Molecular cloning and heterologous expression of an alkaline xylanase from Bacillus pumilus HBP8 in Pichia pastoris

The xynHB gene, encoding alkaline xylanase was cloned from Bacillus pumilus by a shot-gun method. The gene was cloned into vector pHBM905A, and expressed in Pichia pastoris GS115. Xylanase-secreting transformants were selected on plates containing RBB-xylan. Enzymatic activity in the culture supernatants was up to 644?U?mL^?1 and the optimal secretion time was 4 days at 25°C. SDS-PAGE showed two bands, of 32.2?kDa and 29.6?kDa, both larger than the predicted mass of 22.4?kDa based on its amino acid sequence. Zymogram analysis demonstrated that the enzyme in both bands could hydrolyze xylan. Deglycosylation by endoglycosidase H revealed that both were derived from the same protein but contain different extents of glycosylation (30 and 25%). The optimal pH and temperature of the enzyme was pH6–9 and 50°C, respectively.     

CELL-ASSOCIATED PROTEOLYTIC ENZYMES FROM MARINE PHYTOPLANKTON1

Despite their central importance in cell metabolism, little is known about proteases in marine phytoplankton. We surveyed caseinolytic and leucine aminopeptidase (LAP) activities in log-phase cultures of the chlorophyte Dunaliella tertiolecta Butcher, the diatom Thalassiosira weissflogii (Gru.) Fryxell et Hasle, the chrysophyte Isochrysis galbana Parke, the coccolithophorid Emiliania huxleyi (Lohm.) Hay et Mohler, and the cyanobacterium Synechococcus sp. (WH 5701). LAP activity was very low at pH < 6 and peaked between pH 7.5 and 8.5 in all species, whereas caseinolytic activity in most species showed only minor peaks in the pH 4–5 range and broad maxima above pH 8. Thus, acidic vacuolar proteases apparently represented only a small fraction of total protease activity. Attempts to classify proteases using selective inhibitors were inconclusive. Neither the serine/cysteine protease inhibitor leupeptin nor the aspartic protease inhibitor pepstatin. A inhibited caseinolytic or LAP activity in any species. The metalloprotease inhibitor EDTA was only effective against LAP activity in some species, causing average decreases of 30–50%, whereas the cysteine/serine protease inhibitor phenyl methyl sulfonylfluoride achieved at best a 30–60% decrease in caseinolytic activity. Caseinolytic activities were remarkably stable. At pH 7.5 and 25°C, extracts of D. tertiolecta, E. huxleyi, and Synechococcus showed no changes in activity after 24 h, whereas activity declined by less than 50% in the other species. Incubation of cell extracts for 1 h at 25°C in pH 7.5 buffer did not alter patterns of cell proteins, suggesting that endogenous proteases did not effectively degrade endogenous proteins. Casein zymograms were used to identify >200-and <20-kDa proteases in homogenates of log-phase T. weissflogii; only the smaller protease was found in D. tertiolecta. Antibodies to the ATPase subunit (C) of the conserved, chloroplastic Clp protease from Pisum cross-reacted with proteins in Synechococcus, D. tertiolecta, and I. galbana, but no cross-reactions were found for any species with antibodies against the ClpP subunit from either E. coli or Nicotiana. Our results show that phytoplankton contain a diverse complement of proteases with novel characteristics.     

Purification and some properties of the extracellular protease ofBacillus pumilus

Supernatant of a culture ofBacillus pumilus D 78 was precipitated with ethanol and chromatographed on DEAE- and CM-cellulose to isolate and purify a neutral protease with fibrinolytic and caseinolytic activity. Analysis by ultracentrifugation and immunoelectrophoresis indicate the homogeneity of the purified enzyme with the sedimentation constant s_20,w equal to 2.3. The fibrinolytic activity had a lower heat stability and was also more sensitive to pH higher than 8.0. The caseinolytic activity was stable over a wide range of pH (4.5 to 11.0). The enzyme binds acid dyes and is inhibited by Cu²⁺, Zn²⁺, Ca²⁺ and Fe³⁺, as well as byL-cysteine and KCN at a concentration of 10mM. Likewise, EDTA andp-chloromercuribenzoate show an inhibitory effect.     

Purification and Properties of API–2b→API–2c Converting Protease from Streptomyces griseoincarnatus Strain No. KTo–250, an API–2 Producer

Among three alkaline protease inhibitors (API-2a, -2b, -2c) produced by Streptomyces griseoincarnatus strain No. KTo-250, API-2b was converted to API-2c in the growing system.

The cultural conditions were examined exclusively for the production of API-2b→ API-2c converting protease in the culture filtrate. The protease was purified about 1080-folds by salting-out with ammonium sulfate, column chromatography on DEAE-cellulose and gel filtration on Sephadex G–100.

The optimal and maximal caseinolytic activities of the protease were around pH 9.0 and at 28°C, respectively. The protease activity was inhibited by EDTA and DFP, but not by PCMB, o-phenanthroline, TPCK, TLCK, AP-I and S-SI. The protease was a DFP and EDTA-sensitive alkaline protease, and it required Ca²⁺ ion for its activity and stability.     

Cost-effective and concurrent production of industrially valuable xylano-pectinolytic enzymes by a bacterial isolate Bacillus pumilus AJK

Simultaneous production of xylanase and pectinase by Bacillus pumilus AJK under submerged fermentation was investigated in this study. Under optimized conditions, it produced 315?±?16 IU/mL acidic xylanase, 290?±?20 IU/mL alkaline xylanase, and 88?±?9 IU/mL pectinase. The production of xylano-pectinolytic enzymes was the highest after inoculating media (containing 2% each of wheat bran and Citrus limetta peel, 0.5% peptone, 10?mM MgSO₄, pH 7.0) with 2% of 21-hr-old culture and incubated at 37°C for 60?hr at 200?rpm. Xylanase retained 100% activity from pH 6.0 to10.0 after 3?hr of incubation, while pectinase showed 100% stability from pH 6.0 to 9.0 even after 6?hr of incubation. Cost-effective and concurrent production of xylanase and pectinase by a bacterial isolate in the same production media suggests its potential for various biotechnological applications. This is the first report of simultaneous production of industrially important extracellular xylano-pectinolytic enzymes by B. pumilus.     

Purification and characterization of a propanol-tolerant neutral protease from Bacillus sp. ZG20

Abstract

A propanol-tolerant neutral protease was purified and characterized from Bacillus sp. ZG20 in this study. This protease was purified to homogeneity with a specific activity of 26,655?U/mg. The recovery rate and purification fold of the protease were 13.7% and 31.5, respectively. The SDS-PAGE results showed that the molecular weight of the protease was about 29?kDa. The optimal temperature and pH of the protease were 45?°C and 7.0, respectively. The protease exhibited a good thermal- and pH stability, and was tolerant to 50% propanol. Mg²⁺, Zn²⁺, K⁺, Na⁺ and Tween-80 could improve its activity. The calculated K_m and V_max values of the protease towards α-casein were 12.74?mg/mL and 28.57?µg/(min mL), respectively. This study lays a good foundation for the future use of the neutral protease from Bacillus sp. ZG20.     

Purification and Characterization of an Extracellular Alkaline Serine Protease with Dehairing Function from <Emphasis Type="Italic">Bacillus pumilus</Emphasis>

An extracellular alkaline serine protease (called DHAP), produced by a Bacillus pumilus strain, demonstrates significant dehairing function. This protease is purified by hydrophobic interaction chromatography, ion exchange, and gel filtration. DHAP had a pI of 9.0 and a molecular weight of approximately 32,000 Dalton. It shows maximal activity at pH 10 and with a temperature of 55°C; the enzyme activity can be completely inhibited by phenylmethylsulfonyl fluoride (PMSF) and diisopropyl fluorophosphates (DFP). The first 20 amino acid residues of the purified DHAP have been determined with a sequence of AQTVPYGIPQIKAPAVHAQG. Alignment of this sequence with other alkaline protease demonstrates its high homology with protease from another B. pumilus strain. Received: 17 April 2002 / Accepted: 24 May 2002     

A new kumamolisin-like protease from Alicyclobacillus acidocaldarius: an enzyme active under extreme acidic conditions

A new serine-carboxyl proteinase, called kumamolisin-ac, was purified from the thermoacidophilic bacterium Alicyclobacillus acidocaldarius. The enzyme is a monomeric protein of 45?kDa, active over a wide temperature range (5.0–70°C) and extremely acidic pHs (1.0–4.0), showing maximal proteolytic activity at pH?2.0 and 60°C. Interestingly, kumamolisin-ac displayed a significant proteolytic activity even at 5°C, thus suggesting a sort of cold-adaptation for this enzyme. The protease was remarkably stable at high temperatures (t_1/2 at 80°C, 10?h, pH?2.0) and over a broad range of pH (2.0–7.0). Substrate analysis indicated that kumamolisin-ac was active on a variety of macromolecular substrates, such as haemoglobin, hide powder azure, and azocoll. In particular, a high specific activity was detected towards collagen. The corresponding gene was cloned, expressed and the recombinant protease, was found to be homologous to proteases of the ‘S53’ family. From the high identity with kumamolisin and kumamolisin-As, known as collagenolytic proteases, kumamolisin-ac can be considered as the third collagenolytic affiliate within the ‘S53’ family. Cleavage specificity investigation of kumamolisin-ac revealed a unique primary cleavage site in bovine insulin B-chain, whereas a broad specificity was detected using bovine α-globin as substrate. Thus, kumamolisin-ac could represent an attractive candidate for industrial-scale biopeptide production under thermoacidophilic conditions.     

Production,optimization and purification of a novel extracellular protease from the moderately halophilic bacterium <Emphasis Type="Italic">Halobacillus karajensis</Emphasis>

The production of a protease was investigated under conditions of high salinity by the moderately halophilic bacterium Halobacillus karajensis strain MA-2 in a basal medium containing peptone, beef extract, maltose and NaCl when the culture reached the stationary growth phase. Effect of various temperatures, initial pH, salt and different nutrient sources on protease production revealed that the maximum secretion occurred at 34°C, pH 8.0–8.5, and in the presence of gelatin. Replacement of NaCl by various concentrations of sodium nitrate in the basal medium also increased the protease production. The secreted protease was purified 24-fold with 68% recovery by a simple approach including a combination of acetone precipitation and Q-Sepharose ion exchange chromatography. The enzyme revealed a monomeric structure with a relative molecular mass of 36 kDa by running on SDS-PAGE. Maximum caseinolytic activity of the enzyme was observed at 50°C, pH 9.0 and 0.5 M NaCl, although at higher salinities (up to 3 M) activity still remained. The maximum enzyme activity was obtained at a broad pH range of 8.0–10.0, with 55 and 50% activity remaining at pH 6 and 11, respectively. Moreover, the enzyme activity was strongly inhibited by phenylmethylsulfonyl fluoride (PMSF), Pefabloc SC and EDTA; indicating that it probably belongs to the subclass of serine metalloproteases. These findings suggest that the protease secreted by Halobacillus karajensis has a potential for biotechnological applications from its haloalkaline properties point of view.     

Production and characterization of collagenase from a new Amazonian Bacillus cereus strain

Abstract

A new collagenase producing a strain of Bacillus cereus, isolated from the pollen of a bee of Amazon Region (Brazil), had its enzyme characterized and the production medium composition and culture conditions enhanced. A two-level design on three factors, namely initial medium pH, the substrate (gelatin) concentration and agitation intensity, allowed identifying the first two variables as the most significant ones, while a central composite design (CCD) was subsequently used to identify their optimal levels. Statistics highlighted maximized collagenolytic activity when substrate concentration and initial medium pH were selected at their highest levels (positive effects), whereas agitation intensity at the lowest (negative effect). Triplicate runs performed under predicted optimal conditions (pH 7.8 and 1.7% gelatin concentration) yielded a collagenolytic activity (305.39?±?5.15?U) 4.6- to 15-fold those obtained with the preliminary design. The enzyme displayed optimum activity at 45?°C and pH 7.2, was stable over wide ranges of pH values and temperatures (7.2–11.0 and 25–50?°C, respectively) and was strongly inhibited by 10?mM phenylmethylsulphonyl fluoride. The zymogram showed two prominent bands at 50 and 76?kDa. These results are a first attempt to elucidate the features of this new collagenase, its production conditions, and possible scale-up.     

Production,extraction, and thermodynamics protease partitioning from Aspergillus tamarii Kita UCP1279 using PEG/sodium citrate aqueous two-phase systems

Abstract

The protease from Aspergillus tamarii Kita UCP1279 extraction by aqueous two-phase PEG-Citrate (ATPS) systems, using a factorial design 2⁴, was investigated. Then, the variables studied were polyethylene glycol (PEG) molar mass (M_PEG), concentrations of PEG (C_PEG) and citrate (C_CIT), and pH. The responses analyzed were the partition coefficient (K), activity yield (Y) and purification factor (PF). The thermodynamic parameters of the ATPS partition were estimated as a function of temperature. ATPS was able to pre-purify the protease (PF = 1.6) and obtained 84% activity yield. The thermodynamic parameters ΔG°_m (?10.89?kJ mol^?1), ΔH_m (?5.0?kJ?mol^?1) and partition ΔS_m (19.74?J mol^?1 K^?1) showed that the preferential migration of almost all protein contaminants of the crude extract to the salt-rich phase, while the preferred protease was the PEG rich phase. The extracted enzyme presents optimum temperature and pH at range of 40–50?°C and 9.0–11.0, respectively. Moreover, the enzyme was identified as serine protease based on inhibition profile. ATPS showed the satisfactory performance as the first step for Aspergillus tamarii Kita UCP1279 protease pre-purification.     

Aggregation properties of cold-active lipase produced by a psychrotolerant strain of Pseudomonas palleroniana (GBPI_508)

Abstract

The present study aims to exploit microbial potential from colder region to produce lipase enzyme stable at low temperatures. A newly isolated bacterium GBPI_508 from Himalayan environment, was investigated for the production of cold-active lipase emphasizing on its aggregation properties. Plate based assays followed by quantitative production of enzyme was estimated under different culture conditions. Further characterization of partially purified enzyme was done for molecular weight determination and activity and stability under varying conditions of pH, temperature, and in presence of organic solvents, inhibitors, and metal ions. The psychrotolerant bacterium was identified as Pseudomonas palleroniana following 16S rRNA gene sequencing. Maximum lipase production by GBPI_508 was recorded in 7?days at 25?°C utilizing yeast extract as nitrogen source and olive oil as substrate in the lipase production medium. Triton X-100 (1%) in the medium as emulsifier significantly enhanced the lipase production. Lipase produced by bacterium showed aggregation which was confirmed by dynamic light scattering and native PAGE. SDS-PAGE followed by zymogram analysis of partially purified enzyme showed two active bands of ～50?kDa and ～54?kDa. Optimum activity of partially purified enzymatic preparation was recorded at 40?°C while the activity remained nearly consistent from pH 7.0 to 12.0, whereas, maximum stability was recorded at pH values 7.0 and 11.0 at 25?°C. Interestingly, lipase in the partially purified fraction retained 60% enzyme activity at 10?°C. Medium chain pNP ester (C10) was the most preferred substrate for the lipase of GBPI_508. The lipase possessed >50% residual activity when incubated with different organic solvents (25% v/v) except toluene and dichloromethane which inhibited the activity below 50%. Partially purified enzyme was also stable in the presence of metal ions and inhibitors. The study suggests applicability of GBPI_508 lipase in low temperature conditions such as cold-active detergent formulations and cold bioremediation.     

Subtilisin-like proteinase secreted by the <Emphasis Type="Italic">Bacillus pumilus</Emphasis> KMM 62 strain at different growth stages

Proteinase secreted in the environment by bacilli on different growth stages was isolated by ion chromatography from the culture medium of Bacillus pumilus KMM 62. According to the hydrolysis character of specific chromogenic substrates and inhibition type, the enzyme belongs to subtilisin-like serine proteinases. The isolated proteinase with the molecular mass of 30 kDa displays maximum activity on hydrolysis of the peptide substrate Z-Ala-Ala-Leu-pNA at pH 8.0–8.5 and temperature 30°C. The protein is stable in the range of pH 7.5–10.0. It was shown that subtilisin-like serine proteinase from B. pumilus KMM 62 possessed thrombolytic activity.     

Novel extremely acidic lipases produced from Bacillus species using oil substrates

The extremely acidophilic microorganisms Bacillus pumilus and Bacillus subtilis were isolated from soil collected from the commercial edible oil and fish oil extraction industry. Optimization of conditions for acidic lipase production from B. pumilus and B. subtilis using palm oil and fish oil, respectively, was carried out using response surface methodology. The extremely acidic lipases, thermo-tolerant acidic lipase (TAL) and acidic lipase (AL), were produced by B. pumilus and B. subtilis, respectively. The optimum conditions for B. pumilus obtaining the maximum activity (1,100 U/mL) of TAL were fermentation time, 96 h; pH, 1; temperature, 50 °C; concentration of palm oil, 50 g/L. After purification, a 7.1-fold purity of lipase with specific activity of 5,173 U/mg protein was obtained. The molecular weight of the TAL was 55 kDa. The AL from B. subtilis activity was 214 U/mL at a fermentation time of 72 h; pH, 1; temperature, 35 °C; concentration of fish oil, 30 g/L; maltose concentration, 10 g/L. After purification, an 11.4-fold purity of lipase with specific activity of 2,189 U/mg protein was obtained. The molecular weight of the extremely acidic lipase was 22 kDa. The functional groups of lipases were determined by Fourier transform-infrared (FT-IR) spectroscopy.     

Single-site substitutions improve cold activity and increase thermostability of the dehairing alkaline protease (DHAP)

To engineer dehairing alkaline protease (DHAP) variants to improve cold activity and increase thermostability so these variants are suitable for the leather processing industry. Based on previous studies with bacterial alkaline proteases, double-site mutations (W106K/V149I and W106K/M124L) were introduced into the DHAP from Bacillus pumilus. Compared with the wild-type DHAP hydrolytic activity, the double-site variant W106K/V149I showed an increase in specific hydrolytic activity at 15 °C by 2.3-fold toward casein in terms of hydrolytic rate and 2.7-fold toward the synthetic peptide AAPF-pN by means of k_cat/K_m value. The thermostability of the variant (W106K/V149I) was improved with the half-life at 60 and 70 °C increased by 2.7- and 5.0-fold, respectively, when compared with the thermostability of the wild-type DHAP. Conclusively, an increase in the cold activity and thermostability of a bacterial alkaline protease was achieved by protein engineering.     

Biochemical properties of a serine protease from Aspergillus flavus and application in dehairing

The proteases are enzymes produced by several filamentous fungi with important biotechnological applications. In this work, a protease from Aspergillus flavus was characterized. The culture filtrate of A. flavus was purified to homogeneity by Sephacryl S-200 column chromatography followed by CM–cellulose. The molecular weight of the purified enzyme was estimated to be approximately 32?kDa by SDS–PAGE. The enzyme hydrolysed BTpNA (N-α-benzoyl-dl-tyrosyl-p-nitroanilide), azo-casein and casein as substrates. Optimal temperature and pH were 55?°C and 6.5, respectively. The enzyme was stimulated by Mg²⁺, Ca²⁺, Zn²⁺ and inhibited by Hg²⁺ and Ag²⁺ and Cu²⁺. The protease showed increased activity with detergents, such as Tween 80 and Triton X, and was stable to the reducing agents, such as β-mercaptoethanol. The protease activity was strongly inhibited in the presence of phenylmethylsulfonyl fluoride, indicating it is a serine protease. The enzyme entrapped in calcium alginate beads retained its activity for longer time and could be reused up to 10 times. The thermostability was increased after the immobilization and the enzyme retained 100% of activity at 45?°C after 60?min of incubation, and 90% of residual activity at 50?°C after 30?min. In contrast, the free enzyme only retained 10% of its residual activity after 60?min at 50?°C. The enzymatic preparation was demonstrated to be efficient in the capability of dehairing without destruction of the hide. The remarkable properties such as temperature, pH and immobilization stability found with this enzyme assure that it could be a potential candidate for industrial applications.     

The high expression of Aspergillus pseudoglaucus protease in Escherichia coli for hydrolysis of soy protein and milk protein

Abstract

The hydrolysates of soy protein and milk protein are nutritional and functional food ingredients. Aspergillus pseudoglaucus aspergillopepsin I (App) is an acidic protease, including signal peptide, propeptide, and catalytic domain. Here, we cloned the catalytic domain App with or without propeptide in Escherichia coli. The results showed that the App without propeptide was not expressed or did not exhibit activity and App with propeptide (proApp) was highly expressed with a specific activity of 903?U/mg. Moreover, the denaturation temperature of proApp was 4.1?°C higher than App’s. The proApp showed 104?U/mg and 252?U/mg hydrolysis activities towards soy protein and milk protein under acidic conditions. By RP-HPLC analysis, the peptides obtained from the hydrolysates of soy protein and milk protein were hydrophilic peptides. This work first demonstrates efficient proteolysis of soy protein and milk protein through the functional expression of full-length proApp, which will likely have valuable industrial applications.     

Production,characterization, and immobilization of partially purified surfactant–detergent and alkali-thermostable protease from newly isolated Aeromonas caviae

Proteolytic Aeromonas caviae P-1-1 growing at wide-ranging pH (7.0–11.0) and moderate salinity (0–5% NaCl) was isolated from cattle shed of Thanjavur, India. It produced lipase, gelatinase, and polyhydroxybutyrate. Different culture conditions, incubation time, carbon and nitrogen sources, vitamins, amino acids, surfactants, and metal ions for optimal growth and protease production of P-1-1 were examined. Maximum protease (0.128?U/mL) production was achieved with 1% fructose, 1% yeast extract, 0.1% ammonium sulfate, 3% NaCl, 0.1% CaCl₂?·?2H₂O, 1% glycine, 0.1% vitamin E, and 0.1% Tween-40 at pH 8.0 after 42?hr of incubation at 37°C. It was active over broad range of pH (7.0–12.0), temperature (15–100°C), and salinity (0–9% NaCl) with optima at pH 10.0, 55°C, and 3% NaCl. It retained 65 and 48% activities at pH 12.0 and 100°C, respectively. Partially purified protease was highly stable (100%) within pH range 7.0–12.0 and salinities of 0–5% NaCl for 48?hr. Cu²⁺, Mn²⁺, Co²⁺, and Ca²⁺ did not inhibit its activity. Its stability at extreme pHs, temperatures, and in the presence of surfactants and commercial detergents suggests its possible application in laundry detergents. Partially purified protease was immobilized and reused. This is the first report of alkali-thermotolerant, surfactant–detergent-stable partially purified extracellular protease from A. caviae.     

Extraction,purification, and biochemical characterization of serine protease from leaves of Abrus precatorius

A protease from fresh leaves of Abrus precatorius was purified using two classical chromatography techniques: ion-exchange (DEAE-Sepharose) and Gel filtration (Sephadex G-75). The purified protease showed a molecular weight of ～?28?kDa on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The optimum pH and temperature for the purified protease was 8 and 40°C, respectively. The purified protease was stable throughout a wide temperature range from 10 to 80°C and pH from 2 to 12. Protease activity was inhibited in the presence of Co²⁺, Ni²⁺, Hg²⁺, and Zn²⁺ while its activity has increased in the presence of Ca²⁺ and Mg²⁺. The protease was highly specific to casein when compared to its specificity for gelatin, bovine serum albumin, hemoglobin, and defatted flour of Ricinodendron heudelotii. Its V_max and K_m determined using casein as a substrate were 94.34?U/mL and 349.07?µg/mL respectively. Inhibition studies showed that this purified protease was inhibited by both phenylmethane sulfonyl fluoride and aprotinin which are recognized as competitive inhibitors of serine proteases.     

Fermentative production of extracellular amylase from novel amylase producer,Tuber maculatum mycelium,and its characterization

Abstract

Truffles are symbiotic hypogeous edible fungi (form of mushroom) that form filamentous mycelia in their initial phase of the growth cycle as well as a symbiotic association with host plant roots. In the present study, Tuber maculatum mycelia were isolated and tested for extracellular amylase production at different pH on solid agar medium. Furthermore, the mycelium was subjected to submerged fermentation for amylase production under different culture conditions such as variable carbon sources and their concentrations, initial medium pH, and incubation time. The optimized conditions after the experiments included soluble starch (0.5% w/v), initial medium pH of 7.0, and incubation time of 7 days, at room temperature (22?±?2?°C) under static conditions which resulted in 1.41?U/mL of amylase. The amylase thus obtained was further characterized for its biocatalytic properties and found to have an optimum activity at pH 5.0 and a temperature of 50?°C. The enzyme showed good thermostability at 50?°C by retaining 98% of the maximal activity after 100?min of incubation. The amylase activity was marginally enhanced in presence of Cu²⁺ and Na⁺ and slightly reduced by K⁺, Ca²⁺, Fe²⁺, Mg²⁺, Co²⁺, Zn²⁺, and Mn²⁺ ions at 1?mM concentration.