The interaction between hemoglobin and two surfactants with different charges

The interactions of hemoglobin (Hb) with sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (DTAB) are investigated by several methods. We observed the formation of hemichrome below the critical micelle concentration (cmc) of surfactant and the release of heme from Hb above the cmc. When pH value of Hb/surfactant system is lower than isoelectric point (pI) of Hb, the interaction of SDS with Hb is both electrostatic and hydrophobic, while the interaction of DTAB with Hb is hydrophobic mainly. On the contrary, when pH > pI, the interaction of SDS with Hb is hydrophobic mainly, while the interaction of DTAB with Hb is both electrostatic and hydrophobic. In the case where both the electrostatic interaction and hydrophobic interaction exist, the electrostatic interaction plays a more important role. Thus, SDS tends to interact with Hb more obviously than DTAB does when pH < pI and the interaction between DTAB and Hb is stronger when pH > pI.     

Nontransferrin-bound Iron in Serum of Patients Receiving Bone Marrow Transplants

Nontransferrin-bound iron (NTBI) and other parameters of iron status were measured in 40 patients undergoing bone marrow transplantation (BMT) prior to conditioning therapy (between day −10 and −7), at the time of BMT (day 0), and 2 weeks later (day +14). Serum iron and transferrin saturation values were normal before conditioning therapy. At day 0 serum iron values were high and median transferrin saturation was 98% (changes in the values of both serum iron and transferrin saturation, p < .0001). Transferrin saturation values were still elevated 2 weeks posttransplant (day +14 vs. baseline values, p = .0001). Starting at low NTBI levels pretransplant (median 0.4 , range 0–4.2 , controls: ≤ 0.4 ), all patients revealed high levels on day 0 (median 4.0 , range 1.9–6.9 , p < .0001) and 2 weeks posttransplant (median 2.7 , range 0–6.2 , p < .0001). These observations indicate that the plasma iron pool in patients undergoing BMT increases to a level at which the normal ability to sequestrate iron becomes exhausted and considerable amounts of NTBI appear in serum. This “free” form of iron can mediate the production of reactive oxygen species and may cause organ toxicity in the early posttransplantation period. © 1997 Elsevier Science Inc.     

Purification and properties of a major isoform of β-1,3-glucanase from pearl millet seedlings

A major isoform of β-1,3-glucanase from pearl millet seedlings was purified following ammonium sulfate precipitation, ion-exchange chromatography and gel filtration techniques. The enzyme had a molecular weight of 20.5 kDa on SDS–PAGE and was highly basic with a pI of 9.6. It was thermostable with a broad temperature optima for activity ranging from 37 to 70°C and had an optimum pH of 5.2. Mercuric chloride and para-chloromercuric benzoate inhibited completely the enzyme while manganese chloride activated it. Antibodies raised against the purified β-1,3-glucanase identified another protein with an apparent molecular weight of 30 kDa in western reactions. Significance of this enzyme in pearl millet–downy mildew host–pathogen interaction is discussed.     

Protocol to determine accurate absorption coefficients for iron-containing transferrins

An accurate protein concentration is an essential component of most biochemical experiments. The simplest method to determine a protein concentration is by measuring the A₂₈₀ using an absorption coefficient (ε) and applying the Beer-Lambert law. For some metalloproteins (including all transferrin family members), difficulties arise because metal binding contributes to the A₂₈₀ in a nonlinear manner. The Edelhoch method is based on the assumption that the ε of a denatured protein in 6 M guanidine-HCl can be calculated from the number of the tryptophan, tyrosine, and cystine residues. We extend this method to derive ε values for both apo- and iron-bound transferrins. The absorbance of an identical amount of iron-containing protein is measured in (i) 6 M guanidine-HCl (denatured, no iron), (ii) pH 7.4 buffer (nondenatured with iron), and (iii) pH 5.6 (or lower) buffer with a chelator (nondenatured without iron). Because the iron-free apoprotein has an identical A₂₈₀ under nondenaturing conditions, the difference between the reading at pH 7.4 and the lower pH directly reports the contribution of the iron. The method is fast and consumes approximately 1 mg of sample. The ability to determine accurate ε values for transferrin mutants that bind iron with a wide range of affinities has proven to be very useful; furthermore, a similar approach could easily be followed to determine ε values for other metalloproteins in which metal binding contributes to the A₂₈₀.     

Comparison of different techniques for the analysis of metallothionein isoforms by capillary electrophoresis

We have investigated free-solution capillary electrophoresis (FSCE) and micellar electrokinetic capillary chromatography (MECC) separations of metallothionein (MT) isoforms conducted in uncoated and surface-modified fused-silica capillaries. At alkaline pH, FSCE rapidly resolves isoforms belonging to the MT-1 and MT-2 charge classes. At acidic pH, additional resolution of MT isoforms is achieved. The use of high-ionic-strength (0.5 M) phosphate buffers can result in high peak efficiencies and increased resolution for some MT isoforms. Interior capillary surface coatings such as polyamine and linear polyacrylamide polymers permit separation of MT isoforms with enhanced resolution through their effects on electroosmotic flow (EOF) and protein-wall interactions. Improvements in MT isoform resolution can also be achieved by MECC using 100 mM borate buffer pH 8.4 containing 75 mM SDS. Deproteinization of tissue cytosol samples with acetonitrile (60–80%) or perchloric acid (7%) produces extracts that can be subjected to direct analysis of MT by FSCE or MECC. We conclude that optimal separation of MT isoforms by capillary electrophoresis (CE) can be achieved with the appropriate combination of different capillaries, buffers and sample preparation techniques.     

Receptor-modulated iron release from transferrin: differential effects on N- and C-terminal sites

Iron release to PPi from N- and C-terminal monoferric transferrins and their complexes with transferrin receptor has been studied at pH 7.4 and 5.6 in 0.05 M HEPES or MES/0.1 M NaCl/0.01 M CHAPS at 25 degrees C. The two sites exhibit kinetic heterogeneity in releasing iron. The N-terminal form is slightly less labile than its C-terminal counterpart at pH 7.4, but much more facile in releasing iron at pH 5.6. At pH 7.4, iron removal by 0.05 M pyrophosphate from each form of monoferric transferrin complexed to the receptor is considerably slower than from the corresponding free monoferric transferrin. However, at pH 5.6, complexation of transferrin to its receptor affects the two forms differently. The rate of iron release to 0.005 M pyrophosphate by the N-terminal species is substantially the same whether transferrin is free or bound to the receptor. In contrast, the C-terminal form releases iron much faster when complexed to the receptor than when free. Urea/PAGE analysis of iron removal from free and receptor-complexed diferric transferrin at pH 5.6 reveals that its C-terminal site is also more labile in the complex, but its N-terminal site is more labile in free diferric transferrin. Thus, the newly discovered role of transferrin receptor in modulating iron release from transferrin predominantly involves the C-terminal site. This observation helps explain the prevalence of circulating N-terminal monoferric transferrin in the human circulation.     

Some properties of mitochondria, mitoplasts and submitochondrial particles of different polarities from plant tissues

Irrespective of the starting material, i.e. washed mitochondria, purified mitochondria or mitoplast from Solanum tuberosum L., submitochondrial particles of well-defined polarities can be generated by French press treatment in low-salt medium or by sonication in high-salt medium. The first treatment will result in submitochondrial particles which are more than 80% right-side-out (right-side-out particles), the second in submitochondrial particles that are more than 80% inside-out (inside-out particles). The isoelectric point (pI = 4.0) of the inside-out particles measured by cross-partition is distinctly different from the isoelectric points of the other mitochondrial membranes which exhibit pI values between 4.5 and 4.7. The surface charge density measured by 9-aminoacridine fluorescence varies in the same order from −27 mC · m⁻² for Percoll-purified mitochondria to −51 mC · m⁻² for both right-side-out and inside-out particles. Even though the charge densities for the two surfaces of the inner membrane are similar, inside-out particles are much more negatively charged at pH 7.0, since they are 6-times larger. These results clearly demonstrate that it is possible to obtain submitochondrial particles of various polarities and sizes which in turn constitute valuable tools for the study of lateral and transverse asymmetry of the inner mitochondrial membrane.     

The contribution of ionic interactions to the conformational stability and function of polygalacturonase from A. niger

Aspergillus niger produces multiple forms of polygalacturonases with molecular masses ranging from 30 to 60 kDa. The high molecular weight polygalacturonase (61 ± 2 kDa) from A. niger possesses a pH optimum of 4.3 and a pI of 3.9. The enzyme exhibited high sensitivity, both in terms of activity and structure, in the pH range of 4.3–7.0. The enzyme was irreversibly inactivated at pH 7.0. The enzyme is predominantly rich in parallel β structure. There is unfolding of the enzyme molecule between 4.3 and 7.0 resulting in irreversible loss of secondary and tertiary structure with the exposure of hydrophobic surfaces. ANS binding measurements, intrinsic fluorescence and acrylamide quenching measurements have confirmed the unfolding and exposure of hydrophobic surfaces. The midpoint of pH transition for both activity and secondary structure is 6.2 ± 0.1. The pH-induced changes of polygalacturonase confirm the role of histidine residues in structure and activity of the enzyme. The irreversible nature of inactivation is due to the unfolding induced exposure of hydrophobic surfaces leading to association/aggregation of the molecule. Size exclusion chromatography measurements have established the association of enzyme at higher pH. Urea induced unfolding measurements at pH 4.3 and 7.0 have confirmed the loss in stability as we approach neutral pH.     

Headspace solid-phase microextraction with 1-pyrenyldiazomethane on-fibre derivatisation for analysis of fluoroacetic acid in biological samples

Four molecular forms of transferrins with different iron-binding states were separated by HPLC using a pyridinium polymer column. The elution order was monoferric transferrin bound to the C-site, holotransferrin, apotransferrin and monoferric transferrin bound to the N-site. Human sera were also analyzed with the column, and ICP-MS combined with HPLC was used to detect iron in each peak. Transferrin peaks separated by HPLC were also confirmed by an immunological method. The percentages of iron saturation in transferrins obtained by the HPLC method were compared with the values calculated from clinical data.     

Optimization of ion-exchange protein separations using a vector quantizing neural network

In this work, a previously proposed methodology for the optimization of analytical scale protein separations using ion-exchange chromatography is subjected to two challenging case studies. The optimization methodology uses a Doehlert shell design for design of experiments and a novel criteria function to rank chromatograms in order of desirability. This chromatographic optimization function (COF) accounts for the separation between neighboring peaks, the total number of peaks eluted, and total analysis time. The COF is penalized when undesirable peak geometries (i.e., skewed and/or shouldered peaks) are present as determined by a vector quantizing neural network. Results of the COF analysis are fit to a quadratic response model, which is optimized with respect to the optimization variables using an advanced Nelder and Mead simplex algorithm. The optimization methodology is tested on two case study sample mixtures, the first of which is composed of equal parts of lysozyme, conalbumin, bovine serum albumin, and transferrin, and the second of which contains equal parts of conalbumin, bovine serum albumin, tranferrin, beta-lactoglobulin, insulin, and alpha -chymotrypsinogen A. Mobile-phase pH and gradient length are optimized to achieve baseline resolution of all solutes for both case studies in acceptably short analysis times, thus demonstrating the usefulness of the empirical optimization methodology.     

Induction of peroxidases and superoxide dismutases in transformed embryogenic calli of alfalfa (Medicago sativa L.)

Peroxidase (POD) and superoxide dismutase (SOD) enzyme activities were analyzed in non-regenerative transformed embryogenic lines of alfalfa (Medicago sativa L.) carrying wound-inducible oryzacystatin I (OC-I), wound-inducible oryzacystatin I antisense (OC-Ias), or hygromycin phosphotransferase (hpt) genes. All of the transformed lines analyzed had elevated levels of all POD isoforms. Three POD isoforms with pI values of approximately 4.5, 4.8, and 8.4, and one additional pair of isoforms with a pI value of approximately 8.8 were separated from tissue extracts of all transgenic lines. Isoelectrofocusing patterns revealed the induction of one isoform of SOD with a pI of about 5.6 in all transgenic lines compared with non-transformed embryogenic tissue. These results indicate that the process of transformation may disrupt redox homeostasis in alflalfa tissues.     

Separation of three mouse metallothionein isoforms by free-solution capillary electrophoresis

We have used free-solution capillary electrophoresis (FSCE) to separate three distinct mouse metallothionein (MT) isoforms, MT-1, MT-2 and MT-3. FSCE was conducted in an uncoated fused-silica capillary (57 cm × 50 μm I.D., 50 cm to detector) using 50 mM sodium phosphate buffer adjusted to pH 7.0 or 2.0. At neutral pH, each of the three isoform peaks were well resolved from a mixture with the order of migration (MT-1> MT-2> MT-3) related to the net negative charge on the protein. At acidic pH, the migration order was reversed with MT-3 migrating fastest, suggesting MT-3 had a higher net positive charge than MT-2 or MT-1. UV absorbance spectra (190–300 nm) confirmed the presence of Zn in MT-1 and MT-2. MT-3, which was saturated with Cd to stabilize the protein, gave a spectrum characteristic of the Cd---S charge transfer (shoulder at ca. 250 nm). At pH 2.0, the absorbance spectra for all three mouse MTs were characteristic of the metal-free form of the protein (apothionein). Thus, FSCE conducted at neutral pH separates MT isoforms with their metals intact, whereas at pH 2.0, both the Zn and the Cd dissociate from the protein during the run.     

A thermostable alkaline active endo-β-1-4-xylanase from Bacillus halodurans S7: Purification and characterization

A thermostable, alkaline active xylanase was purified to homogeneity from the culture supernatant of an alkaliphilic Bacillus halodurans S7, which was isolated from a soda lake in the Ethiopian Rift Valley. The molecular weight and the pI of this enzyme were estimated to be around 43 kDa and 4.5, respectively. When assayed at 70 °C, it was optimally active at pH 9.0–9.5. The optimum temperature for the activity was 75 °C at pH 9 and 70 °C at pH 10. The enzyme was stable over a broad pH range and showed good thermal stability when incubated at 65 °C in pH 9 buffer. The enzyme activity was strongly inhibited by Mn²⁺. Partial inhibition was also observed in the presence of 5 mM Cu²⁺, Co²⁺ and EDTA. Inhibition by Hg²⁺ and dithiothreitol was insignificant. The enzyme was free from cellulase activity and degraded xylan in an endo-fashion.     

Extraction and purification of phycocyanin from Calothrix sp.

The extraction and purification of phycocyanin from Calothrix sp., cyanobacteria isolated from rice fields in Cuernavaca, Morelos, Mexico is described. Phycocyanin was extracted with 2 mg of lysozyme/g wet biomass, and purified by anion chromatography using Q-Sepharose fast-flow (Pharmacia^®, 1.5 cm×10 cm) column and hydrophobic interaction chromatography with methyl macro-prep (Bio-Rad^®, 1.5 cm×20 cm) column. The purified protein showed a pI of 5.2 and has two subunits with apparent molecular mass of 21–17 kDa each. The estimated molecular mass of native purified phycocyanin was 114 kDa, suggesting a stereochemistry of (β)₃.     

Production, purification and characterization of an endopolygalacturonase from Mucor rouxii NRRL 1894

An extracellular polygalacturonase (PGase) from Mucor rouxii NRRL 1894 was purified to homogeneity by two chromatographic steps using CM-Sepharose and Superdex 75. The purified enzyme was a monomer with a molecular weight of 43100 Da and a pI of 6. The PGase was optimally active at 35 °C and at pH 4.5. It was stable up to 30 °C and stability of PGase decrease rapidly above 60 °C. The extent of hydrolysis of different pectins was decreased with increasing of degrees of esterification. Except Mn²⁺, all the examined metal cations showed inhibitory effects on the enzyme activity. The apparent K_m and V_max values for hydrolyze of polygalacturonic acid (PGA) were 1.88 mg/ml and 0.045 μmol/ml/min, respectively. The enzyme released a series of oligogalacturonates from polygalacturonic acid indicating that it had an endo-action. Its N-terminal sequence showed homologies with the endopolygalacturonase from the psychrophilic fungus Mucor flavus.     

The influence of pH on the equilibrium distribution of iron between the metal-binding sites of human transferrin.

The dependence of the metal-binding properties of transferrin on pH in the pH 6--9 range was investigated by urea/polyacrylamide-gel electrophoresis. Equations are presented for calculating the relative values of the four conditional site constants for the stepwise binding of iron to the two sites of transferrin and for calculating the equilibrium distribution of the protein among the four principal forms, apotransferrin, the C-terminal and N-terminal monoferric transferrins and diferric transferrin. The relative affinity of iron for the two sites and the co-operativity of iron-binding follow characteristic "pH titration' curves. A mathematical model that can account for the former behaviour is presented. In both cases the metal-binding sites are affected by the ionization of functional groups with apparent pKa values near physiological pH approx. 7.4. There is strong positive co-operatively in the release of protons from these groups. The results indicate that pH must be accurately controlled in studies of the differential properties of the two sites of the transferrin molecule.     

Stimulation of reduced lysozyme regeneration by transferrin and lactoferrin

Human serum transferrin, bovine lactoferrin, and hen conalbumin were investigated with respect to the ability of the bound metal to catalyze thiol oxidation. All three proteins were able to stimulate the oxidation of thiols in both reduced lysozyme and reduced glutathione. The efficiency of the metal in catalyzing thiol oxidation was not decreased by binding to transferrin, suggesting that transferrin-bound metals are completely available to both low and high molecular weight thiols. A 5 × 10^?7m concentration of transferrin isolated from serum was able to catalyze the formation of 70% of the theoretical lysozyme activity from reduced inactive lysozyme by 60 min. Increased rates of lysozyme activity formation were observed with copper-saturated transferrin. Decreased lysozyme regeneration rates were observed with the iron-saturated molecule compared to native transferrin. The results presented suggest that metalloproteins may aid in the maintenance of the steady-state cellular concentrations of low molecular weight disulfide by catalyzing the autooxidation of thiols.     

Oxidation of 1,4-alkanediols into γ-lactones via γ-lactols using Rhodococcus erythropolis as biocatalyst

Whole cells of Rhodococcus erythropolis DSM 44534 grown on ethanol, (R)- and (S)-1,2-propanediol were used for biotransformation of racemic 1,4-alkanediols into γ-lactones. The cells oxidized 1,4-decanediol (1a) and 1,4-nonanediol (2a) into the corresponding γ-lactones 5-hexyl-dihydro-2(3H)-furanone (γ-decalactone, 1c) and 5-pentyl-dihydro-2(3H)-furanone (γ-nonalactone, 2c), respectively, with an EE(R) of 40–75%. The transient formation of the γ-lactols 5-hexyl-tetrahydro-2-furanol (γ-decalactol, 1b) and 5-pentyl-tetrahydro-2-furanol (γ-nonalactol, 2b) as intermediates was observed by GC–MS. 1,4-Pentanediol (3a) was transformed into 5-methyl-dihydro-2(3H)-furanone (γ-valerolactone, 3c) whereas (R)- and (S)-2-methyl-1,4-butanediol (4a) was converted to the methyl-substituted γ-butyrolactones 4-methyl-dihydro-2(3H)-furanone (4c₁) and 3-methyl-dihydro-2(3H)-furanone (4c₂) in a ratio of 80:20 with a yield of 55%. Also cis-2-buten-1,4-diol (5a) was transformed resulting in the formation of 2(5H)-furanone (γ-crotonolactone, 5c). At the higher pH values of 8.8 the yield of lactone formed was improved; however, the enatiomeric excesses were slightly higher at the lower pH of 5.2.     

Release of iron from the two iron-binding sites of transferrin by cultured human cells: modulation by methylamine

We have investigated the effect of increasing concentrations of methylamine (5, 10, and 25 mM) on the removal of iron from the two iron-binding sites of transferrin during endocytosis by human erythroleukemia (K562) cells. The molecular forms of transferrin released from the cells were analyzed by polyacrylamide gel electrophoresis in 6 M urea. Endocytosis of diferric transferrin was efficient since greater than 10% of surface-bound protein escaped endocytosis and was released in the diferric form. Although transferrin exocytosed from control cells had been depleted of 80% of its iron and contained 65-70% apotransferrin, iron-bearing species were also released (15% C-terminal monoferric; 10% N-terminal; 10% diferric). The ratio of the two monoferric species (C/N) was 1.32 +/- 0.12 (mean +/- SD; n = 4), suggesting that iron in the N-terminal site was more accessible to cells. In the presence of methylamine there was a concentration-dependent increase in the proportion of diferric transferrin release (less than 80% at 25 mM) and a concomitant decrease in apotransferrin. Small amounts of the iron-depleted species, especially apotransferrin, appeared before diferric transferrin, suggesting that these were preferentially released from the cells. The discrepancy between the proportions of the monoferric transferrin species noted with control cells was enhanced at all concentrations of methylamine, most markedly at 10 mM when the C/N ratio was 2.4. The N-terminal site of transferrin loses its iron at a higher pH than the C-terminal site, and so by progressively perturbing the pH of the endocytic vesicle we have increased the difference between the two sites observed with control cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Isolation and characterization of a (1 → 3)-β-glucan endohydrolase from germinating barley (Hordeum vulgare): amino acid sequence similarity with barley (1 → 3, 1 → 4)-β-glucanases

A (1 → 3)-β-glucan 3-glucanohydrolase (EC 3.2.1.39) has been purified approx. 190-fold from extracts of germinating barley. The enzyme has an apparent M_r 32 000, a pI of 8.6, and a pH optimum of 5.6. Analysis of hydrolysis products released from the (1 → 3)-β-glucan, laminarin, shows that the enzyme is an endohydrolase. Sequence analysis of the 46 NH₂-terminal amino acids of the (1 → 3)-β-glucanase reveals 54% positional identity with barley (1 → 3,1 → 4)-β-glucanases (EC 3.2.1.73) and suggests a common evolutionary origin for these two classes of β-glucan endohydrolases. The barley (1 → 3)-β-glucanase also exhibits significant similarity with a (1 → 3)-β-glucanase from tobacco.