Biomedical subjects

D M Kurtz

Publications and source records attributed to D M Kurtz.

At least 55 records · Page 3Linked to original sources

PubMed1991

Intrapeptide sequence homology in rubrerythrin from Desulfovibrio vulgaris: identification of potential ligands to the diiron site.

Two regions in the amino acid sequence of the 21.5 kDa subunit of rubrerythrin from Desulfovibrio vulgaris (Hildenborough) are shown to be homologous. Rubrerythrin contains a non-heme, non-sulfur diiron site, and the internally homologous regions share homology with at least one proposed iron binding region of the component A alpha subunit of methane monooxygenase, which also contains a non-heme, non-sulfur diiron site. Comparison of the rubrerythrin sequences with those of the B2 subunit of E. coli ribonucleotide reductase, whose diiron site ligands have been identified, suggests that two glutamate and two histidine residues at positions 53, 56, 129, and 131 within the rubrerythrin sequence furnish ligands to the diiron site. A pair of EXXH sequences appears to represent a diiron binding motif in all three aforementioned proteins. No propene monooxygenase activity was detected with rubrerythrin using the assay designed to test activity of methane monooxygenase component A in the absence of other protein components.

Amino Acid Sequence↗

PubMed1991

Cloning and sequencing of the gene for rubrerythrin from Desulfovibrio vulgaris (Hildenborough).

The gene coding for rubrerythrin from the sulfate-reducing bacterium Desulfovibrio vulgaris (Hildenborough) has been cloned and sequenced. Rubrerythrin is known to contain two types of iron sites: one rubredoxin-like FeS4 center in each of the two identical subunits and one hemerythrin-like diiron site per dimer [LeGall, J., et al. (1988) Biochemistry 27, 1636-1642]. The gene encodes a polypeptide of 191 amino acids, and a normal ribosome binding site is located 11-6 base pairs upstream from the translational start of the gene. There is no evidence for the presence of a leader sequence, suggesting a cytoplasmic location for the protein. The rubrerythrin gene is not part of any other known transcriptional unit in the D. vulgaris genome. The nucleotide sequence encodes four Cys residues, the minimum required for ligation to iron in rubredoxin. The pairs of Cys residues occur in Cys-X-X-Cys sequences as they do in rubredoxin, but the 12-residue spacing between the Cys pairs in rubrerythrin is less than half that in rubredoxins. A pair of Arg residues flanking one Cys residue may contribute to the much more positive reduction potential of the rubredoxin-like site in rubrerythrin compared to that of rubredoxin. While the amino acid sequence of rubrerythrin shows no significant overall homology with that of any known protein, the C-terminal region does share some homology with rubredoxin sequences. If folding of the rubredoxin-like amino acid sequence domain in rubrerythrin is similar to that in rubredoxins, then three His residues are brought into proximity.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1991

Two distinct subunits of hemerythrin from the brachiopod Lingula reevii: an apparent requirement for cooperativity in O2 binding.

Reported are results on the subunit composition of octameric hemerythrin (Hr) from the brachiopod Lingula reevii. Unlike most other Hrs, L. reevii Hr shows cooperativity in O2 binding. Purified L. reevii Hr was found to consist of two different subunits in approximately equimolar proportions. These two subunits differ in molecular weight by approximately 1000. Amino acid sequence data for the first 24 residues of the two subunits, labeled alpha and beta, show 70% identity with each other. Comparisons to amino acid sequences of other Hrs show approximately 50% identity in the first 24 residues and that both the alpha and beta subunits of L. reevii Hr have one residue deleted at their amino termini. Very recently, one other Hr, that from the brachiopod Lingula unguis, was also shown to contain equimolar proportions of two different subunits [Satake, K., Yugi, M., Kamo, M., Kihara, H., & Tsugita, A. (1990) Protein Seq. Data Anal. 3, 1-5], and this Hr also shows cooperativity in O2 binding. An alpha 4 beta 4 octamer is, therefore, proposed to be a common feature of those Hrs that show such cooperativity. Likely arrangements of alpha and beta subunits within an alpha 4 beta 4 octamer having the same configuration of subunits as that in other octameric Hrs are proposed. The most probable arrangements can be readily derived from physically reasonable restrictions on the types of intersubunit interactions and on transmission of allosteric effects.

Allosteric Regulation↗

PubMed1991

Reconstitution of the diiron sites in hemerythrin and myohemerythrin.

The first reconstitutions of functional diiron sites in the nonheme O2-carrying proteins hemerythrin (Hr) and myohemerythrin (myoHr) have been achieved. Both proteins are reconstituted under anaerobic conditions, and the procedure consists of (i) denaturation of the native met form with 6 M guanidinium chloride in the presence of sodium dithionite and 2,2'-dipyridyl, (ii) separation of the apoprotein from the other reagents and products, (iii) addition of an iron(II) stock solution to the apoprotein in the presence of 2-mercaptoethanol, and (iv) several cycles of slow dilution and reconcentration by ultrafiltration to remove excess reagents. Iron analyses indicate that the apoproteins have been essentially completely freed of iron and that reconstituted Hr contains its full complement of iron, i.e., approximately 2 Fe/subunit. Ferrous rather than ferric iron appears to be necessary for recovery of the native structures for both myoHr and Hr. In the case of Hr, reconstitution was successful only when iron(II) was added to apoHr prior to removal of denaturant. ApoHr is essentially insoluble at pH 7 in the absence of denaturants but remains soluble when denaturant is removed in the presence of ferrous iron, which leads to recovery of the octameric structure containing all of its diiron sites. Iron(II) apparently stabilizes the native or a nearly native structure during reconstitution. OxymyoHr and oxyHr are the major initial products of reconstitution. The yield of oxymyoHr from apomyoHr was approximately 87%. In contrast to reconstituted oxymyoHr, where essentially all of the iron appears to be functional, approximately 30% of the diiron sites in the reconstituted oxyHr are unable to bind O2 at ambient p(O2).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Purification and properties of a membrane-bound NADH-cytochrome-b5 reductase from erythrocytes of the sipunculid worm, Phascolopsis gouldii.

The purification to homogeneity of the membrane-bound NADH-cytochrome-b5 reductase from erythrocytes of the sipunculid, Phascolopsis gouldii is reported. This highly purified reductase has allowed more detailed characterizations of its molecular and kinetic properties than was possible in a previous study (Utecht, R.E. and Kurtz, D.M., Jr. (1988) Biochim. Biophys. Acta 953, 164-178). The reductase has a molecular weight of 34,000 and contains FAD as the prosthetic group. In aqueous solution containing 0.5 vol% Triton X-100, the reductase forms an aggregate of Mr approximately 220,000. A higher purity preparation of P. gouldii erythrocyte b5 was also obtained. The combination of purified, solubilized reductase and cytochrome b5 was shown to catalyze the quantitative two-electron reduction of [Fe(III),Fe(III)]methemerythrin to [Fe(II),Fe(II)]deoxyhemerythrin by NADH. The P. gouldii NADH-cytochrome b5 reductase is the first from hemerythrin-containing erythrocytes to be purified and characterized. This methemerythrin reduction system appears to be analogous to methemoglobin reductases from vertebrate erythrocytes.

Animals↗

PubMed1989

Structure of hexakis(N-methylimidazole-N')iron(II) tetraphenylborate dichloromethane solvate.

[Fe(C4H6N2)6][B(C6H5)4]2.CH2Cl2, Mr = 1271.88, monoclinic, P2(1)/c, a = 20.5320 (6), b = 15.113 (3), c = 23.611 (7) A, beta = 109.01 (3) degrees, V = 6926.9 (1) A3, Z = 4, Dx = 1.23 g cm-3, mu(Mo K alpha) = 0.70966 A, graphite monochromator, mu = 3.37 cm-1, F(000) = 2680, T = 298 K, R = 0.057 for 2310 unique observed reflections. The asymmetric unit consists of a single [Fe(C4H6N2)6]2+ cation, two distinct BPh-4 anions, and one CH2Cl2. The Fe atom is octahedrally coordinated by six N-methylimidazole ligands. The six Fe-N bond distances per cation range from 2.174 (11) to 2.228 (10) A, the average being 2.207 (12) A. This average distance is within the range previously observed for high-spin ferrous complexes. The ring planes of N-methylimidazole ligands that lie trans to each other have interplane angles of 17.56, 18.48 and 157.56 degrees. These ring planes lie approximately either parallel or perpendicular to equatorial Fe-N bonds.

Boron Compounds↗

PubMed1989

Roentgenographic findings in pigmented villonodular synovitis of the knee.

Twenty-nine cases of pigmented villonodular synovitis (PVS) of the knee in 27 patients were reviewed to determine characteristic roentgenographic findings. All cases met strict histologic criteria for diagnosis. Four cases were localized PVS (LPVS), and 25 cases were diffuse PVS (DPVS). Roentgenographic findings were largely in the soft tissues. Cystic invasion of bone or degenerative changes were rare, although present in some cases. When present in DPVS, these changes were most pronounced in the patellofemoral articular surface. In the cases of DPVS, large posterior tumefactions did not correlate with extraarticular extension. Clinical behavior of PVS was governed more by anatomic site and form of disease than by the severity of histologic or roentgenographic findings.

Adolescent↗

PubMed1988

31P NMR probes of sipunculan erythrocytes containing the O2-carrying protein hemerythrin.

Reported are the first examinations by 31P NMR of erythrocytes containing the non-heme iron O2-carrying protein hemerythrin (Hr). Intact coelomic erythrocytes from the sipunculids Phascolopsis gouldii and Themiste zostericola were shown by 31P NMR to contain O-phosphorylethanolamine and 2-aminoethylphosphonate as the major soluble phosphorus metabolites. This combination of major metabolites appears to be unique to sipunculan erythrocytes. Nucleoside triphosphates and mannose 1-phosphate were present in lower concentrations. The concentration of O-phosphorylethanolamine within P. gouldii erythrocytes was established to be greater than 20 mM. T. zostericola erythrocytes contained relatively high levels of 2-aminoethylphosphonate (on the order of 0.1 M) and lower levels of O-phosphorylethanolamine compared with those of P. gouldii. For P. gouldii and T. zostericola the intracellular pHs were determined to be 7.2 +/- 0.1 and 7.1 +/- 0.1, respectively, in air-equilibrated erythrocytes, and 6.5 +/- 0.1 in anaerobic P. gouldii erythrocytes. O-Phosphorylethanolamine was found to bind weakly to P. gouldii metHr (Kf approximately 7 M-1). This interaction is best characterized by either negative cooperativity or nonspecific binding. O-Phosphorylethanolamine strongly inhibits azide binding to the iron site of P. gouldii metHr at pH 7.2. The rate of azide binding decreases by approximately 85-fold in the presence of 0.33 M O-phosphorylethanolamine. However, neither O-phosphorylethanolamine nor 2-aminoethylphosphonate at 0.33 M was found to have any significant effect on O2 affinity of P. gouldii deoxyHr. Alternative functions for the two metabolites are suggested.

Animals↗

PubMed1988

Cytochrome b5 and NADH-cytochrome-b5 reductase from sipunculan erythrocytes; a methemerythrin reduction system from Phascolopsis gouldii.

We report the purification and characterization of a soluble cytochrome b5 from coelomic erythrocytes of the sipunculan worm, Phascolopsis gouldii. We also report the isolation and purification of a membrane-bound NADH-cytochrome-b5 reductase from these erythrocytes. The non-heme iron protein, hemerythrin (Hr), is known to be the oxygen carrier in these erythrocytes. The aforementioned purified cytochrome b5 and reductase together catalyze the reduction of P. gouldii [Fe(III),Fe(III)]metHr to [Fe(II),Fe(II)deoxyHr by NADH. EPR spectroscopy demonstrates that a redox process involving formation of the intermediate [Fe(II),Fe(III)]semi-metHr occurs within intact sipunculan erythrocytes as well as in the system of purified components. The rhombic g-tensor of the EPR signal in both cases resembles that of (semi-met)RHr, the form obtained by one-electron reduction of metHr. These observations suggest that cytochrome b5 and NADH-cytochrome-b5 reductase in sipunculan erythrocytes function to counteract autoxidation of oxyHr. The sequence of electron flow in the system of purified components is: NADH----NADH-cytochrome-b5 reductase----cytochrome b5----metHr. At pH 7.5, the reduction of metHr in this system occurs in two phases, only the first of which is dependent on concentration of cytochrome b5. From an analysis of the kinetics and the EPR time-course, we propose that the two phases represent sequential reduction of met- to semi-metHr and reduction of semi-metHr to deoxyHr. This report represents the first demonstration of a physiological system for reduction of metHr.

Animals↗

PubMed1988

Nitric oxide adducts of the binuclear iron site of hemerythrin: spectroscopy and reactivity.

Nitric oxide forms adducts with the binuclear iron site of hemerythrin (Hr) at [Fe(II),Fe(II)]deoxy and [Fe(II),Fe(III)]semimet oxidation levels. With deoxyHr our results establish that (i) NO binds reversibly, forming a complex which we label deoxyHrNO, (ii) NO forms a similar but distinct complex in the presence of fluoride, which we label deoxyHrFNO, (iii) NO is directly coordinated to one iron atom of the binuclear pair in these adducts, most likely in a bent end-on fashion, and (iv) the iron atoms in the binuclear sites of both deoxyHrNO and deoxyHrFNO are antiferromagnetically coupled, thereby generating unique electron paramagnetic resonance (EPR) detectable species. The novel EPR signal of deoxyHrNO (deoxyHrFNO) with g[[ = 2.77 (2.58) and g = 1.84 (1.80) is explained by the magnetic interaction of the Fe(II) (S' = 2) and [FeNO]7 (S = 3/2) centers observed by Mössbauer spectroscopy. Antiferromagnetic coupling leads to a ground state of Seff = 1/2. Analysis of the EPR parameters using the isotropic spin-exchange Hamiltonian, Hex = 2JS3/2.S2, and including zero-field splitting leads to a coupling constant, -J approximately 23 cm-1, for deoxyHrNO. The resonance Raman spectrum of deoxyHrNO shows features at 433 and 421 cm-1 that shift downward with 15N16O and that are assigned to stretching and bending modes, respectively, of the [FeNO]7 unit. Sensitivity of the bending mode to D2O suggests that bound NO participates in hydrogen bonding. We propose that the terminal oxygen atom of NO is hydrogen bonded to the proton of the mu-hydroxo bridge in the Fe-(OH)-Fe unit. A bent Fe-N-O geometry is supported by spectroscopic and structural comparisons to synthetic complexes and is consistent with a limiting [FeII,FeIIINO-] formulation for deoxyHrNO. Reversibility of NO binding to deoxyHr is demonstrated by bleaching of the optical and EPR spectra of deoxyHrNO upon additions of excess N3- or CNO-. DeoxyHrNO undergoes autoxidation under anaerobic conditions over the course of several hours. The product of this autoxidation appears to be an EPR-silent NO adduct of semimetHr. The formal one-electron oxidations of the binuclear iron site of deoxyHr by NO and by HNO2 can conceivably occur with no net change in charge on the iron site. In contrast, autoxidation of oxy- to metHr requires a change in net charge on the iron site, which may provide a kinetic barrier.

Animals↗

PubMed1987

Comparisons of redox kinetics of methemerythrin and mu-sulfidomethemerythrin. Implications for interactions with cytochrome b5.

We have examined the effects on redox kinetics of changing the reduction potential of the mu-oxo-bridged binuclear iron center in octameric hemerythrin (Hr) from Phascolopsis gouldii. The opportunity to examine such effects is provided by the availability of mu-sulfidomethemerythrin (mu-S2-metHr), whose [Fe(III),Fe(III)]met----[Fe(II),Fe(III)]semi-met reduction potential is approximately 200 mV higher than that of methemerythrin (metHr). We have used, as redox partners to Hr, a set of metal complexes and the heme proteins deoxymyoglobin (Mb) and cytochrome b5. The latter protein from P. gouldii is a presumed physiological redox partner of Hr. Similar kinetics at pH 8 in the presence or absence of the allosteric effector perchlorate suggest reduction of the iron atom closer to the outer surface of each subunit in the Hr octamer during the met----semi-met transformation. For all reducing agents, the experimentally determined ratio of second-order rate constants for reductions of mu-S2-metHr and metHr, k12(mu-S2-met)/k12(met), is close to the value of 40 predicted by the simple Marcus relation for "outer-sphere" electron transfer. For oxidations of (semi-met)RHr and mu-S2-semi-metHr, the predicted value of 40 for k12[(semi-met)R]/k12(mu-S2-semi-met) is closely approximated when Fe(CN)6(3-) is used as oxidant. The ionic strength dependence of the second-order rate constant suggests electrostatic interactions of opposite charges during reduction of metHr by P. gouldii cytochrome b5.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1985

Enzymic synthesis of the 4Fe-4S clusters of Clostridium pasteurianum ferredoxin.

Ex novo enzymic synthesis of the two 4Fe-4S clusters of Clostridium pasteurianum ferredoxin has been achieved by incubation of the apoprotein with catalytic amounts of the sulfurtransferase rhodanese in the presence of thiosulfate, DL-dihydrolipoate and ferric ammonium citrate. This enzymic reconstitution procedure was compared to a chemical one, in which the enzyme was replaced by sodium sulfide. A further comparison was made with the results previously obtained in the enzymic synthesis of the 2Fe-2S cluster of spinach ferredoxin, allowing the following conclusions to be drawn. The nature of the cluster to be inserted into the reconstituted iron-sulfur protein is determined by the apoprotein itself. The refolding of the structure of the iron-sulfur proteins around the newly inserted cluster is the rate-limiting step in both chemical and enzymic reconstitution. Rhodanese appears to play a role in the recovery of the native architecture of the reconstituted iron-sulfur protein(s). The extension to the 4Fe-4S centers of the rhodanese-based biosynthetic system allows this enzymic route to be proposed as a general way to the in vivo synthesis of iron-sulfur structures.

Apoproteins↗

PubMed1985

Hodgkin's disease of the endometrium.

We report a case of Hodgkin's disease of the endometrium. The endometrial stroma is replaced by a polymorphic cellular infiltrate in which Reed-Sternberg cells are seen. The patient also has a history of stage IV-B Hodgkin's disease that was diagnosed and treated three years earlier. To our knowledge, Hodgkin's disease of the endometrium has not been described.

Adult↗

PubMed1984

Sulfide-bridged derivatives of the binuclear iron site of hemerythrin at both met and semi-met oxidation levels.

Exposure of methemerythrin (metHr) to S2- under anaerobic conditions results in a one-electron reduction to the semi-met level and replacement of the mu-oxo bridge between the irons with a single sulfide. The sulfide bridge is maintained upon ferricyanide oxidation of semi-metsulfide to metsulfide hemerythrin and upon subsequent dithionite or S2- reduction back to the semi-met level. Chemical analyses show that metsulfideHr contains one S2- per two Fe. The single quadrupole doublet (delta = 0.50 mm/s; delta Eq = 0.99 mm/s) in the Mössbauer spectrum is consistent with a bridging sulfide geometry. The optical and resonance Raman spectra of metsulfideHr are reminiscent of the [2Fe-2S] iron-sulfur proteins. The optical spectrum exhibits multiple S2----Fe(III) charge-transfer transitions between 400 and 600 nm. The resonance Raman spectrum reveals a series of overtones and combinations of the 431-cm-1 Fe-S-Fe symmetric vibration and the 327-cm-1 asymmetric vibration. The relative energies of the symmetric and asymmetric modes are characteristic of a sulfur-bridged system with a bridge angle of approximately 80 degrees. MetsulfideHr decomposes over several hours in air and over several days in the absence of O2 to metHr and semi-metsulfideHr, respectively. Unlike metHr and semi-metHr, neither the metsulfide nor the semi-metsulfide derivatives form stable adducts with anions such as azide or cyanide. Sulfide bridging confers new properties on the binuclear iron center that are of interest to an understanding of the chemistry of hemerythrin and also of the [2Fe-2S] iron-sulfur proteins.

Animals↗

PubMed1984

Oxidation of deoxyhemerythrin to semi-methemerythrin by nitrite.

In anaerobic phosphate buffer, pH 6.3-7.5, deoxyhemerythrin is oxidized to semi-methemerythrin (semi-met) by excess sodium nitrite. This oxidation is quantitative as judged by EPR spectroscopy. Further oxidation to methemerythrin is not detected. The absorbance changes of hemerythrin during the oxidation are biphasic. The rate of the faster first phase is linearly dependent on [H+] and [NO2-] suggesting that the oxidant is nitrous acid rather than nitrite. During the slower second phase, the characteristic EPR spectrum of semi-methemerythrin appears. The first phase can be interpreted by a scheme in which nitrous acid transforms deoxyhemerythrin (FeIIFeII) to the semi-met nitrosyl adduct (FeIIFeIIINO) and hydroxide. Independent experiments confirm that the combination of semi-met plus NO produces an EPR-silent adduct. The rates of the absorbance changes for the second phase are nearly independent of nitrite concentration and pH in the range 6.3-7.5. This slower phase involves the transformation of the EPR-silent intermediate to the semi-met nitrite adduct (FeIIFeIIINO2-) and is consistent with rate-limiting dissociation of nitric oxide followed by rapid attachment of nitrite. Nitrite appears to be a unique oxidant of deoxyhemerythrin in that when employed in excess, the final, stable product is semi-met- rather than methemerythrin. The lack of reactivity of ethyl nitrite with deoxyhemerythrin suggests that HONO oxidizes deoxyhemerythrin via an "inner-sphere" process in contrast to oxidants such as Fe(CN)6(3-). A proposed generalization is that excesses of "inner-sphere" oxidants convert deoxy to (semi-met)R, which is stabilized with respect to (semi-met)R, which is stabilized with respect to (semi-met)0 and met because the oxidant and/or a product of the oxidant can bind to the iron site.

Animals↗

PubMed1984

Chromatographic separation of extruded iron-sulfur cores from the apoproteins of Clostridium pasteurianum and spinach ferredoxins in aqueous Triton X-100/urea.

Iron-sulfur core extrusions from spinach [( 2Fe-2S]) and Clostridium pasteurianum (2[4Fe-4S]) ferredoxins in aqueous Triton X-100/urea containing excess benzenethiol yield quantitatively [FenSn(SPh)4]2- with n = 2 and n = 4, respectively. The iron-sulfur cluster can be separated from the corresponding apoprotein by rapid passage of the extrusion mixture over a small anaerobic column of Whatman DE-52 anion-exchange cellulose. Essentially quantitative recovery of [FenSn (SPh)4]2- is achieved in the eluate. The apoprotein remaining on the column can be eluted with 0.5 M NaCl. Most of the residual Triton X-100 and benzenethiol can be removed by passage of the apoprotein eluate over a small column of Bio-Beads SM-2, a hydrophobic polystyrene adsorbent. Apoprotein recovery is comparable to that obtained by other chromatographic methods. At least with spinach ferredoxin, the apoprotein prepared in this fashion can be reconstituted. The procedures developed in this work are potentially most applicable to selective removal of [2Fe-2S] and [4Fe-4S] centers from a multicenter enzyme without irreversible denaturation.

Apoproteins↗

PubMed1983

Semi-met oxidation level of chalcogenide derivatives of methemerythrin. Mössbauer and EPR studies.

Conclusive evidence is presented for an S = 1/2 spincoupled pair of high spin ferric and ferrous ions in the major reaction product of sulfide with the met form of the non-heme iron oxygen-carrying protein hemerythrin. Evidence for an analogous selenide derivative is also reported. Mössbauer and EPR spectroscopy establish (a) the charge and spin states of the individual iron atoms in sulfidehemerythrin as Fe(III), S = 5/2, and Fe(II), S = 2, and (b) the existence of an antiferromagnetic exchange interaction that couples the two spins to a resultant spin S = 1/2. The combined Mössbauer and EPR data confirm the correctness of the formulation first proposed for semi-methemerythrin by Harrington, P.C., de Waal, D.J.A., and Wilkins, R.G. ((1978) Arch. Biochem. Biophys. 191, 444-451) and furthermore show that a majority of the iron centers in the protein can be stabilized at this oxidation level. The results also demonstrate a new route to semi-methemerythrin. A titration of methemerythrin with selenide indicates that this derivative forms by a two step process consisting of first, reduction to the semi-met oxidation level by selenide and second, binding of selenide to either one or both irons.

Animals↗

PubMed1982

Kinetics and equilibria of active site core extrusion from spinach ferredoxin in aqueous N,N-dimethylformamide/Triton X-100 solutions.

The nature and reactivity of each species participating in core extrusion of the [2Fe-2S] cluster of spinach ferredoxin by benzenethiol have been investigated in a novel aqueous medium containing 10-40% (v/v) N,N-dimethylformamide (DMF) plus 5-6% (v/v) of the nonionic detergent, Triton X-100. By use of visible absorption and circular dichroism spectroscopies, it is found that prior to addition of benzenethiol, modifications of the ferredoxin induced by DMF are reversible. Both the extent and rate of modification are dependent on DMF concentration and on ionic strength. At ferredoxin concentrations near 0.1 mM, complete modification by DMF is shown to be unnecessary for quantitative core extrusion provided that the benzenethiol concentration is greater than or equal to 50 mM and the DMF concentration is greater than or equal to 10% (v/v). When benzenethiol is added after DMF, core extrusion occurs in two phases. Our interpretation is that the rapid phase corresponds to core extrusion of that portion of the ferredoxin already modified by DMF and that the slower phase corresponds to modification of the remaining portion of the ferredoxin by DMF. When DMF solutions containing benzenethiol and Triton X-100 in various ratios are mixed with aqueous solutions of spinach ferredoxin, the rate of core extrusion appears to be determined both by the rate at which the ferredoxin is modified and by the Triton/benzenethiol ratio. Under all conditions examined we observe significantly faster rates of core extrusion from spinach ferredoxin in aqueous Triton/DMF than in 80/20 (v/v) hexamethylphosphoramide/water, a previously used core extrusion medium. Our results suggest a catalytic role for the micellar phase.

Binding Sites↗