal fucoidan

A β-N-acetylglucosaminidase produced by a novel fungal source, the moderately thermophilic aerobic ascomycete Talaromyces emersonii, was purified to apparent homogeneity. Submerged fermentation of T. emersonii, in liquid medium containing algal fucoidan as the main carbon source, yielded significant amounts of extracellular N-acetylglucosaminidase activity. The N-acetylglucosaminidase present in the culture-supernatant was purified by hydrophobic interaction chromatography and preparative electrophoresis. The enzyme is a dimer with molecular weight and pI values of 140 and 3.85, respectively. Substrate specificity studies confirmed the glycan specificity of the enzyme for N-acetylglucosamine. Michaelis-Menten kinetics were observed during enzyme-catalyzed hydrolysis of the fluorescent substrate methylumbelliferyl-β-D-N-acetylglucosaminide at 50°C, pH 5.0 (K_m value of 0.5 mM). The purified N-acetylglucosaminidase displayed activity over broad ranges of pH and temperature, yielding respective optimum values of pH 5.0 and 75°C. The T. emersonii enzyme was less susceptible to inhibition by N-acetylglucosamine and other related sugars than orthologs from other sources. The enzyme was sensitive to Hg²⁺, Co²⁺ and Fe³⁺.