Determination of iron in heme compounds

Hierarchy of globin complexes. The quaternary structure of the extracellular chlorocruorin of Eudistylia vancouverii

The molecular dimensions of the extracellular, hexagonal bilayer chlorocruorin of the polychaete Eudistylia vancouverii, determined by scanning transmission electron microscopy (STEM) of negatively stained specimens, were diameter of 27.5 nm and height of 18.5 nm. STEM mass measurements of unstained, freeze-dried specimens provided a molecular mass (Mm) of 3480 +/- 225 kDa. The chlorocruorin had no carbohydrate and its iron content was 0.251 +/- 0.021 wt%, corresponding to a minimum Mm of 22.4 kDa. Mass spectra and nuclear magnetic resonance spectra of the prosthetic group confirmed it to be protoheme IX with a formyl group at position 3. SDS/polyacrylamide gel electrophoresis, reversed-phase chromatography and N-terminal sequencing suggested that the chlorocruorin consists of at least three chains of approximately 30 kDa and five chains of approximately 16 kDa; the two types of subunits occur in the ratio 0.26:0.74(+/- 0.08). Complete dissociation of the chlorocruorin at neutral pH in the presence of urea or guanidine hydrochloride, followed by gel filtration, produced elution profiles consisting of three peaks, B, C and D. Fractions B and C consisted of the approximately 16 kDa chains and fraction D consisted of the approximately 30 kDa subunits. Mass measurements of particles in STEM images of unstained, freeze-dried fractions B and C provided Mm of 208 +/- 23 kDa and 65 +/- 12 kDa, respectively, in agreement with 191 +/- 13 kDa and 67 +/- 5 kDa obtained by gel filtration. Particles with Mm = 221 +/- 21 kDa were also observed in STEM images of unstained, freeze-dried chlorocruorin. These results imply that the chlorocruorin structure, in addition to the approximately 30 kDa linker subunits that have 0.26 to 0.47 heme groups/chain, comprises approximately 65 kDa tetramers and approximately 200 kDa dodecamers (trimers of tetramers) of globin chains. The stoichiometry of the tetramer and linker subunits calculated from molar amino acid compositions was 34 +/- 4 and 43 +/- 9. The complete dissociation of the chlorocruorin was accompanied by a 50 to 75% loss of the 55 +/- 14 Ca2+/mol protein, and was decreased to approximately 35% by the presence of 10 to 25 mM-Ca2+. Reassociation of dissociated chlorocruorin was maximal in the presence of 2.5 to 5 mM-Ca2+. The dodecamer and/or tetramer subunits in the absence or presence of Ca2+ exhibited very limited (less than 10%) reassociation into hexagonal bilayer structures, only in the presence of the linker subunit.(ABSTRACT TRUNCATED AT 400 WORDS)

Iron and heme contents of the extracellular hemoglobins and chlorocruorins of annelids

1. A survey of the literature on the extracellular hemoglobins and chlorocruorins of over 30 species of annelids, covering the last 30 years, shows that the range of iron content is 0.211-0.265 wt.% (mean = 0.228 +/- 0.013, N = 28) and the range of the heme content is 1.83-3.64 wt.% (mean = 2.60 +/- 0.38, N = 29). 2. There is relatively little scatter in the values of the experimental iron contents and only one of the 28 values is clearly outside the standard deviation range. 3. The values of heme contents are much more scattered, with seven values, clearly outside the standard deviation limits. 4. The aberrant cases are discussed and it is noted that the mean heme content of 2.60 wt.% corresponds to an iron content of 0.236 wt.% in excellent agreement with the mean iron content of 0.228 wt.%. 5. This result suggests strongly that experimental values of iron and heme contents outside the ranges of 0.211-0.243 and 2.3-2.7 wt.%, respectively, corresponding to a minimum molecular mass outside the range 23,000-26,000, should be regarded with caution.

Quaternary structure of the giant extracellular hemoglobin of the leech Macrobdella decora

The molecular dimensions of the extracellular hemoglobin of the leech Macrobdella decora, determined by scanning transmission electron microscopy were 29.8 nm x 19.5 nm (diameter x height) for negatively stained specimens. Measurements of molecular mass (Mm) of unstained specimens with the microscope gave Mm = 3560 +/- 160 kDa. Small-angle X-ray scattering measurements gave a diameter of 28.0(+/- 0.5) nm, radius of gyration 10.5(+/- 0.2) nm and volume 7500(+/- 300) nm3. The hemoglobin had no carbohydrate and its iron content was found to be 0.23(+/- 0.02)% (w/w), corresponding to a minimum Mm of 24,000(+/- 1300) kDa. SDS/polyacrylamide gel electrophoresis of the unreduced hemoglobin showed that it consisted of three subunits, which have apparent Mm values of 12 (1), 25 (2) and 29 kDa (3). The reduced hemoglobin consisted of four subunits, I (12 kDa), II (14 kDa), III (26 kDa) and IV (30 kDa). Subunit 1 corresponded to subunit I, subunit 2 to subunits III and IV and subunit 3 to subunit II. Partial N-terminal sequences were obtained for subunit 1, the two chains of subunit 2 and one of the two chains of subunit 3, suggesting that the hemoglobin consists of at least five different polypeptide chains. The percentage fraction of the three unreduced subunits was determined by densitometry of SDS/polyacrylamide gel patterns and quantitative determination of Coomassie R-250 dye bound to the individual bands in reduced and unreduced patterns to be, monomer (subunit I) : non-reducible subunit (subunit 2) : reducible dimer (subunit 3) = 0.35 : 0.29 : 0.35 (S.D. = +/- 0.05). This corresponded to a stoichiometry of 74 +/- 11 : 37 +/- 5 : 38 +/- 6, assuming the molecular masses to be 17 kDa, 30 kDa and 34 kDa, taking into account the anomalously high mobility of annelid globins in SDS-containing gels. The stoichiometry calculated from the amino acid compositions of the hemoglobin and the three subunits was 82 +/- 12 : 29 +/- 4 : 40 +/- 8. Gel filtration of the hemoglobin at pH 9.8, at neutral pH subsequent to dissociation at pH 4 and at neutral pH in the presence of urea and Gu.HCl provided no evidence for the existence of a putative 1/12 of the whole molecule (Mm approx. 300 kDa). Furthermore, the largest subunits obtained had Mm of 60 to 100 kDa and had a much decreased content of subunit 2, suggesting that the hemoglobin was not a simple multimeric protein. Three-dimensional reconstruction from microscope images provided a model of Macrobdella hemoglobin that is very similar to the reconstruction of Lumbricus hemoglobin: the radial mass distribution curves are virtually superimposable.(ABSTRACT TRUNCATED AT 400 WORDS)

Iron acquisition by Haemophilus influenzae

The mechanisms for acquisition of iron by Haemophilus influenzae and their role in pathogenesis are not known. Heme and nonheme sources of iron were evaluated for their effect on growth of type b and nontypable strains of H. influenzae in an iron-restricted, defined medium. All 13 strains acquired iron from heme, hemoglobin, hemoglobin-haptoglobin, and heme-hemopexin. Among nonheme sources of protein-bound iron, growth of H. influenzae was enhanced by partially saturated human transferrin but not by lactoferrin or ferritin. Purified ferrienterochelin and ferridesferrioxamine failed to provide iron to H. influenzae, and the supernatants of H. influenzae E1a grown in iron-restricted medium failed to enhance iron-restricted growth of siderophore-dependent strains of Escherichia coli, Salmonella typhimurium, and Arthrobacter terregens. Marked alterations in the profile of outer membrane proteins of H. influenzae were observed when the level of free iron was varied between 1 microM and 1 mM. Catechols were not detected in the supernatants of strain E1a; however, iron-related hydroxamate production was detected by two biochemical assays. We conclude that the sources of iron for H. influenzae are diverse. The significance of hydroxamate production and iron-related outer membrane proteins to H. influenzae iron acquisition is not yet clear.

The dissociation of the extracellular hemoglobin of Tubifex tubifex at extremes of pH and its reassociation upon return to neutrality

The dissociation of the extracellular hemoglobin of Tubifex tubifex at alkaline and acid pH, and its reassociation upon return to neutral pH, was investigated using gel filtration, ultracentrifugation, and polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS-PAGE). Tubifex hemoglobin dissociated at pH above 8 and below 6; both dissociations appeared to be equilibrium processes. The extent of dissociation increased as the pH moved away from neutrality; although dissociation was virtually complete at pH 11, its extent at acid pH did not exceed 50-60% at pH 4. Ca(II), Mg(II), and Sr(II) cations over the range 1-100 mM decreased the extent of the dissociation only at alkaline pH. The visible absorption spectrum of the oxyhemoglobin remained unaltered in the pH range 4-9. At more extreme pH, it changed with time, altering irreversibly to that of the aquo ferri form. Gel filtration of the hemoglobin at both extremes of pH showed that it dissociated into two heme-containing fragments; one consisting of subunit 1 (Mr approximately 17,000) and the other containing subunits 2, 3, and 4 of the hemoglobin (Mr approximately 60,000). Upon return to neutral pH, the dissociated fragment reassociated to the extent of 50 to 80% to whole hemoglobin molecules. The reassociation decreased with increase in alkaline pH, and with decrease in acid pH to which the hemoglobin had been exposed; it increased in the presence of Ca(II), Sr(II), and Mg(II) only subsequent to dissociation at alkaline pH. The SDS-PAGE patterns, gel-filtration elution volumes, and alpha-helical contents, determined from circular dichroism at 222 nm, of the reassociated whole molecules were identical to those of the native hemoglobin.

X-ray fluorescence spectrometric determination of the metal content of the extracellular hemoglobin of Tubifex tubifex

X-ray fluorescence spectrometric analysis of Tubifex hemoglobin, using the energy-dispersive and wavelength-dispersive spectrometers, demonstrated that in addition to Fe, Ca and small amounts of Zn, Cu and Pb were also present. Assuming that Tubifex hemoglobin contains 160 atoms of Fe per molecular weight of 3.9 10(6), the molar ratios of Fe:Ca:Zn:Cu:Pb were calculated to be 160:70:2:8: 2. The iron content of the hemoglobin was also determined spectrophotometrically using the formation of Fe(II)-1,10-phenanthroline and Fe(II)-4-(2-pyridylazo)-resorcinol complexes: it was found to be 0.240+/-0.010% by weight and was unaffected by prior dialysis against 10 mM EDTA solution at neutral pH. Thus, it is unlikely that Tubifex hemoglobin contains any non-heme iron.

The erythrocruorin of Eisenia fetida. I. Properties and subunit structure

The erythrocruorin of Eisenia fetida possesses a relative molecular mass, determined by sedimentation equilibrium, of (3.82 +/- 0.05) . 10(6). According to the iron and heme contents, 0.218 +/- 0.008% and 2.34 +/- 0.02% by mass, respectively, it contains 144 hemes per molecule. The dimensions of the molecule observed by electron microscopy are 25.0 X 16.5 nm (diameter X height). SDS-polyacrylamide gel electrophoresis indicates that the erythrocruorin consists of six subunits (Mr 14,900, 15,300, 17,200, 19,700, 31,600 and 40,000). Oxygen binding studies showed that E. fetida erythrocruorin has a high oxygen affinity (P50 = 2.8 Torr at pH 7.5), exhibits a slight bohr effect and possesses a high cooperativity with the Hill coefficient h = 3.7-4.8. Treatment of the erythrocruorin either by freezing and thawing or by aging or exposure to alkaline pH converts it irreversibly into a state of lower cooperativity with h = 2.0-2.6. A model of the subunit structure of the erythrocruorin is proposed which takes into account the physiochemical and oxygen-binding properties of the erythrocruorin and the subunits obtained upon its dissociation.

Molecular architecture of annelid erythrocruorins. Extracellular hemoglobin of Arenicola marina (Polychaeta)

The respiratory protein, erythrocruorin, of the annelid Arenicola marina was investigated. Spectral properties show many analogies with those of vertebrate hemoglobine. 144 heme groups (ferroprotoporphyrin) were found in the whole molecule, which has a relative molecular mass of 3.56 X 10(6), as determined by sedimentation equilibrium, and an isoelectric point of 4.69. Protein dissociation patterns were analysed by electrophoresis after denaturation in the presence of dodecylsulfate, with and without 2-mercaptoethanol. A tentative model associating molecular mass of the native molecule, heme content, molecular mass of the polypeptide chains and functional properties is proposed. A twelfth subunit of A. marina erythrocruorin would contain twelve heme groups arranged in three functional units made up of four protomers, half of these being covalently bound to non-heme chains; two structural chains would be spatially arranged as bonds between the subunits.

Subunits of the extracellular hemoglobin of Arenicola marina

The molecular weight of the extracellular hemoglobin of Arenicola marina determined by equilibrium sedimentation is 3.74 +/- 0.12 . 10(6). Its iron content is 0.244 +/- 0.005 wt.% corresponding to a minimum molcular weight of 22 900 +/- 500. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate showed that the hemoglobin dissociated into three subunits: 14 000 (subunit 1), 31 000 (subunit 2) and 49 000 (subunit 3); in the presence of 2-mercaptoethanol four subunits were observed, 14 000 (subunit I), 16 000 (subunit II), 31 000 (subunit III), and 35 000 (subunit IV). Two-dimensional electrophoresis showed that subunit 1 produced subunit I, and subunits 2 and 3 produced subunits I, II and variable amounts of subunits III and IV. Gel filtration of reduced and alkylated A. marina hemoglobin in 6 M guanidinium hydrochloride suggests that the molecular weight of subunits I and II is 17 500 +/- 1000. A. marina hemoglobin appears to consist of at least 5--7 polypeptide chains. It is proposed that some of the polypeptide chains can associate to form dimers (subunits 2, III, IV) or trimers (subunit 3).

Structure-volume relationships in hemoglobin. A densitometric and dilatometric study of the oxy leads to deoxy transformation

Partial volume measurements have been performed for human hemoglobin, both on the oxygenated and deoxygenated forms. Density measurements (by pycnometry) give vHbO2 = 0.752 +/- 0.002 and vHb =0.753 +/- 0.006 for the partial specific volume and do not distinguish between the two different structures. Differential measurements, by dilatometry, however, show a signigicantly higher molal volume (of about 50 cm3/mol hemoglobin tetramer) for the deoxy over the oxygenated from at pH 7. The same reaction, at pH 9, gives a much smaller increase or even a decrease of volume. The different volume changes at pH 7 and at pH 9 are not due to the so-called Bohr ionization but to the weakening, at pH 9 compared to pH 7, of stabilising salt linkages in the deoxy structure.

Physical properties and subunits of Haemopis grandis erythrocruorin

The erythrocruorin of the leech Haemopis grandis possessed an S20,w of 57 S at neutral pH, its isoelectric point at pH 6.0 and exhibited a slightly sigmoid oxygenation curve with n approximately 2.1 and P50 = 11.2 mm at pH 7.4. A minimum molecular weight of 24000 +/- 1500 per heme group was determined from the iron and heme contents, 0.22 +/- 0.01 and 2.73 +/- 0.14 weight %, respectively. The subunit composition of the erythrocruorin was investigated using gel filtration in sodium dodecyl sulfate and polyacrylamide gel electrophoresis in sodium dodecyl sulfate at neutral pH. Haemopis erythrocruorin dissociated in the presence of sodium dodecyl sulfate into four subunits (1 through 4) possessing molecular weights of about 27000, 23000, 21000 and 13500, respectively. When the erythrocruorin was reduced with mercaptoethanol prior to sodium dodecyl sulfate electrophoresis, three subunits were observed, possessing molecular weights of about 13000 (I), 16500 (II) and 28000 (III). Sodium dodecyl sulfate electrophoresis of the isolated subunits 1 through 4 showed that subunit I was provided by subunits 1 and 4, subunit II was provided by subunit 1 and subunit III was provided by both subunit 2 and subunit 3. Haemopis erythrocruorin thus appeared to consist of at least five different polypeptide chains. It is likely that not all of the constituent polypeptide chains were associated each with a heme group. The shape of the Haemopis erythrocruorin observed by electron microscopy appeared to be consistent with the two-tiered hexagonal array characteristic of annelid erythrocruorins and chlorocruorins.

Physical properties and subunits of DNA dubia erythrocruorin

The erythrocruorin of the freshwater leech Dina dubia possessed an S20,w of 61 S and exhibited a slightly sigmoid oxygenation curve with n congruent to 1.6 and P50 = 2.4 mm at pH 7.4. A minimum mol. wt of 23 000 +/- 2100 per heme group was determined from the iron and heme contents, 0.22 +/- 0.02 and 2.92 +/- 0.35 weight %. The subunit composition of this erythrocruorin was investigated using polyacrylamide gel electrophoresis and gel filtration in sodium dodecyl sulfate at neutral pH and gel filtration at pH 9. Sodium dodecyl sulfate electrophoresis of Dina erythrocruorin revealed the presence of five subunits (1-5) with mol. wts of about 13 000, 21 000, 23 000, 25 000 and 31 000, respectively. When the erythrocruorin was reduced with mercaptoethanol prior to dodecyl sulfate electrophoresis, three subunits (I-III) were observed, two possessing molecular weights in the range 12 000-14 000 (I and II) and one possessing a molecular weight of about 28 000. One of the subunits I, II, was provided by the dissociation of the 31 000 subunit. Subunit III (28 000) consisted of subunits 2, 3, and 4. It is likely that not all of the polypeptide chains constituting Dina erythrocruorin are associated each with a heme group.