A screening test for detecting iron overload in population studies

A simple, robust, inexpensive and reasonably accurate screening test, which involves colorimetric assessment of the unsaturated iron-binding capacity, was used to detect significant degrees of iron overload in a field setting. It was used in a survey of 152 men aged over 40 years who had previously been identified as having serum ferritin values above 400 micrograms/l and who were therefore potentially homozygous carriers of the HLA-linked iron-loading gene responsible for the clinical disorder idiopathic haemochromatosis (IHC). Such individuals almost always have a raised transferrin saturation and the screening test was compared with a standard method of measurement. The screening test accurately identified 7 out of 10 subjects with transferrin saturations above 62%. It also accurately identified 137 out of 142 subjects with saturation below 62%. There were 5 false-positive results; in all these subjects saturations were at the upper limit of normal or marginally raised. The test thus had a sensitivity of 77%, a specificity of 97%, an accuracy of 95% and a positive predictive accuracy of 67%. The test successfully identified a subgroup of subjects with serum ferritin values above 400 micrograms/l who appeared to have more severe degrees of iron overload. The screening test, which requires only 200 microliter serum and costs only 6.2 c, should not only be of potential value in identifying subjects at risk of developing the clinical manifestations of IHC but may prove even more useful in defining the prevalence of significant iron overload in the rural black population of South Africa.

Phenotypic expression of the HLA-linked iron-loading gene in the Afrikaner population of the western Cape

A previous study conducted on a group of Afrikaans-speaking subjects in the south-western Cape indicated a high frequency (0.115) of the HLA-linked iron-loading gene which causes idiopathic haemochromatosis. The results of phenotypic and genotypic studies on the first degree relatives of identified homozygotes and heterozygotes are now reported. There was considerable heterogeneity of phenotypic expression in the group of heterozygotes, with overlap between the homozygous and heterozygous subjects. The heterozygous relatives of heterozygous index cases, who had been identified on the basis of a serum ferritin concentration greater than 400 micrograms/l, appeared to have more frequent and more marked abnormalities of iron measurements than the heterozygote relatives of homozygous index cases (serum ferritin value greater than 400 micrograms/l, percentage transferrin saturation greater than 60). This suggests that the screening test was identifying a group of more significantly affected heterozygotes, with biochemical abnormalities that overlapped with the identified homozygotes. The index cases were followed up over a period of 5 years and during this time the 7 subjects diagnosed as heterozygotes showed a progressive increase in serum ferritin concentrations, which suggests some iron accumulation. Individual pedigrees included instances of gene recombination within the major histocompatibility complex, and of probable false-positive genotype assignment. The overall results confirm a high frequency of the gene in this particular community.

Thermodynamics of carbon monoxide binding to monomeric cytochrome c'

The thermodynamic parameters for carbon binding to monomeric Rhodopseudomonas palustris cytochrome c' are determined. An enthalpy change for CO(aq) binding to the cytochrome is measured directly by titration calorimetry as -6.7 +/- 0.2 kcal/mol of heme, the CO binding equilibrium constant is measured at 35 degrees C as (1.96 +/- 0.05) X 10(5) M-1, and the binding equilibrium constant at 25 degrees C is calculated from the van't Hoff equation as (2.8 +/- 0.1) X 10(5) M-1. Comparison of the results to the known energetics of CO binding to dimeric cytochrome c', where the CO binding site is buried in the protein interior, indicates that the heme binding site on the monomer form is, in contrast, more exposed.

Amino acid sequences of cytochrome c-554(548) and cytochrome c' from a halophilic denitrifying bacterium of the genus Paracoccus

The amino acid sequences of the cytochromes c-554(548) and c' from the moderately halophilic bacterium Paracoccus sp., I.A.M. 203 (= A.T.C.C. 12084, N.C.I.B. 8669) have been determined. Cytochrome c-554(548) consists of a single polypeptide chain of 83 residues, and dimerizes strongly. The most similar protein of known sequence is the N-terminal half of the dihaem cytochrome c4, and other related proteins include the cytochrome c-554(547) of Thiobacillus neapolitanus and the cytochrome c-553 of Desulfovibrio vulgaris. Cytochrome c', which has a single polypeptide chain of 132 residues, is similar in sequence to cytochromes c' from phototrophic and denitrifying bacteria, but only shows about 36% sequence identity to the most similar protein of known sequence. Both of the Paracoccus proteins have a considerable excess of acidic amino acid side chains over basic ones, and a higher proportion of their basic amino acids is arginine than is usual in cytochromes c. Both these characteristics seem to be adaptations to increase the stability of the proteins in an environment of high ionic strength. Detailed evidence for the amino acid sequences of the proteins has been deposited as Supplementary Publication 50140 (24 pp.) at the British Library (Lending Division), Boston Spa, Yorkshire LS23 7BQ, U.K. from which copies are available on prepayment.

Kinetics of reduction by free flavin semiquinones of algal cytochromes and plastocyanin

It had been shown that plastocyanin and cytochrome c-553 are functionally interchangeable in algae and that the physiological electron transfer reactions are sensitive to ionic strength. The isoelectric points of these proteins range from very acidic to basic depending upon species, and naturally occurring amino acid substitutions of charged residues have been shown to affect the kinetics of electron transfer, presumably through alteration of protein net charge. We have now shown that these naturally occurring amino acid substitutions also affect the kinetics of nonphysiological electron transfer reactions, and that we can quantitate the extent of nonconservation of charge. The reduction of plant and algal proteins by FMN semiquinone is sensitive to ionic strength and the effects can be correlated with net protein charge with regard to sign, but not to magnitude, with the charge at the site of electron transfer varying from +3 through 0 to -3. We had previously observed in a large variety of electron transfer proteins from bacteria (G. Tollin, T. E. Meyer, and M. A. Cusanovich (1986) Biochim. Biophys. Acta 853, 29-41) that charge localized at the site of electron transfer, rather than net protein charge, was more likely to affect kinetics. This also appears to be the case with the algal proteins. By comparison of protein structures, we have been able to predict which substitutions are likely to be responsible for the kinetic effects in the algal proteins and to discuss the implications of such changes for function.

The amino acid sequence of the cytochrome c2 from the phototrophic bacterium Rhodopseudomonas globiformis

The amino acid sequence of the principal soluble cytochrome c from the phototrophic acidophilic bacterium Rhodopseudomonas (or Rhodopila) globiformis was determined. By the criteria of percentage sequence identity and fewness of internal insertions and deletions it is more similar in sequence to some mitochondrial cytochromes c than to any known bacterial cytochrome. The organism does not have any properties that commend it as being particularly similar to postulated prokaryotic precursors of the mitochondrion. We consider that the relatively high degree of sequence similarity is an instance of convergence, and is an example of the limitations that are imposed on attempts to deduce distant evolutionary relationships from sequence information. Detailed evidence for the amino acid sequence of the protein has been deposited as Supplementary Publication SUP 50136 (12 pages) at the British Library Lending Division, Boston Spa, West Yorkshire LS23 7BQ, U.K., from whom copies are available on prepayment [see Biochem. J. (1987) 241, 5].

The HLA linked iron loading gene in an Afrikaner population

The serum ferritin concentration was used as a screening test to identify the presence of iron overload in 599 Afrikaans subjects (300 males and 299 females) living in the South Western Cape, South Africa. Seventeen of the males with concentrations greater than 400 micrograms/l were reevaluated three and five years later. Serum ferritin concentrations were measured again and further diagnostic procedures were carried out. These included an assessment of alcohol intake and measurements of serum gamma glutamyltransferase, the percentage saturation of transferrin, and HLA-A,-B,-C, and -DR loci typing on the subjects as well as their families. Liver biopsies were performed on some affected subjects. Of the original 16 index subjects, four were diagnosed as homozygous for the HLA linked iron loading gene which is responsible for the clinical disease idiopathic haemochromatosis. Six appeared to be heterozygotes, three were heterozygotes who were also abusing alcohol, and two did not fit into any of the diagnostic groups. The calculated gene frequency was 0.082, with an expected heterozygote frequency of 0.148. The fact that no females were identified in the study suggested that the diagnostic criteria for homozygosity (serum ferritin greater than 400 micrograms/l and % saturation greater than 60%) were set too high. The data were therefore recalculated for the 300 males; when this was done the gene frequency was 0.115 and the heterozygote frequency 0.024. Two subjects were diagnosed as homozygotes in the study of family members and 37 as heterozygotes (33 definite and four probable). Both the homozygotes and nine of the heterozygotes showed mild to moderate disturbances of iron metabolism. There was considerable overlap between the phenotype expression in these nine heterozygotes and the homozygotes, probably as a result of setting the threshold for the serum ferritin concentrations at the relatively high value of 400 microgram/ml. By doing this a small subset of heterozygotes with biochemical abnormalities was identified. The results of the present pilot study suggest a high frequency of the HLA linked iron loading gene in the Afrikaner population of South Western Cape.

Idiopathic haemochromatosis. Family studies and results of a pilot prevalence survey

HLA typing for the A and B loci was carried out in conjunction with measurements of iron status on 38 of the first-degree relatives of 8 patients suffering from idiopathic haemochromatosis (IHC). Seven of the 8 probands had the A3 allele, 2 in the homozygous form. Seven had the B7 allele and in 5 it was present with the A3 allele on the same chromosome. Six family members were diagnosed as homozygous for the HLA-linked iron-loading gene on the basis of genotype assessment, while 27 were assessed as being heterozygous. Five of the 6 homozygous subjects had developed significant iron overload. In contrast, disturbances of iron metabolism in the heterozygotes were mild and present in only 33%. Pseudodominant inheritance of IHC was noted in one family, presumably as the result of a homozygous/heterozygous mating. A pilot epidemiological survey was carried out on 222 Afrikaans-speaking men in an attempt to find affected individuals using the serum ferritin concentration as the screening test. One homozygous subject was identified (genotype A3, B7/A3, B7) and further family studies confirmed the presence of the HLA-linked iron-loading gene. These preliminary results suggest a disease frequency of about 4-5/1,000 in the Afrikaner population.

Crystallization and preliminary analysis of crystals of cytochrome c2 from Rhodopseudomonas capsulata

Two crystal forms of the cytochrome c2 isolated from Rhodopseudomonas capsulata have been obtained. One crystal form (type I), grown from ammonium sulfate solutions at pH 7.5, belongs to the space group R32 with unit cell dimensions of a = b = 100.0 A, and c = 162.2 A in the hexagonal setting. These crystals most likely contain two molecules in the asymmetric unit. The other crystal form (type II) was obtained from polyethylene glycol 6000 solutions at pH 6.5. Type II crystals belong to the space group P3(1)21 or P3(2)21 with one molecule per asymmetric unit and unit cell dimensions of a = b = 52.4 A, and c = 87.9 A. Both crystal forms diffract to at least 1.8 A resolution and appear to be resistant to radiation damage.

The complete amino acid sequence of rubredoxin from the green phototrophic bacterium Chlorobium thiosulphatophilum strain PM

A complete amino acid sequence for the rubredoxin from the photosynthetic bacterium Chlorobium thiosulphatophilum is proposed. The sequence, a single polypeptide chain of 53 amino acids, was deduced from the sequences of peptides obtained by chymotryptic, tryptic, thermolytic or mild acid digestion. The rubredoxin shows a high degree of sequence homology with rubredoxins from non-photosynthetic bacteria, and the evolutionary implications of this are considered.

Laser flash photolysis studies of electron transfer between ferredoxin-NADP+ reductase and several high-potential redox proteins

Complex formation and the kinetics of electron transfer between ferredoxin-NADP+ reductase (FNR) and two structurally homologous acidic 4Fe-4S high-potential ferredoxins (HiPIP's) from Ectothiorhodospira halophila (HP1 and HP2) and two structurally homologous cytochromes c2 from Paracoccus denitrificans and Rhodospirillum rubrum (PC2, and RC2, respectively) have been investigated by gel filtration and laser flash photolysis techniques. Gel filtration studies indicated that complex formation occurred between FNRox and HP1ox or HP2ox at low ionic strength (10 mM) and that the complexes were completely dissociated at high ionic strength (310 mM). Laser flash photolysis using lumiflavin as the reductant demonstrated that both free HP1ox and HP2ox reacted primarily with the anionic form of fully reduced lumiflavin (LFH-), whereas FNR was unreactive. Second-order rate constants of 1 X 10(6) and 0.8 X 10(6) M-1 s-1 were obtained for these reactions at 10 mM ionic strength. Increasing the ionic strength to 310 mM resulted in an approximately 1.5-fold increase in the rate constant. Inclusion of stoichiometric amounts of FNRox into the reaction mixture at low ionic strength led to a 2.5-fold increase in the rate constants. The reaction of 5-deazariboflavin semiquinone (5-dRf.) with the oxidized HiPIP's was also investigated by laser flash photolysis. Second-order rate constants of 3.0 X 10(8) M-1 s-1 (HP1) and 2.5 X 10(8) M-1 s-1 (HP2) were obtained for the free proteins at 10 mM ionic strength. Under the same conditions, 5-dRf. reacted with free FNRox, resulting in the formation of the neutral protein-bound semiquinone (FNR.), with a second-order rate constant of 6 X 10(8) M-1 s-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of a water-soluble, yellow protein isolated from a halophilic phototrophic bacterium that has photochemical activity analogous to sensory rhodopsin

A water-soluble yellow protein, previously discovered in the purple photosynthetic bacterium Ectothiorhodospira halophila, contains a chromophore which has an absorbance maximum at 446 nm. The protein is now shown to be photoactive. A pulse of 445-nm laser light caused the 446-nm peak to be partially bleached and red-shifted in a time less than 1 microsecond. The intermediate thus formed was subsequently further bleached in the dark in a biphasic process occurring in approximately 20 ms. Finally, the absorbance of native protein was restored in a first-order process occurring over several seconds. These kinetic processes are remarkably similar to those of sensory rhodopsin from Halobacterium, and to a lesser extent bacteriorhodopsin and halorhodopsin; although these proteins are membrane-bound, they have absorbance maxima at about 570 nm, and they cycle more rapidly. In attempts to remove the chromophore for identification, it was found that a variety of methods of denaturation of the protein caused transient or permanent conversion to a form which has an absorbance maximum near 340 nm. Thus, by analogy to the rhodopsins, the absorption at 446 nm in the native protein appears to result from a 106-nm red shift of the chromophore induced by the protein. Acid denaturation followed by extraction with organic solvents established that the chromophore could be removed from the protein. It is not identical with all-trans-retinal and remains to be identified, although it could still be a related pigment. The E. halophila yellow protein has a circular dichroism spectrum which indicates little alpha-helical secondary structure (19%).(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization of a high potential iron-sulfur protein from the halophilic phototrophic bacterium Ectothiorhodospira halophila

Crystals of the high-potential iron-sulfur protein from Ectothiorhodospira halophila strain BN 9626 have been grown from 3.4 to 3.5 M ammonium sulfate solutions at pH 7.5. The crystals belong to the space group P21 with unit cell dimensions of a = 60.00 A, b = 31.94 A, c = 40.27 A, and beta = 100.5 degrees. There are 2 molecules/asymmetric unit. The crystals diffract to at least 1.8 A, are stable in the x-ray beam, and are suitable for a high resolution x-ray crystallographic analysis.

Electron-transfer reactions between flavodoxin semiquinone and c-type cytochromes: comparisons between various flavodoxins

As an extension of previous work from this laboratory using Clostridium pasteurianum flavodoxin [Tollin, G., Cheddar, G., Watkins, J. A., Meyer, T. E., & Cusanovich, M. A. (1984) Biochemistry 23, 6345-6349], we have measured the rate constants as a function of ionic strength for electron transfer from the semiquinones of Clostridium MP, Anacystis nidulans, and Azotobacter vinelandii flavodoxins to the following oxidants: cytochrome c from tuna and horse, Paracoccus denitrificans cytochrome c2, Pseudomonas aeruginosa cytochrome c-551, and ferricyanide. The rate constants extrapolated to infinite ionic strength (k infinity) for the C. MP flavodoxin are all slightly smaller than for the C. pasteurianum flavodoxin, as would be predicted on the basis of the higher redox potential of the C. MP protein. This indicates that there is a close similarity between the surface topographies of the two proteins in the vicinity of the coenzyme binding site. Moreover, the electrostatic interactions between the two flavodoxins and the various oxidants are also approximately the same. These studies justify our previous use of the crystallographic structure of the C. MP flavodoxin to interpret kinetic results obtained with the structurally uncharacterized C. pasteurianum flavodoxin. Despite their lower redox potentials, both Anacystis and Azotobacter flavodoxins are appreciably less reactive toward all of these oxidants (as much as 2 orders of magnitude in some cases) than are the Clostridium flavodoxins.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallographic characterization of a photoactive yellow protein with photochemistry similar to sensory rhodopsin

A photoactive yellow protein purified from the phototrophic bacterium Ectothiorhodospira halophila, has been crystallized by vapor diffusion from ammonium sulfate solution. The hexagonal crystals are in space group P6(3) with unit cell dimensions a = b = 66.89, c = 40.68 A and appear to have one 15,000-dalton protein in the asymmetric unit. Photoactive yellow protein contains a chromophore with retinal-like properties; its color can be reversibly bleached, by visible light, with kinetics similar to those of sensory rhodopsin. The crystals can also be bleached by an intense visible light source without cracking, but are not bleached by x-rays. This suggests that structures can be obtained for both bleached and colored conformations of the protein-bound chromophore. The crystals diffract strongly to at least 1.3 A resolution, are resistant to radiation damage, and are suitable for a high resolution structure determination. The covalently bound chromophore and photobleaching characteristics of the protein offer unique opportunities to study protein conformational change and refolding as well as to understand the mechanisms of light-induced conformational change at atomic resolution.

Reduction kinetics of the ferredoxin-ferredoxin-NADP+ reductase complex: a laser flash photolysis study

The kinetics of reduction of spinach ferredoxin (Fd), ferredoxin-NADP+ reductase (FNR), and the Fd-FNR complex have been investigated by the laser flash photolysis technique. 5-Deazariboflavin semiquinone (5-dRf), generated in situ by laser flash photolysis under anaerobic conditions, rapidly reduced both oxidized Fd (Fdox) (k = 2 X 10(8) M-1 s-1) and oxidized FNR (FNRox) (K = 6.3 X 10(8) M-1 s-1) at low ionic strength (10 mM) at pH 7.0, leading to the formation of reduced Fd (Fdred) and FNR semiquinone (FNR.), respectively. At higher ionic strengths (310 and 460 mM), the rate constant for the reduction of the free Fdox increased about 3-fold (k = 6.7 X 10(8) M-1 s-1 at 310 mM and 6.4 X 10(8) M-1 s-1 at 460 mM). No change in the second-order rate constant for reduction of the free FNRox was observed at high ionic strength. At low ionic strength (10 mM), 5-dRf. reacted only with the FAD center of the preformed 1:1 Fdox-FNRox complex (k = 5.6 X 10(8) M-1 s-1), leading to the formation of FNR.. No direct reduction of Fdox in the complex was observed. No change in the kinetics occurred in the presence of excess NADP+. The second-order rate constant for reduction of Fdox by 5-dRf. in the presence of a stoichiometric amount of fully reduced FNR at low ionic strength was 7 X 10(6) M-1 s-1, i.e., about one-thirtieth the rate constant for reduction of free Fdox.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient kinetics of reduction of blue copper proteins by free flavin and flavodoxin semiquinones

Rate constants have been determined for the electron-transfer reactions between reduced free flavins and flavodoxin semiquinone and several blue copper proteins. Correlations between these values and redox potentials demonstrate that spinach plastocyanin, Pseudomonas aeruginosa azurin, Alcaligenes sp. azurin, and Alcaligenes sp. nitrite reductase have the same intrinsic reactivities toward free flavins, whereas stellacyanin is more reactive (3.3 times) and laccase considerably less reactive (approximately 12 times). Electrostatic interactions between the negatively charged flavin mononucleotide (FMN) and the copper proteins show that the interaction site charges for laccase and nitrite reductase are opposite in sign to the net protein charge and that the signs and magnitudes of the charges are consistent with the known three-dimensional structures for plastocyanin and the azurins and with amino acid sequence homologies for stellacyanin. The results demonstrate that the apparent interaction site charge with flavodoxin is larger than that with FMN for plastocyanin, nitrite reductase, and stellacyanin but smaller for Pseudomonas azurin. This is interpreted in terms of a larger interaction domain for the flavodoxin reaction, which allows charged groups more distant from the actual electron-transfer site to become involved. The intrinsic reactivities of plastocyanin and azurin toward flavodoxin are the same, as was the case with FMN, but both stellacyanin and nitrite reductase are considerably less reactive than expected (approximately 2 orders of magnitude). This result suggests the involvement of steric factors with these latter two proteins which discriminate against large reactants such as flavodoxin.

Redox pathways in electron-transfer proteins: correlations between reactivities, solvent exposure, and unpaired-spin-density distributions

The relative reactivities toward reduction by free flavin semiquinones of cytochromes (c-type cytochromes, cytochrome b5, c'-type cytochromes) iron-sulfur proteins (high-redox-potential ferredoxins, rubredoxins, low-redox-potential ferredoxins), and blue copper proteins (plastocyanin, azurins) are shown to correlate with calculations of the solvent exposure of the various prosthetic groups. In the case of the c-type cytochromes, one of the major centers of exposure is the sulfur atom of the thioether bridge that covalently links heme ring C to the protein. Charge-iterative extended Hückel calculations on a heme c model indicate that both porphyrin pi and Fe(III)d pi orbitals can delocalize onto the bridging sulfur atom. Unpaired spin densities are comparable to those obtained for individual aromatic porphyrin ring carbon atoms. Thus, the exposed sulfur of ring C may act to facilitate electron transfer.

Crystallization and preliminary analysis of crystals of high potential iron-sulfur protein from Rhodospirillum tenue

Large single crystals of the high potential iron-sulfur protein isolated from Rhodospirillum tenue strain 3761 have been obtained. They belong to the space group P2(1) with unit cell dimensions of a = 36.7 A, b = 52.6 A, c = 27.6 A, and beta = 90.8 degrees. There are two molecules in the asymmetric unit. Based on oscillation photographs, the crystals diffract to at least 1.6 A resolution. They are stable in the x-ray beam and appear suitable for a high resolution x-ray structure analysis.

Kinetics of electron transfer between cytochromes c' and the semiquinones of free flavin and clostridial flavodoxin

Rate constants have been measured for the reactions of a series of high-spin cytochromes c' and their low-spin homologues (cytochromes c-554 and c-556) with the semiquinones of free flavins and flavodoxin. These cytochromes are approximately 3 times more reactive with lumiflavin and riboflavin semiquinones than are the c-type cytochromes that are homologous to mitochondrial cytochrome c. We attribute this to the greater solvent exposure of the heme in the c'-type cytochromes. In marked contrast, the cytochromes c' are 3 orders of magnitude less reactive with flavodoxin semiquinone than are the c-type cytochromes. We interpret this result to be a consequence of the location of the exposed heme in cytochrome c' at the bottom of a deep groove in the surface of the protein, which is approximately 10-15 A deep and equally as wide. While free flavins are small enough to enter the groove, the flavin mononucleotide (FMN) prosthetic group of flavodoxin is apparently prevented by steric constraints from approaching the heme more closely than approximately 10 A without dynamic structural rearrangements. Most cytochromes c' are dimeric, but a few are monomeric. The three-dimensional structure of the Rhodospirillum molischianum cytochrome c' dimer suggests that the heme should be more exposed in the monomer than in the dimer, but no relationship is observed between intrinsic reactivity toward free flavin semiquinones and the aggregation state of the protein. Likewise, there is no evidence that the spin state or ligand field of the iron has any effect on intrinsic reactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

The amino acid sequence of cytochrome c-555 from the methane-oxidizing bacterium Methylococcus capsulatus

The amino acid sequence of the cytochrome c-555 from the obligate methanotroph Methylococcus capsulatus strain Bath (N.C.I.B. 11132) was determined. It is a single polypeptide chain of 96 residues, binding a haem group through the cysteine residues at positions 19 and 22, and the only methionine residue is a position 59. The sequence does not closely resemble that of any other cytochrome c that has yet been characterized. Detailed evidence for the amino acid sequence of the protein has been deposited as Supplementary Publication SUP 50131 (12 pages) at the British Library Lending Division, Boston Spa, West Yorkshire LS23 7BQ, U.K., from whom copies are available on prepayment.