Size-exclusion chromatography with on-line light-scattering, absorbance, and refractive index detectors for studying proteins and their interactions

Techniques of using size-exclusion chromatography (SEC) with on-line light-scattering, uv absorbance, and refractive index detectors to characterize the polypeptide molecular weights of simple proteins or glycoproteins or to determine the stoichiometry of protein complexes are described. Two unique advantages of this approach over conventional SEC are that the molecular weight measurement is independent of elution position and can exclude the contributions from carbohydrates. When a protein or complex contains no carbohydrates, a two-detector method, i.e., light scattering combined with refractive index, can be used to calculate the molecular weight. When a protein contains carbohydrates, a three-detector method is used to calculate the molecular weight of polypeptide alone. Finally, a self-consistent three-detector method is used to determine the stoichiometry of a protein complex containing carbohydrates. Example applications for all these methodologies are described.

ErbB receptor activation, cell morphology changes, and apoptosis induced by anti-Her2 monoclonal antibodies

A panel of mAbs were generated against the purified soluble form of erbB2/Her2 receptor, corresponding to the extracellular region of the receptor, and examined for their ability to mimic the receptor ligand. Some of the mAbs strongly induced tyrosine phosphorylation of 180-185 kDa proteins, including not only Her2 but also Her3 and Her4 receptors, when they were expressed on the surface of breast cancer cells. These mAbs do not cross-react with Her3 or Her4 as demonstrated by competition study. Receptor phosphorylation was also observed with the cell lines transfected with Her2 or a chimeric receptor consisting of the extracellular domain of Her2 and the transmembrane and cytoplasmic domains of epidermal growth factor receptor. Selected mAbs were tested for their ability to change cell morphology, and one specific mAb, mAb74, induced cell morphology changes and apoptosis.

Oxidative stability and alpha-tocopherol retention in turkey burgers during refrigerated and frozen storage as influenced by dietary alpha-tocopheryl acetate

1. The effect of vitamin E (alpha-tocopheryl acetate) in turkey diets on the oxidative stability of raw and cooked turkey burgers and on the retention of alpha-tocopherol during refrigerated (4 degrees C) or frozen (-20 degrees C) storage was investigated. One hundred and two, one-day-old T-8s turkey poults were divided at random into 3 groups of 34 animals each and fed on either a basal diet (normal commercial turkey diet) supplemented with 20 mg alpha-tocopheryl acetate/kg (control) or fed an alpha-tocopherol supplemented diet containing 300 (E300) or 600 (E600) mg alpha-tocopheryl acetate/kg for 21 weeks. 2. Dietary supplementation with alpha-tocopheryl acetate significantly reduced TBARS numbers in both raw and cooked burgers during refrigerated and frozen storage. 3. The mean values of alpha-tocopherol in raw and cooked burgers stored at 4 degrees C did not change during storage. 4. In the case of both raw and cooked samples stored at -20 degrees C, the alpha-tocopherol values decreased from 5.67 to 3.54 and from 3.56 to 2.30 micrograms/g in the raw burgers from turkeys from the E600 and E300 treatments, respectively, after 4 months storage. The values decreased from 5.60 to 2.88 and from 3.29 to 1.85 micrograms/g in cooked burgers from turkeys from the E600 and E300 treatments, respectively, after 5 months storage.

Characterization of keratinocyte growth factor binding to heparin and dextran sulfate

Binding of keratinocyte growth factor (KGF) with heparin (molecular weight of 5000) and dextran sulfate (molecular weight of 8000) was studied using an online monitoring of size-exclusion chromatography with light scattering, refractive index, and uv absorbance detectors. This technique allows the determination of the molecular weight of KGF eluting as complexes with the above polymers. When mixtures of KGF with heparin were injected into the column, two peaks of heparin/KGF complexes were observed. The first peak corresponded to, on average, 3.4 KGF per complex and the second peak to an average of about 2 KGF per complex. These results suggest that the heparin/ KGF complex is heterogeneous, consisting of 1,2,3, and 4 KGF molecules per complex. To calculate the number of heparin molecules in these complexes, the rate of disappearance of free KGF was determined as heparin was added. The average number of KGF bound to 1 mol of heparin was calculated to be about 2 mol, suggesting that only one heparin molecule is present in these complexes. The heparin binding of two KGF mutants, i.e., (C1, 15S)KGF (with substitutions of serine for cysteines 1 and 15) and d28KGF (lacking 28 N-terminal amino acid residues), was essentially identical to that of the native sequence KGF. A similar experiment was carried out for KGF binding to dextran sulfate. The molecular weight of the complex corresponded to 2 to 2.6 KGF molecules per complex. The rate of disappearance of free KGF as the dextran sulfate added showed 2-3 mol of KGF bound to 1 mol of dextran sulfate, consistent with the idea that the complex contains only 1 dextran sulfate molecule.

Inguinal lymph node foreign body granulomas after placement of a silicone rubber (Silflex) implant of the first metatarsophalangeal joint

We describe a woman who developed foreign body granulomas 8 years after metatarsophalangeal (MTP) joint silicone rubber implantation for hallux rigidus. She developed overnight swelling and tenderness in the inguinal region on the ipsilateral side. Histological evaluation of the lymph node showed foreign body granulomas. Electron microscopy and dispersion x-ray analysis identified the foreign body material in the inguinal lymph node and fibrous capsule surrounding the implant as silicone. Foreign body granulomas of surrounding tissues and regional lymph nodes are infrequently reported postoperative complications in patients with small joint silicone rubber implants. A fractured or eroded implant surface attributed to wear is usually noted at prosthesis excision. Granulomas in normal or enlarged lymph nodes have been described in both symptomatic and asymptomatic patients, with intact or fractured prostheses. Many related joint and lymph node symptoms resolve with removal of the silicone rubber prosthesis.

Complete sequence, subunit structure, and complexes with pancreatic alpha-amylase of an alpha-amylase inhibitor from Phaseolus vulgaris white kidney beans

The complete amino acid sequence of a white kidney bean (Phaseolus vulgaris) alpha-amylase inhibitor (PHA-I), which is composed of two kinds of glycopolypeptide subunits, alpha and beta, was established by conventional methods. The polypeptide molecular weight of PHA-I determined by the light-scattering technique, considered together with the sequence molecular weights revealed for the subunits, indicated that PHA-I has the subunit stoichiometry of (alpha beta)2 complex. Inhibition test of PHA-I with increasing amounts of porcine pancreatic alpha-amylase (PPA) suggested that an inactive 2:1 complex is formed between PPA and PHA-I. In fact, two complexes differing from each other in the molar ratio of PPA to PHA-I were separated by gel filtration, and molecular weight estimation by the light-scattering technique confirmed that they are complexes of PHA-I with one or two PPA molecules. The binding of PPA to PHA-I appeared to follow simple binomial statistics, suggesting that two binding sites on PHA-I are independent and of high affinity for PPA.

Pulmonary malakoplakia in acquired immunodeficiency syndrome: an ultrastructural study of morphogenesis of Michaelis-Gutmann bodies

Malakoplakia is an unusual inflammatory reaction to a variety of infections, characterized by the accumulation of macrophages containing the target-like calcospherites, the Michaelis-Gutmann body (MGB). We report three patients with acquired immunodeficiency syndrome with pulmonary malakoplakia associated with Rhodococcus equi infection; two patients were diagnosed at autopsy and one by examination of a transbronchial biopsy specimen. All three patients had pulmonary bacterial cultures and light and electron microscopic examination. The patients were 33-, 41-, and 43-year old men, human immunodeficiency virus-positive for 2, 6, 8 years, respectively. The two patients diagnosed at autopsy had cavitary lesions, and the patient diagnosed by biopsy specimen had nodular lesions on chest radiographs. Histologically, the lungs had well-circumscribed areas of infiltration with benign macrophages with granular cytoplasm, scattered MGBs, and numerous gram-positive coccobacilli. Electron microscopic examination showed intracellular coccobacilli, from 990 X 702 to 972 X 648 nm in diameter, with thick, homogenous cell walls, trilaminar cytoplasmic membranes, and dense cytoplasm with from one to five vacuoles. Electron microscopic studies showed that the bacteria within the pulmonary macrophages had thicker cell walls, less prominent nucleoid areas, and more vacuoles than the bacteria in cultures from the sputum and blood. The mature MGB ultrastructurally had a concentric, trilaminate structure with central mineralized core and was without recognizable bacterial forms. Early MGBs, however, consisted of a circular, electron-dense core containing bacteria, ultrastructurally similar to the R. equi seen in the culture. Pulmonary malakoplakia in patients with the acquired immunodeficiency syndrome might thus represent an acquired macrophage dysfunction of the intracellular digestion of phagocytized bacteria. The bacteria within the macrophages, however, seemed to have thicker cell walls compared with those in culture, and thus might be protected from enzyme digestion. It seems that MGBs are formed around the undigested bacteria as an alternative pathway for bacterial destruction, because R. equi was identified within the cores of early MGBs but not the mature or late stage MGBs.

Dimerization of the extracellular domain of granuloycte-colony stimulating factor receptor by ligand binding: a monovalent ligand induces 2:2 complexes

Granulocyte-colony stimulating factor (G-CSF) binds to a specific cell surface receptor and induces signals for growth and differentiation in cells of granulocyte hematopoietic lineage. In order to understand how G-CSF binding initiates signals into these cells, we have studied its interactions with the entire extracellular domain of the receptor (sG-CSFR). The sG-CSFR was purified from CHO cell conditioned media with a G-CSF affinity column, resting in a preparation fully competent for ligand binding. However, when sG-CSFR was purified by conventional means, i.e., without affinity chromatography, only about half was competent. Therefore, all studies were carried out using affinity-purified material. The sG-CSFR exhibited a weak self-association into a dimer with a dissociation constant of 200microM in the absence of G-CSF. Addition of G-CSF dimerizes the receptor, with a preferred stoichiometry of 2 G-CSF molecules plus 2 receptors. Unexpectedly, receptor-receptor interactions rather than through two receptors binding to the same G-CSF molecule; i.e., G-CSF is a monovalent ligand. G-CSF binding to the receptor monomer occurs with high affinity. The binding of G-CSF also enhances the receptor-receptor dimerization; when G-CSF is bound to both receptors, dimerization is enhanced 2000-fold, while the interaction of a 1:1 receptor-ligand complex with a second ligand-free receptor is enhanced 80-fold. Thus, the mechanism of receptor dimerization is fundamentally different than that of related cytokine receptors such as growth hormone and erythropoietin receptors. Circular dichroic spectra showed a small but significant conformational change of receptor upon binding G-CSF. This is consistent with the idea that G-CSF binding alters the conformation of the receptor, resulting in an increase in receptor-receptor interactions.

Human stem cell factor dimer forms a complex with two molecules of the extracellular domain of its receptor, Kit

Stem cell factor (SCF) is a cytokine that is active toward hematopoietic progenitor cells and other cell types, including germ cells, melanocytes, and mast cells, which express its receptor, the tyrosine kinase, Kit. SCF exists as noncovalently associated dimer at concentrations where it has been possible to study its quaternary structure; it stimulates dimerization and autophosphorylation of Kit at the cell surface. We have used recombinant versions of human SCF and human Kit extracellular domain (sKit) to study SCF-Kit interactions. By size exclusion chromatography, plus various physical chemical methods including light scattering, sedimentation equilibrium, and titration calorimetry, we demonstrate the formation of complexes containing a dimer of SCF (unglycosylated SCF1-165) plus two molecules of sKit. The concentrations of SCF and sKit in these studies were in the range of 0.35-16.2 microM. The data are analyzed and discussed in the context of several possible models for complex formation. In particular, the sedimentation data are not consistent with a model involving cooperative binding. The Kd estimate for SCF-sKit interaction, obtained by sedimentation equilibrium, is about 17 nm at 25 degrees C. With glycosylated SCF1-165, the Kd is considerably higher.

Isolation of anti-leukemia compounds from Citrus reticulata

In vitro effects of medicinal plant extracts from the pericarpium of Citrus reticulata (cv Jiao Gan) (PCRJ) on the growth and differentiation of a recently characterized murine myeloid leukemic cell clone WEHI 3B (JCS) were investigated. Extracts of PCRJ not only inhibited the proliferation of JCS cells in a dose dependent manner, but also induced differentiation of JCS cells into macrophages and granulocytes. Morphological differentiation of PCRJ treated JCS cells was associated with an increase in phagocytic activity of the cells. Furthermore, both in vitro clonogenicity and in vivo growth of PCRJ treated JCS leukemic cells in syngeneic BALB/c mice were significantly reduced. The survival rate of mice receiving PCRJ treated JCS tumour cells was also increased. Using 1H-NMR, 13C-NMR, and GC/MS, two active components isolated from PCRJ were identified as nobiletin and tangeretin.

Dimerization of the extracellular domain of the erythropoietin (EPO) receptor by EPO: one high-affinity and one low-affinity interaction

Although there is considerable evidence that signaling by the erythropoietin (EPO) receptor is initiated when it is dimerized by binding EPO, it has been previously reported that the soluble extracellular domains of the EPO receptor (sEPOR) are not dimerized in the presence of EPO and are able to form only 1:1 complexes with EPO. We have now shown unambiguously by light scattering, sedimentation equilibrium, and titration calorimetry that two molecules of sEPOR can bind to a single EPO monomer but that the binding of the second sEPOR is approximately 1000-fold weaker than that of the first. Because this second binding interaction is quite weak (Kd of approximately 1 microM), the 2:1 sEPOR.EPO complexes are easily dissociated during chromatography (forming the 1:1 complexes reported previously) and cannot be isolated in pure form. Global analysis of the sedimentation equilibrium data has enabled us to determine the binding constants and is consistent with a model in which EPO has two independent binding sites for sEPOR but cannot exclude anticooperative or sequential binding models. The influence of glycosylation of EPO and/or sEPOR on the binding affinities has also been investigated. Titration calorimetry is consistent with the sedimentation data and shows that the weaker binding site has a more negative delta H. The relation of these results to the binding of EPO to membrane-bound receptors and to the phenomenon of apparent high-affinity and low-affinity classes of receptors is discussed.

Exploring the allowed sequence space of a membrane protein

We present a comprehensive view of the tolerance of a membrane protein to sequence substitution. We find that the protein, diacylglycerol kinase from Escherichia coli, is extremely tolerant to sequence changes with three-quarters of the residues tolerating non-conservative changes. The conserved residues are distributed with approximately the same frequency in the soluble and transmembrane portions of the protein, but the most critical active-site residues appear to residue in the second cytoplasmic domain. It is remarkable that a unique structure of the membrane embedded portion of the protein can be encoded by a sequence that is so tolerant to substitution.

Absorption by 1-year-old children of an iron supplement given with cow's milk or juice

A paucity of data are available on toddlers for the evaluation of optimal strategies of Fe supplementation. In this study, we used a two-tracer stable isotope technique to determine Fe absorption from a 5-mg dose of stable isotopically enriched (57Fe or 58Fe) ferrous sulfate given with cow's milk (CM) compared with the same dose given with apple juice. Ten children (age 13 +/- 1 mo, weight 10.8 +/- 1.1 kg) who had recently discontinued formula feeding and begun on CM were studied. Red blood cell (RBC) iron incorporation of the isotope was determined 14 d after dosing with 57Fe and 58Fe. Fe absorption was calculated based on the assumption that 90% of absorbed Fe is incorporated into RBC. Absorption of Fe was significantly greater (13.7 +/- 6.4%) when given with juice than with milk (5.7 +/- 4.0%), p < 0.01 by paired t tests. Fe absorption from the dose given with juice was significantly negatively correlated with serum ferritin (n = 9, r = -0.70, p < 0.05). These results indicate that 1) a small supplement of Fe is better absorbed when given with juice than with CM, and 2) a large variability in Fe absorption exists in healthy 1-y-old infants, which is related to their existing Fe stores.

A patient-derived cytotoxic T-lymphocyte clone and two peptide-dependent monoclonal antibodies recognize HLA-B27-peptide complexes with low stringency for peptide sequences

HLA-B27 molecules expressed on the T2 mutant cell line do not have peptides. Such empty HLA-B27 molecules were not recognized by an HLA-B27-restricted cytotoxic T-lymphocyte (CTL) clone (auto-1) derived from synovial fluid. To test for peptide dependency of the clone, B27-T2 cells were incubated with a panel of 48 different peptides. This lack of stringency was compared with that of a peptide-dependent monoclonal antibody, B27.M2. Positive B27.M2 reactivity resulted when the B27-T2 cells were incubated with two peptides: RRKAMFEDI and RRMGPPVGHR, derived from Chlamydia HSP60 and human ribonucleoprotein, respectively. Because of the limited availability of CTL versus monoclonal antibody, the specificity of B27.M2 was studied in greater detail. The importance of the HLA-B27 heavy chain in antibody recognition of class I-peptide complexes was demonstrated by site-directed mutagenesis. The stringency of the peptide residues was tested by making analogs of each of the nine residues in RRKAMFEDI, creating a panel of 180 analogs. Although stringency was highest for the sixth position, as many as six different amino acids provided positive reactivity. These results indicate that immune recognition of HLA-B27-peptide complexes might have rather low stringency for the peptide sequences. In theory, then, pathogen-derived peptides which induce autoimmunity by generating autoreactive CTL might not share much sequence similarity with the responsible self peptides.

PYY and GLP-1 contribute to feedback inhibition from the canine ileum and colon

To explore mechanisms whereby unabsorbed nutrients in the ileum inhibit the upper gut ("ileal brake"), we perfused the canine ileum or colon and monitored phase 3 in the duodenum. Fasting motility was recorded when the ileum or colon was perfused with 154 mM NaCl, a mixed isotonic nutrient solution (Ensure), or individual nutrients (maltose, casein hydrolysates, or sodium oleate). Blood samples were collected before and during the perfusions. The ileum was also perfused with 154 mM NaCl while peptide YY (PYY) was infused by vein. In both sets of experiments, plasma levels of PYY, neurotensin, and glucagon-like peptide-1 (GLP-1) were measured. Ileal or colonic perfusion of Ensure delayed phase 3 [migrating motor complexes (MMC)] in the duodenum, inhibited ileal motility, and increased plasma levels of PYY and GLP-1. Ileal casein and oleate and colonic casein also delayed the duodenal MMC. The MMC cycle length and plasma levels of PYY were closely correlated. Intravenous PYY prolonged the MMC cycle; an intravenous dose of 100 pmol.kg-1.h-1 of PYY mimicked the effects of ileal Ensure. These results support the hypothesis that PYY, and possibly GLP-1, participate in the ileal brake. This negative feedback loop also affects the distal small bowel. The proximal colon also triggers the feedback inhibition of gut motility (colonic brake).

Binding of Neu differentiation factor with the extracellular domain of Her2 and Her3

The interaction of neu differentiation factor (NDF) with the extracellular domains of Her2 (sHer2) and Her3 (sHer3) have been studied using native gels, light scattering, and sedimentation equilibrium. The full-length NDF beta 2 was shown to bind sHer3 with a dissociation constant of 26 +/- 9 nM, while it showed a 1000-fold weaker binding to sHer2. Taken together, these results demonstrate that NDF is a high affinity ligand for Her3, but not for Her2. No increase in affinity of the NDF beta 2 for sHer3 was observed upon addition of sHer2 to the NDF beta 2-sHer3 mixture. Binding of NDF beta 2 to sHer3 did not induce receptor dimerization or oligomerization, the stoichiometry being one sHer3 per one NDF molecule. This finding suggests that transmembrane and/or intracellular domains of receptor family members or perhaps additional unidentified components may be involved in NDF induced dimerization and autophosphorylation, or alternatively, that dimerization is not the mechanism for Her3 autophosphorylation and signal transduction.

A new strategy for enhancing the stability of lyophilized protein: the effect of the reconstitution medium on keratinocyte growth factor

PURPOSE

Protein stabilization during lyophilization has previously focused on optimization of the formulation as well as the freezing and dehydration process parameters. However, the effect of the reconstitution medium has been largely neglected. We have investigated its effect on aggregate formation using recombinant keratinocyte growth factor (KGF).

METHODS

The protein was lyophilized under suboptimal conditions to induce aggregation and precipitation upon reconstitution with water. A series of additives were examined by UV spectrophotometry and size exclusion chromatography (SEC-HPLC) for their effects on decreasing the degree of KGF aggregation and precipitation by the increase in recovery of soluble monomer.

RESULTS

Several additives resulted in a significant reduction of aggregation, including sulfated polysaccharides, surfactants, polyphosphates, and amino acids. A similar effect was achieved by adjusting the ionic strength of the reconstitution medium. SEC-HPLC indicated that the amount of soluble monomer was also increased by these additives suggesting that the recovery of the soluble protein correlates with the native, monomeric protein.

CONCLUSIONS

These results suggest that optimization of reconstitution conditions will be a useful methodology for increasing the recovery of soluble, active proteins and that for KGF, the recovery of the soluble protein correlates with the native, monomeric form.

Features of cholesterol structure that regulate the clearance of chylomicron-like lipid emulsions

Cholesterol is an absolute requirement for the clearance from plasma of the remnants of triglyceride-rich lipoproteins. Our laboratory previously established that cholesterol was essential for the hepatic uptake of remnant particles after intravenous injection of chylomicron-like lipid emulsions (1). The aim of the present study was to determine the structural features of the cholesterol molecule that regulate the metabolism of chylomicrons. Chylomicron-like lipid emulsions, which reflect the size and composition and mimic the physiology of lymph chylomicrons, were prepared with tracer amounts of labeled triolein ([14C]TO) and cholesteryl oleate ([3H]CO) to follow the hydrolysis of triglyceride and the uptake of chylomicron remnant particles by the liver. Sterols selected as cholesterol congeners with functional group variations were incorporated into the emulsions in place of cholesterol and injected intravenously in rats. Control emulsions contained either no cholesterol or approximately 1% (by weight) cholesterol. The effects of the different sterol structures on lipolysis and hepatic remnant uptake were compared with controls to determine the significance of various functional groups. Clearance of emulsion CO was impaired when cholesterol was absent or replaced by cholesteryl chloride, cholesteryl formate, or 3-keto-cholesterol. Clearance of emulsions containing epicholesterol, where the OH group at the 3-position is in the alpha configuration, was similar to control emulsions containing cholesterol. Congeners with an additional hydroxyl group, viz. 7 alpha-hydroxycholesterol, 7 beta-hydroxycholesterol, or 25-hydroxycholesterol, reduced CO clearance. Androstenol, which lacks the side chain at the C17-position, also retarded CO clearance from plasma. In contrast, emulsions incorporating congeners with side chain variations such as campesterol, beta-sitosterol, stigmasterol, or saturated congeners of cholesterol such as cholestanol, coprostanol and its epimer, epicoprostanol, all were cleared similarly to emulsions containing cholesterol. In conclusion, for physiological clearance of a chylomicron-like emulsion, the presence of a hydroxyl (-OH) group at the 3-position and an alkyl side chain at the C17-position of cholesterol are essential, while the structure of the side chain and the saturation of the ring structure are not critical. The mechanism of the specificity of sterols on the metabolism of protein-free emulsions is unclear, but does not relate to changes in microfluidity of the surface lipids, nor to the amount or isoform of associated apolipoproteins.

Elastoderma

Elastoderma is an exceedingly rare condition that is characterized clinically as an acquired localized laxity of skin resembling cutis laxa and histologically as an excessive accumulation of pleomorphic elastic structures within the dermis. We report the case of a 27-year-old white man with a 2-year history of lax, extensible, wrinkled skin with areas of protrusion and ulceration localized to the anterior aspect and back of the neck. Histologic examination of specimens from the affected area revealed increased masses of intertwined thin, elastic fibers without calcification in the papillary and upper reticular dermis. Transmission and scanning electron microscopy demonstrated irregular deposition of elastic material at the periphery of elastic tissue fibers, with grapelike globular structures. To our knowledge, this is the second reported case of elastoderma.

Expression of whey acidic protein (WAP) genes in tissues other than the mammary gland in normal and transgenic mice expressing mWAP/hGH fusion gene

Whey acidic protein (WAP) is a major whey protein secreted in rodents' milk. Murine WAP (mWAP) genes have been assumed to be expressed solely in the mammary gland. However, several heterologous genes fused with the mWAP promoter and artificially introduced into animal genomes as transgene were expressed not only in the mammary gland but also in other tissues as well. In the present study, we investigated, by means of the reverse transcription polymerase chain reaction (RT-PCR), the patterns of expression of endogenous WAP genes in tissues of normal mice and in transgenic mice carrying hGH gene coupled to the mWAP promoter sequence. The results revealed that the genes driven by the mWAP promoter, regardless of whether they are endogenous genes or transgenes, were transcribed in a variety of tissues other than the mammary gland of lactating normal female mice, although the expression levels are generally low. The expression of WAP genes in the cerebrum and the liver is regulated, as in the mammary gland, according to the reproductive stages. However, the tissue distribution of endogenous WAP gene expression in mature virgin transgenic female mice was the same as that in lactating normal female mice.

The importance of Arg40 and 45 in the mitogenic activity and structural stability of basic fibroblast growth factor: effects of acidic amino acid substitutions

High-affinity binding of basic fibroblast growth factor (bFGF) to the tyrosine kinase receptor requires cell-surface heparan sulfate proteoglycan or exogenous addition of heparin. The crystal structure of bFGF shows Arg40 and 45 on the surface opposite to the heparin-binding region, suggesting that these charged residues may be involved in the receptor binding. Therefore, these amino acids were mutated to aspartic acid separately or simultaneously, and also a simultaneous mutation to glutamic acid was introduced. These mutants displayed a mitogenic activity decreased greater than tenfold compared to the wild-type protein. Addition of heparin had no effect on the activity, while these mutants showed heparin-binding characteristics resembling those of the native sequence protein. The mutants exhibited decreased stability compared to the native sequence protein. Gradual changes in conformation were observed by circular dichroic and infrared spectroscopy. Heparin chromatography also showed the presence of denatured form for these mutants. However, in the presence of multivalent anions such as citrate, sucrose octasulfate, and heparin, the conformation of the mutants resembled that of the wild-type protein, as revealed by X-ray crystallography and circular dichroism spectra of the mutant with a Arg40-->Asp substitution.

Interaction of human gallbladder mucin with calcium hydroxyapatite: binding studies and the effect on hydroxyapatite formation

Calcium hydroxyapatite (HAP) crystals formed in vitro in the presence of polymeric human gallbladder mucin (1.0 mg/mL) were smaller (0.75 +/- 0.39 microns) than control crystals (7.86 +/- 2.76 microns), but the mucin did not affect the kinetics of crystal formation or alter the amount of mineral phase present at equilibrium. In contrast, glycopeptide subunits produced by proteolysis of the native mucin had no effect on HAP crystal size. Both native mucin and glycopeptides bound to mature HAP crystals, but the glycopeptides were much more readily displaced by phosphate ions. Therefore, in experiments where HAP was being formed, the phosphate ions inhibited the interaction of glycopeptides with the nascent HAP. These results indicate that gallbladder mucin may modulate HAP formation in vivo, and that this ability may be altered during pathological states, such as neutrophil infiltration or bacterial colonization, that may cause the release of proteinases capable of digesting mucin.

Studies on the structure and function of glycosylated and nonglycosylated neu differentiation factors. Similarities and differences of the alpha and beta isoforms

Comparative analyses of both glycosylated and nonglycosylated neu differentiation factor (NDF) isoforms revealed significant similarities and differences of their overall structures and functions. Biophysical analyses confirmed that all NDF isoforms are monomeric, but have an extended ellipsoidal shape in solution. All full-length NDFs are similar in secondary and tertiary structures and they contain no alpha-helix but are abundant in beta-strand structures. A small NDF fragment containing only the epidermal growth factor domain is also rich in beta-strand structures, but exhibits tertiary structure different from the long NDF forms. Monoclonal antibodies that selectively recognize epidermal growth factor domains of human NDF-alpha and -beta can specifically bind the respective NDF-alpha and -beta isoforms independent of NDF origins. Western blot analysis and quantitative binding assays further identify that an NDF preparation produced naturally from Rat1-EJ cells contains both alpha and beta isoforms in a 3 to 2 ratio. In receptor-binding competition experiments, human and rat NDF-beta isoforms have higher affinity than NDF-alpha isoforms. NDF-beta isoforms can dramatically enhance the stimulation of DNA synthesis for transfected NIH3T3 cells that overexpress HER-3 and HER-4 receptors, while NDF-alpha isoforms can only stimulate proliferation of HER-4-transfected cells with lower activity. Taken together, NDF-alpha and -beta isoforms share similar gross protein conformations but are biologically distinct.

Cultivation of Ehrlichia chaffeensis in mouse embryo, Vero, BGM, and L929 cells and study of Ehrlichia-induced cytopathic effect and plaque formation

We successfully propagated Ehrlichia chaffeensis in mouse embryo, Vero, BGM, and L929 cells inoculated with host cell-free ehrlichiae, indicating that E. chaffeensis is capable of entry, survival, and growth in a relatively wide range of cell types derived from different species. We demonstrated rapid adaptation of E. chaffeensis in these cell lines, so that typical morulae could be detected as early as 5 days after inoculation. E. chaffeensis-induced cytopathic effect with different morphological characteristics in mouse embryo, Vero, and L929 cells. The earliest cytopathic effect appeared in untreated and irradiated mouse embryo cells at 4 days postinoculation. As the infected foci gradually expanded, the center of the foci showed necrotic cells with pyknotic nuclei and degraded morulae. E. chaffeensis caused cell lysis in untreated and irradiated L929 cells, with formation of distinct, round macroscopic plaques at 18 days postinoculation. In untreated and irradiated Vero cells, E. chaffeensis produced infected foci composed of loosely interwoven necrotic cells, spaces of detached cells, cells filled with morulae, and uninfected cells, resulting in characteristic reticular foci. Irradiated cells generally contained many large morulae and presented larger cytopathic foci. DH82 and BGM cells did not develop obvious cytopathic foci under the conditions employed. The findings reported herein offer the opportunity to study the pathogenic mechanism of cell injury by E. chaffeensis, the basis for quantification of infectious E. chaffeensis, improved approaches for recovery of ehrlichiae from human patients and tick hosts, and additional methods for cultivation of E. chaffeensis for molecular analysis.

High level expression of human leukemia inhibitory factor (LIF) from a synthetic gene in Escherichia coli and the physical and biological characterization of the protein

LIF is a multi-functional cytokine that elicits effects on a broad range of cell types. In this report, we present the high level expression of human LIF (hLIF) from a chemically synthesized gene template in Escherichia coli where it comprises up to 25% of the cellular protein. The recombinant hLIF, after purification and folding, was examined using CD, FTIR spectroscopy and light scattering. CD and FTIR spectra showed that the hLIF is an alpha-helical protein and has a distinct tertiary structure. The IFTR spectrum resembles that of other four helical bundle proteins including G-CSF and IL-6. Light scattering analysis indicated that it is a monomeric protein, distinguishing it from M-CSF and interferon gamma, which also belong to the class of four helical bundle proteins but are dimeric. Recombinant hLIF was assayed for its activity on the murine leukemic cell line, M-1 as well as on human leukemic cell line, ML-1. It inhibited the growth of M-1 cells and differentiated them towards macrophages. However, it did not have any differentiation inducing effect on human leukemic cell lines alone or in combination with other cytokines.

Application of magnetic sector thermal ionization mass spectrometry to studies of erythrocyte iron incorporation in small children

The optimal evaluation of iron metabolism requires the administration of two isotopes of iron. However, high-precision measurement of isotopic ratios from blood samples obtained after administration of two stable isotopes of iron to human subjects has not previously been reported. Using a cation-exchange system to isolate iron from blood samples, we found that high-precision (< 0.2%) measurements of 58Fe/56Fe and 57Fe/56Fe could be performed using magnetic sector thermal ionization mass spectrometry. Clinical studies in four 1-year-old infants showed that this technique could be used to demonstrate a lower rate of iron absorption in small children given an iron supplement (57Fe) with milk compared to those given iron (58Fe and ferrous sulfate) with ascorbic acid. This technique will enable the evaluation of iron metabolism in populations in whom the use of radioactive iron tracers is not appropriate.

Interactions of neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and the NT-3.BDNF heterodimer with the extracellular domains of the TrkB and TrkC receptors

Interactions of three neurotrophin dimers, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and a NT-3.BDNF heterodimer with extracellular, soluble TrkB and TrkC receptors were studied using native gels, light scattering, and sedimentation equilibrium. These three neurotrophins showed binding of two TrkB receptors per neurotrophin dimer, with a tendency to dissociate into one TrkB per dimer for NT-3 and the heterodimer, as determined by native gels, light scattering, and sedimentation equilibrium. For TrkC, native gels suggested binding of NT-3, heterodimer, and BDNF but not of nerve growth factor. Sedimentation equilibrium revealed that all three neurotrophin molecules bind to TrkC at two receptors per dimer but that BDNF binds much more weakly and that the heterodimer has an intermediate binding strength. Light scattering/size exclusion chromatography showed complexes with two TrkC receptors per NT-3 dimer and one TrkC per heterodimer but did not detect binding of BDNF to TrkC. This latter result is not inconsistent with the sedimentation data, because the weak binding of BDNF to TrkC may be easily dissociated by nonspecific interactions of BDNF with the size exclusion column. The relative binding constants for these neurotrophins and the soluble receptor extracellular domains, as determined by sedimentation equilibrium, are correlated with their biological activity. However, the magnitude of these binding constants is insufficient by approximately 3 orders of magnitude to promote receptor dimerization at physiologically active concentrations.

The effect of mutant beta 2-microglobulins on the conformation of HLA-B27 detected by antibody and by CTL

The arthritis-predisposing HLA-B27 consists of a heavy chain, a small peptide, and the monomorphic beta 2-microglobulin (beta 2-m). CTLs and a mAb, Ye-2, which recognize the complex with specificities both for the heavy chain and for the peptide, are available. The beta 2-m is in noncovalent association with the heavy chain at multiple points and is exchangeable with free beta 2-m outside of the complex. The purpose of our experiments was to test whether mutant beta 2-m capable of modulating HLA-B27 activity could be created. Eighteen recombinant mutants of the human beta 2-m were experimentally generated. In 14 of these, mutations were at or near residues that are either contact residues or interface residues with the heavy chain. Relative to the parent beta 2-m, two-thirds of the mutants showed reduced ability to exchange into HLA-B27 complexes. However, at least four of them induced more than 80% decrease in Ye-2 Ab reactivity. Two mutants were able to induce a minor decrease in susceptibility to lysis by four CTL clones. One of the CTL clones was autoreactive. Two of the CTL clones were specific for HLA-B27 cells experimentally infected with arthritis-causing Yersinia enterocolitica. These results indicate that certain beta 2-m residues play an indirect role in peptide presentation, although they are not directly associated with the peptide residues.

The effect of mutant beta 2-microglobulins on the conformation of HLA-B27 detected by antibody and by CTL

The arthritis-predisposing HLA-B27 consists of a heavy chain, a small peptide, and the monomorphic beta 2-microglobulin (beta 2-m). CTLs and a mAb, Ye-2, which recognize the complex with specificities both for the heavy chain and for the peptide, are available. The beta 2-m is in noncovalent association with the heavy chain at multiple points and is exchangeable with free beta 2-m outside of the complex. The purpose of our experiments was to test whether mutant beta 2-m capable of modulating HLA-B27 activity could be created. Eighteen recombinant mutants of the human beta 2-m were experimentally generated. In 14 of these, mutations were at or near residues that are either contact residues or interface residues with the heavy chain. Relative to the parent beta 2-m, two-thirds of the mutants showed reduced ability to exchange into HLA-B27 complexes. However, at least four of them induced more than 80% decrease in Ye-2 Ab reactivity. Two mutants were able to induce a minor decrease in susceptibility to lysis by four CTL clones. One of the CTL clones was autoreactive. Two of the CTL clones were specific for HLA-B27 cells experimentally infected with arthritis-causing Yersinia enterocolitica. These results indicate that certain beta 2-m residues play an indirect role in peptide presentation, although they are not directly associated with the peptide residues.

Pulsed-laser-induced dark traces in LiNbO(3):Zn and LiNbO(3):Mg

Laser-induced dark traces in LiNbO(3):Zn (7.5 mol.%) and LiNbO(3):Mg (6.5 mol.%) crystals have been studied with the crystals as the component for 1064-532-nm second-harmonic generation of Nd:YAG radiation operating in a pulsed mode. The results show that, at high input power density, both crystals suffer from dark traces, which reduce the second-harmonic-generation efficiency. The dark traces are induced by the 532-nm radiation and can be recovered after the laser beam is shut off. The transmission loss spectra show a broad absorption peak near 400 nm.

Rickettsia conorii infection of C3H/HeN mice. A model of endothelial-target rickettsiosis

BACKGROUND

Rickettsial diseases result from disseminated intraendothelial cell infection. The clinically critical conditions, meningoencephalitis and interstitial pneumonia, are associated with multifocal rickettsial vascular injury.

EXPERIMENTAL DESIGN

C3H/HeN mice inoculated intravenously with either 2.25 x 10(3) or 2.25 x 10(5) Rickettsia conorii (Malish 7 strain) were observed for illness with sacrifice of animals for evaluation of pathologic lesions and host responses by light and electron microscopy, rickettsial content and location by plaque assay, immunohistology, and electron microscopy, and immune response by cytokine analyses and serology.

RESULTS

Mice inoculated with a high dose of rickettsiae established disseminated endothelial infection on day 1, became ill with progressive increase in rickettsiae on day 4, and died with vascular injury-based meningoencephalitis and interstitial pneumonia on day 5 or 6. Mice inoculated with the low rickettsial dose became ill on day 5 and recovered by day 10. Clearance of rickettsiae was associated with lymphohistiocytic perivasculitis. Rickettsial infection of Kupffer cells and hepatocytes led to the formation of transient hepatic granulomas. Infection-associated loss of the ability of spleen cells to secrete interleukin-2 on stimulation with concanavalin A suggested transient immunosuppression.

CONCLUSIONS

This experimental infection provides the best available model for rickettsial disease with endothelial infection and injury, immune rickettsial clearance, regeneration of endothelium, and repair of the vascular lesions.

Testing the importance of each residue in a HLA-B27-binding peptide using monoclonal antibodies

When a peptide derived from histone 3.3 was incubated with mouse L cells transfected with HLA-B27, the cells became highly reactive with Ye-2, an anti-HLA-B27 mAb. The critical residues were analyzed by testing analogues in which each of the nine residues in the peptide was consecutively substituted by 19 other amino acids. The conclusions were separately verified using a different HLA-B27-positive cell line. The ability of some of these peptides to bind to HLA-B27 was also assayed by their ability to stabilize HLA-B27 in a mutant cell line which required HLA-B27-binding peptides to express HLA-B27 at 37 degrees C. These experiments showed that in P4, P5, P6, P7, P8, and P9, all 20 different amino acids could be substituted without eliminating the ability of the analogues to bind to HLA-B27. The residues which were responsible for the HLA-B27-peptide complex reacting with the Ye-2 antibody were P8 and P9. The latter might mediate its effect by altering either the surface conformation of the closely associated HLA-B27 heavy chain or the conformation of the peptide itself.

Serum antibodies from patients with ankylosing spondylitis and Reiter's syndrome are reactive with HLA-B27 cells transfected with the Mycobacterium tuberculosis hsp60 gene

HLA-B27-related arthritis is probably mediated by an immune response against HLA-B27 complexed with peptides derived from proteins of arthritis-causing bacteria. Immunogenic proteins with a high degree of homology among bacteria, such as in the hsp60 family, are likely candidates. To create such complexes experimentally, we transfected an HLA-B27 cell line with the Mycobacterium tuberculosis hsp60 gene. Because of previous observations that HLA-B27-peptide complexes can be distinguished by antibodies, we tested the transfected cell line with a panel of sera from 24 HLA-B27+ arthritis patients. Significant antibodies were detected in at least eight of the sera. Several cell lines and peptides were used as negative controls to ensure that the antibody reactivity was specific to HLA-B27-peptide complexes. A panel of nine peptides derived from the sequence of the Mycobacterium hsp60 were synthesized and tested. At least three were identified as being responsible for the serological activities.

A monoclonal antibody that recognizes HLA-B27 in the context of peptides

The T2 mutant cell line is unable to load peptides into the MHC class I Ags inside the cells. These "empty" MHC class I Ags are not expressed on the cell surface unless the cells are cultured at low temperatures. Expression will occur at 37 degrees C only in the presence of peptides that bind to and stabilize the class I Ags. T2 cells transfected with the B*2705 gene were tested with a panel of anti-HLA-B27 mAb. Two of the antibodies, ME1 and KS3, reacted with the "empty" HLA-B27 expressed at low culture temperatures. Three antibodies, B27.M1, B27.M2, and Ye-2, were unreactive with these "empty" HLA-B27. The cells were then incubated with a panel of HLA-B27-binding peptides. One of the antibodies, Ye-2, became reactive when the cells were incubated with a peptide derived from HIV gp120 and to a less degree with a peptide derived from histone H3.3. Mouse L cells transfected with the B*2705 and the human beta 2m genes also reacted very poorly with B27.M1, B27.M2, and Ye-2. Those two peptides were also able to induce high increase in Ye-2 reactivity. Alternately, increase in Ye-2 reactivity was also observed when the L cells were incubated with IFN-gamma or TNF-alpha. These experiments indicate that the Ye-2 anti-HLA-B27 mAb recognizes HLA-B27 in the context of certain residing peptides either added exogenously or expressed endogenously. The B27.M1 and B27.M2 antibodies might share similar characteristics.

Testing the importance of each residue in a HLA-B27-binding peptide using monoclonal antibodies

When a peptide derived from histone 3.3 was incubated with mouse L cells transfected with HLA-B27, the cells became highly reactive with Ye-2, an anti-HLA-B27 mAb. The critical residues were analyzed by testing analogues in which each of the nine residues in the peptide was consecutively substituted by 19 other amino acids. The conclusions were separately verified using a different HLA-B27-positive cell line. The ability of some of these peptides to bind to HLA-B27 was also assayed by their ability to stabilize HLA-B27 in a mutant cell line which required HLA-B27-binding peptides to express HLA-B27 at 37 degrees C. These experiments showed that in P4, P5, P6, P7, P8, and P9, all 20 different amino acids could be substituted without eliminating the ability of the analogues to bind to HLA-B27. The residues which were responsible for the HLA-B27-peptide complex reacting with the Ye-2 antibody were P8 and P9. The latter might mediate its effect by altering either the surface conformation of the closely associated HLA-B27 heavy chain or the conformation of the peptide itself.

A monoclonal antibody that recognizes HLA-B27 in the context of peptides

The T2 mutant cell line is unable to load peptides into the MHC class I Ags inside the cells. These "empty" MHC class I Ags are not expressed on the cell surface unless the cells are cultured at low temperatures. Expression will occur at 37 degrees C only in the presence of peptides that bind to and stabilize the class I Ags. T2 cells transfected with the B*2705 gene were tested with a panel of anti-HLA-B27 mAb. Two of the antibodies, ME1 and KS3, reacted with the "empty" HLA-B27 expressed at low culture temperatures. Three antibodies, B27.M1, B27.M2, and Ye-2, were unreactive with these "empty" HLA-B27. The cells were then incubated with a panel of HLA-B27-binding peptides. One of the antibodies, Ye-2, became reactive when the cells were incubated with a peptide derived from HIV gp120 and to a less degree with a peptide derived from histone H3.3. Mouse L cells transfected with the B*2705 and the human beta 2m genes also reacted very poorly with B27.M1, B27.M2, and Ye-2. Those two peptides were also able to induce high increase in Ye-2 reactivity. Alternately, increase in Ye-2 reactivity was also observed when the L cells were incubated with IFN-gamma or TNF-alpha. These experiments indicate that the Ye-2 anti-HLA-B27 mAb recognizes HLA-B27 in the context of certain residing peptides either added exogenously or expressed endogenously. The B27.M1 and B27.M2 antibodies might share similar characteristics.

Stoichiometry of heparin binding to basic fibroblast growth factor

Fibroblast growth factors (FGFs) strongly bind to heparin and are thereby stabilized against deactivation and proteolytic cleavage. We have investigated the interactions of basic fibroblast growth factor (bFGF) with low- and high-molecular-weight heparin using size exclusion chromatography with on-line light scattering, absorbance, and refractive index detection. When heparin-bFGF mixtures with excess heparin are chromatographed using eluant that does not contain heparin, essentially all the protein is seen to elute as a complex with the heparin, indicating strong binding such that the complex does not dissociate significantly during chromatography (approximately 20 min). Combining the data from the light scattering, absorbance, and refractive index chromatograms allows us to determine the molecular weight of the protein component of the complex, and therefore to measure the number of bFGF molecules bound per heparin. A series of samples were prepared with a constant concentration of bFGF and variable amounts of a low-molecular-weight heparin (LMWH, M(r) = approximately 5000). At bFGF: heparin ratios above 1.5, a mix of complexes containing 3, 2, and 1 bFGF molecules is observed, with an average of 2.2 bFGF molecules per complex. Since the amount of bFGF incorporated into complexes implies an average of 2.5 +/- 0.3 bFGF molecules per heparin, there is only one heparin molecule per complex. The coexistence of complexes of different size when bFGF is in excess implies that the LMWH molecules are heterogeneous with respect to their ability to bind bFGF. When a high-molecular-weight heparin (HMWH, M(r) = 15,000) is used, complexes averaging 6.3 bFGF molecules per HMWH molecule are seen, while the overall amount of bFGF appearing in complexes implies six to seven sites per HMWH. These data show that the protein molecules can be packed very closely together. Both types of heparin give a heparin mass of 2300 Da per bFGF binding site, which corresponds approximately to an octasaccharide.

Characteristics of human immunodeficiency virus and chlorpromazine induced antiphospholipid antibodies: effect of beta 2 glycoprotein I on binding to phospholipid

OBJECTIVE

To investigate the nature of the target epitope for human immunodeficiency virus (HIV) and chlorpromazine (CPZ) induced antiphospholipid antibodies (aPL) by evaluating the effect of the aPL cofactor (beta 2 glycoprotein I) on phospholipid binding and to compare this with known binding patterns of infection induced and autoimmune aPL.

METHODS

aPL positive sera from 17 patients with HIV and 16 patients with schizophrenia treated with CPZ were tested and compared with aPL positive sera from 20 patients with syphilis and 35 with autoimmune disease. Both the sera and either IgG fractions prepared by affinity chromatography or IgM fractions prepared by euglobulin precipitation and gel filtration were tested for binding to cardiolipin (CL) in ELISA in the presence and absence of purified human beta 2 glycoprotein I (beta 2-GPI). Competition studies evaluated biotinylated CPZ IgM aPL binding and the effect on this of added aPL, placental anticoagulant protein I--a phospholipid binding protein that inhibits autoimmune aPL--and CL vesicles.

RESULTS

HIV IgG aPL binding to CL was inhibited by beta 2-GPI (51-53%), like syphilis IgG aPL and in contrast to autoimmune IgG aPL. CPZ IgM aPL, like autoimmune IgM aPL, bound more efficiently in the presence of beta 2-GPI, with binding increases of 31-149%. Binding of biotinylated CPZ IgM aPL to CL was competitively inhibited by autoimmune IgG aPL (47%) and CPZ aPL (92%) but not by HIV IgG aPL or normal IgG. Placental anticoagulant protein I and CL vesicles completely prevented binding of CPZ IgM aPL to CL (100 and 96% inhibition, respectively).

CONCLUSIONS

Findings indicate that CPZ aPL resembles the autoimmune aPL, whereas aPL found in HIV infection do not appear to be of autoimmune type.

Densimetric determination of equilibrium binding of sucrose octasulfate with basic fibroblast growth factor

Fibroblast growth factors (FGFs) strongly bind to heparin and are thereby stabilized against deactivation and proteolytic cleavage. Sucrose octasulfate (SOS), which has a chemical structure resembling the repeating unit of heparin, has also been shown to enhance stability of basic FGF against thermal denaturation and to induce a small conformational change. We have examined SOS binding to bFGF using equilibrium dialysis. The difference in SOS concentration across the dialysis membrane was measured using a precision density meter, since the density of SOS differs greatly from that of water. With care, this densimetric technique can measure binding with a precision of +/- 0.1 mol/mol using about 2 mg/ml of protein. These results show that the binding saturates at 2 mol of SOS per mole of bFGF as the SOS concentration increases to 3.6 mM or higher. The effect of SOS on the thermal stability of bFGF was examined using denaturation at a constant heating rate, by both turbidity and differential scanning calorimetry. Since the thermal denaturation is irreversible, the temperature where aggregation abruptly increases was taken to indicate the onset of denaturation. This temperature increased by approximately 12 degrees C as the SOS concentration increased from 0.018 to 3.6 mM and remained constant above 3.6 mM, consistent with our binding data if the binding is specific to the native state.

Acid-induced unfolding of brain-derived neurotrophic factor results in the formation of a monomeric "a state"

Recombinant human brain-derived neurotrophic factor in acid undergoes a slow loss of tertiary structure as monitored by both near-UV circular dichroism and fluorescence, and appears to retain some secondary structure, as monitored by far-UV circular dichroism and Fourier transform infrared spectroscopy. This loss of tertiary structure parallels a decrease in the weight average molecular weight, from dimer to monomer, when examined using light scattering. Increasing the temperature accelerates this slow reaction. This process may be described most simply as N2 in equilibrium with 2D where N and D are the native and denatured forms of the protein, respectively. However, the acid denaturation strongly depends on the protein concentration, with higher concentration resulting in a lower rate and extent of denaturation. This suggests that the more complicated mechanism N2 in equilibrium with 2N in equilibrium with 2D more accurately describes the denaturation, where the dissociation into a native monomer is the rate-limiting step, and the conversion of N to D occurs relatively rapidly. Size-exclusion chromatography (at neutral pH) at several points during denaturation further demonstrated that the amount of tertiary structure remaining paralleled the dimer concentration and also that the monomer form was long-lived, remaining as monomer during the course of the chromatography. Size-exclusion chromatography and sedimentation velocity determination indicated that the acid-denatured form is a compact molecule. On the basis of the above data, the acid-denatured form may be considered to be a monomeric compact intermediate A state with no tertiary structure but considerable secondary structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Refolding of brain-derived neurotrophic factor from guanidine hydrochloride: kinetic trapping in a collapsed form which is incompetent for dimerization

We have studied the pathway and kinetics of refolding of recombinant human brain-derived neurotrophic factor (BDNF), which is a very tightly-associated dimer in its native state. When BDNF unfolded in 6 M guanidine hydrochloride is diluted 20-fold into phosphate-buffered saline, a partially folded intermediate is rapidly formed (< 1 min). Circular dichroism and fluorescence spectroscopy show that this intermediate has extensive secondary structure, but no well-defined tertiary structure. Size-exclusion chromatography with light scattering detection shows that it is compact and monomeric, and therefore corresponds to what is often called a "collapsed form" or "molten globule". This collapsed form disappears with a half-time of approximately 30 min, simultaneously with the appearance of native dimers, without accumulation of monomeric species with a native tertiary structure. Remarkably, the monomer-dimer association constant of the collapsed form is approximately 10(10) weaker than the native structure, and it has a low tendency to form large aggregates. Given the very large hydrophobic surface present at the dimer interface of nerve growth factor (and presumably in BDNF), these results indicate that these hydrophobic groups are not exposed in the collapsed form, and that it is therefore quite dissimilar from the native structure. A significant conformational change in the collapsed form is necessary to re-expose these hydrophobic groups to form the dimer interface, making this the rate-limiting step in reaching the native conformation.