Paraffin oil protected high resolution hybrid isoelectric focusing for the demonstration of substitutions of neutral amino acids in denatured proteins: The case of four human transthyretin (prealbumin) variants associated with familial amyloidotic polyneuropathy

New sequence variants in patients affected by amyloidosis show transthyretin instability by isoelectric focusing

The plasma protein transthyretin (TTR) can aggregate into insoluble amyloid fibrils causing systemic amyloidosis (ATTR amyloidosis) in patients carrying a variant TTR protein. If new variants arise, it is crucial to clarify whether they are disease-associated or benign. In this study, we further functionally characterize three new and unclassified TTR variants (Thr40Asn, Phe64Val and the described but not functionally assessed variant Leu12Val), using a simplified, fast isoelectric focusing (IEF) approach. After validating the system with known TTR variants, we assessed the sera of five patients carrying these new TTR variants in a heterozygous state. All three variants showed aberrant banding patterns that were similar to those of other well-characterized TTR variants, including the common Val30Met variant that causes ATTR amyloidosis. In addition to a clear band corresponding to monomeric wild-type TTR, we observed an additional variant band at the cathodal side of the IEF gel. These results indicate conformational instability of the new Thr40Asn, Phe64Val and Leu12Val variants. Together with the clinical and immunohistological data of these patients and affected family members, as well as the absence of these variants in human genetic mutation databases, our results strongly hint that these variants are amyloidogenic and therefore probably disease-associated. These findings have implications for patient therapy and for genetic counselling of family members.

Transthyretin mutations in hyperthyroxinemia and amyloid diseases

Over 80 different disease-causing mutations in transthyretin (TTR) have been reported. The vast majority are inherited in an autosomal dominant manner and are related to amyloid deposition, affecting predominantly peripheral nerve and/or the heart. A small portion of TTR mutations are apparently non-amyloidogenic. Among these are mutations responsible for hyperthyroxinemia, presenting high affinity for thyroxine (a TTR ligand). Compound heterozygotic individuals for TTR mutants have been described; noteworthy is the clinically protective effect exerted by a non-pathogenic over a pathogenic mutation. Current TTR mutations and their significance are briefly reviewed here.

Screening and biochemical characterization of transthyretin variants in the Portuguese population

The study of pathogenic and nonpathogenic transthyretin (TTR) variants is very important for the understanding of such TTR-related diseases as hereditary amyloidosis and also to establish a relationship between the structure and function of the molecule. Variants with clinical manifestations can be easily detected, but clinically silent variants can be detected only by population screening programs using specialized techniques. Hybrid isoelectric focusing (HIEF) in extremely flattened immobilized pH gradients (IPG) allows the detection of even neutral amino acid substitutions and has been used to analyze approximately 5,000 samples from the Portuguese population. Comparison with samples from carriers of three known TTR mutations (Met 30 associated with hereditary amyloidosis, Met 119, and Asn 90) was also made. In this study we detected: (1) 8 individuals carriers of TTR Met 30, (2) 35 carriers of TTR Met 119, (3) 12 carriers of TTR Asn 90, (4) 1 compound heterozygote for TTR Met 30/Met 119, and (5) 5 variants that presented a different pattern from the controls used. We also performed DNA sequencing analyses of two of the variants with the different band pattern in HIEF. The individuals were found to be carriers of TTR Ile 122 and TTR Thr 190, respectively. All the mutations detected, except for Asn 90, result from substitutions in CpG hot spots and thus can be rather frequent in the populations. Studies on the clinical evolution of the compound heterozygotes and on the physical-chemical properties of these hybrid TTRs will help to understand the pathogenicity associated with TTR.

Immobiline-based two-dimensional gel electrophoresis: an optimised protocol for resolution of human colonic mucosal proteins

An optimised protocol for the production of two-dimensional protein patterns of human colonic mucosal cells using immobilised pH gradients in the first dimension is presented. We tested a wide variety of solubilising agents and electrophoretic parameters, separately and in combination. Protein solubilisation was found to be best using a lysis solution containing 9 M urea, 2% Triton X-100, 2% 2-mercaptoethanol, 0.8% Pharmalyte pH 3-10 and 8 mM phenylmethylsulfonyl fluoride (PMSF). Horizontal streaking of basic proteins in the first dimension was virtually eliminated by a combination of washing Immobiline strips in 100 mM asorbic acid, pH 4.5, for 24 h before use and isoelectric focusing samples at 40 degrees C. The presence of 2% glycerol in the first dimension resulted in tighter resolution throughout the entire pH range. The use of these conditions may prove to have broad applicability to the generation of optimally resolved Immobiline-based two-dimensional protein patterns of many other tissues.

Electrically neutral microheterogeneity of human plasma transthyretin (prealbumin) detected by isoelectric focusing in urea gradients

Mutants of the human plasma transthyretin (TTR, prealbumin) have attracted interest due to their rather frequent association with the autosomal dominant disease familial amyloidotic polyneuropathy (FAP). Some three quarters of known TTR mutations produce electrically neutral amino acid substitutions undetectable via separation by charge. We have developed an electrophoretic procedure sensitive to differences in the stability of tetramers and monomers under partially denaturing conditions. The differential folding states were found to be fully reversible. Applying the procedure we found 14 electrically silent mutants of TTR among 2000 plasma samples from German donors. We demonstrate that the normal TTR monomer exists in different forms of variable stability and/or charge due to binding of sulfhydryls from plasma to the unique cysteine at position 10 of the primary structure as well as due to modification by treatment with an oxidant. We found that reduction of Cys10 increases the stability of the folded monomeric and tetrameric conformations. The conformational changes of TTR induced by isoelectric focusing in a urea gradient were found to be associated by a gain of three positive charge units. Using published crystallographic data we present structural sites in the TTR molecule which could explain the observed effects.

A simple screening test for variant transthyretins associated with familial transthyretin amyloidosis using isoelectric focusing

Variant forms of the plasma protein transthyretin (TTR) are associated with the most frequently occurring type of familial systemic amyloidosis. Organ system involvement in transthyretin type amyloidosis (ATTR) is often similar to that which occurs in light chain amyloid disease (AL). The proper diagnosis of ATTR is important since treatment (liver transplantation) differs from that in AL (chemotherapy). We present a two-step test to screen sera for variant TTRs using non-denaturing gel electrophoresis performed in 7.5% acrylamide (PAGE) followed by isoelectric focusing (IEF) between pH 4.0 and 7.0 in 2.5 M urea. Serum samples from 110 patients with amyloidosis and their relatives were tested using this IEF technique and compared to genetic mutation results. Sera from patients with ATTR who underwent liver transplantation were also examined prior to and following surgery. IEF analysis showed the presence of both wild-type and variant TTR in 74 of the 110 serum samples tested. Genomic DNA from peripheral blood was used to identify TTR gene mutations in 77 of the 110 patients. Fifteen variants including Val122Ile, preponderant in the African-American population, could be demonstrated by IEF. The sensitivity of IEF was 96% (74/77) and the specificity was 100% (33/33). The predictive values for a positive or negative result were 100% (74/74) and 92% (33/36), respectively. There were no false-positive results and 4% (3/77) false-negative results. In sera from patients with ATTR who underwent liver transplantation, variant TTR was detected by IEF before, but not after, surgery. A simple, accurate, sensitive method is presented as a useful screening test for variant transthyretins associated with ATTR.

Homozygosity and heterozygosity for the transthyretin Leu64 mutation: clinical, biochemical and molecular findings

Transthyretin gene point mutations cause hereditary amyloidosis with an autosomal dominant pattern of inheritance. The disease usually manifests itself in heterozygous patients, although a few homozygotes have been reported. We describe two unrelated patients carrying the Leu64 mutation, one of whom presents a homozygous genotype (Family B). Homozygosity was confirmed by sequence analysis, RG-PCR and double one-dimensional electrophoresis of the plasma protein. Although the clinical picture of the homozygous patient of Family B was more severe than that shown by the heterozygous members of Family A, the variability often displayed by FAP patients does not allow any firm conclusion about the role of homozygosity in the seriousness of the disease.

Isoelectric focusing as a tool for the investigation of post-translational processing and chemical modifications of proteins

It has been demonstrated that good agreement may be observed between computed and experimental isoelectric point (pI) values when proteins of known sequence are focused under denaturing conditions on immobilized pH gradient IPG slabs, at least in the pH range 4-7.5. Hence, discrepancies between expected and found in this experimental set-up may be reliably ascribed to some kind of post-transcriptional processing, or chemical modification, having taken place in the sample. This evaluation is made easier when the comparison is set between the pI of a parent molecule and that (or those) of one to several of its derivatives as resolved in a single experiment (for instance, as a spot row in two-dimensional maps); no previous knowledge is required in these cases about the amino acid composition of the primary structure. The effects on protein surface charge are discussed in this review mainly for two biologically relevant processes, glycosylation and phosphorylation. Then, the pI shifts are analysed for some protein modifications that may occur naturally but can also be artefactually elicited, such as NH2 terminus blocking, deamidation and thiol redox reactions. Finally, carboxymethylation and carbamylation are used to exemplify chemical treatments often applied in connection with electrophoretic techniques and involving charged residues. Procedures to be applied in order to verify whether a given modification has occurred, and often relying on the focusing of a treated specimen, are detailed in each section. Numerical examples on model proteins are also discussed. As an important field of application of the above concepts may be genetic engineering, an exhaustive bibliographic list dealing with pI evaluation and structural assessment on recombinant proteins is included.

Purification and characterization of amyloid-related transthyretin associated with familial amyloidotic cardiomyopathy

Analysis of amyloid fibril material associated with familial amyloidotic cardiomyopathy revealed that it contains a mixture of transthyretin-related polypeptides. The major protein band in SDS/polyacrylamide gel corresponding to a molecular mass of 14.5 kDa, consists of transthyretin fragments starting at positions 46, 49 and 59, the latter not previously identified, and one blocked fragment derived from the N-terminal part of transthyretin. In reverse-phase HPLC, the major fragment recovered was that starting at Thr49, indicating a trypsin-like cleavage (Lys at position 48). Two minor bands, corresponding to 17 kDa and 35 kDa, contained proteins with blocked N-termini, and migrated as monomeric and dimeric transthyretin, respectively. A 13-kDa protein band was found to contain transthyretin with a ragged N-terminus, mainly starting at positions 2 and 5. Three more bands, corresponding to 10, 25 and 29 kDa, consist of transthyretin molecules with blocked N-termini and most likely of aggregates of truncated molecules. A point mutation of amyloid transthyretin was identified at position 111 (Met instead of Leu in normal serum transthyretin) which confirms the mutation found for Danish siblings with familial amyloidotic cardiomyopathy. However, the presence of a non-variant amyloid transthyretin was also observed, indicating that the Danish kindred is heterozygous with respect to this point mutation. Isoelectric focusing of the amyloid fibril material resolved multiple protein bands ranging over pH 4.5-6.5, confirming heterogeneities. Methanol extraction of the cardiac amyloid fibril material prior to the purification steps reveals a methanol-soluble substance amounting to about 10% (by mass dry material) of the amyloid fibril material. A yellow substance in this fraction shows absorbance maxima (270, 280 and 430 nm) similar to those observed for transthyretin in normal serum. Gas chromatography/mass spectrometry of the methanol extract revealed the presence of saturated fatty acids (C14:0, C16:0 and C18:0 in the corresponding ratio 2:8:5) and polyunsaturated fatty acids (C16:1, C18:1, C18:2 and C20:4 in the corresponding ratio of 1:2:1:1) as further constituents of the amyloid fibril material.

Solute focusing techniques for bioseparations

The growth of industrial biotechnology has had a major impact on the research and development of both analytical and preparative focusing techniques. Pioneering methods such as isoelectric focusing are being modified to overcome the limitations of batch mode operation, scale up difficulties, high power requirement, and excessive heat generation. present advances in focusing techniques have been made by either overcoming the limitations of standard techniques or by investigating new focusing systems. Here we review the background and history of isoelectric focusing and discuss several new focusing techniques including recycle isoelectric (RIEF), counteracting chromatographic electrophoresis (CACE), and countercurrent gradient chromatography (CGC).

Mannitol prevents methionine sulphoxidation mediated electrophoretic heterogeneity of apolipoprotein A-I

Hybrid isoelectric focusing of apolipoprotein A-I in polyacrylamide gels with immobilized pH-gradients under non-denaturing conditions resulted in the occurrence of additional bands which could prevent the specific and sensitive detection of genetic variants. Hybrid isoelectric focusing of two chromatographically distinguishable apolipoprotein A-I isoforms that differ by sulphoxidation of methionine residues, apo A-I(Met) and apo A-I(MetSO), revealed that the additional bands were caused by this post-translational modification. Several antioxidative additives and conditions were compared for their ability to prevent methionine sulphoxidation in apolipoprotein A-I. In the presence of 200 g/L mannitol in the gel, apolipoprotein A/I focused as a single band. Since methionine sulphoxidation in proteins is a general phenomenon either taking place in vivo or in vitro by isoelectric focusing, we conclude that isoelectric focusing in the presence of mannitol will improve the quality of resolution of many proteins in gels with immobilized pH-gradients.

Transthyretin mutations in health and disease

To date, over 40 different mutations in transthyretin (TTR) have been associated with amyloid deposition. The major unresolved problem is the correlation between the clinical heterogeneity and the genetic heterogeneity. For instance, whereas some mutations produce neuropathy and some give rise to cardiomyopathy, others produce vitreous opacities, the vast majority being neuropathic. Moreover, some mutations are not amyloidogenic but are responsible to hyperthyroxinemias (by virtue of the protein function in thyroid transport), whereas others are apparently nonpathogenic. The study of TTR variants is very important to the understanding of the amyloid formation process and to establish a relationship between the structure and function of the molecule. The results of current TTR mutation screening programs and their characterization are summarized.

Electrophoretic artifacts arising from the use of thiol-containing reagents

Thiol reagents migrate as a curtain behind the salt front when loaded with the sample solution onto disc-electrophoresis gels. In immobilized pH gradients (IPG) the same compounds are driven by electrophoresis and electroosmosis from the alkaline to the neutral and acidic regions of the gradients. In either case, a dose-dependent sideways spreading results in spurious reduction between adjacent lanes if samples with and without reducing agent are loaded side by side.

Cardiac amyloidosis: a review and report of a new transthyretin (prealbumin) variant

Cardiac amyloidosis is caused by amyloid deposits derived from different human plasma proteins. It can lead to cardiac conduction disturbances, restrictive cardiomyopathy, and low output heart failure. The heart is variably involved during the development of systemic amyloidosis and seems to be more frequently affected in immunoglobulin (primary) than in reactive (secondary) amyloidosis. Amyloid is common in the elderly. Isolated atrial amyloid, for which a major subunit is the atrial natriuretic peptide, seems to be three times more frequent than senile cardiac amyloid, which is derived from normal prealbumin (transthyretin). Like polyneuropathy, cardiac amyloidosis is a prominent clinical feature of hereditary amyloidosis, namely of the autosomal dominant transthyretin (TTR) type. All 28 cases of TTR amyloidoses reported so far were heterozygotes for a single nucleotide change in the gene for TTR that resulted in amino acid substitutions in the mature protein. A new TTR genetic variant is reported in a German family where the index patient presented at the age of 63 with anginal pain and arrhythmia. Electrocardiography was suggestive of a pseudoinfarction pattern, and echocardiography and cardiac catheterisation showed signs of hypertrophic nonobstructive cardiomyopathy with increased ventricular filling pressures and a prominent "a" wave. Amyloid of the TTR type was identified by immunohistochemistry in the endomyocardial biopsy specimen. Hybrid isoelectric focusing established heterozygosity by showing normal TTR protein and an electrically neutral TTR variant differing from all known TTR variants so far. The patient died in an accident before investigations were complete. Electrophoretic analysis of the plasma from his first degree relatives (son, daughter, brother, and mother) identified the asymptomatic 22 year old son as an apparently heterozygous carrier of the mutant TTR protein. Comparative tryptic peptide mapping and sequencing showed that isoleucine at position 68 of the amino acid sequence was replaced by leucine.

Slowly progressive familial dementia with recurrent strokes and white matter hypodensities on CT scan

We describe 2 normotensive sisters presenting slowly progressive dementia associated with acute or subacute focal neurological symptoms, unilateral or bilateral motor signs, and dysarthria. Their father, who died in the seventh decade, had a similar clinical picture. Computerized axial tomography (CT) scan of the head showed symmetrical hypodensities in the periventricular white matter and mild to moderate hydrocephalus. In these patients a diagnosis of Binswanger's disease was based on the clinical features supported by white matter changes on CT scan. Our study suggests that genetic factors may play a role in the etiology of Binswanger's disease.

Two transthyretin variants (TTR Ala-49 and TTR Gln-89) in two Sicilian kindreds with hereditary amyloidosis

We report the biochemical and molecular characterization of two new transthyretin (TTR) variants in two Italian families with hereditary amyloidosis. Both families presented neuropathy and cardiomyopathy but they differ in other clinical features. These TTR variants were previously detected by isoelectric focusing (IEF); one is a neutral TTR variant and the other one is basic. By protein and DNA analysis the neutral variant was found to have a substitution of an alanine for a threonine residue at position 49 (TTR Ala-49) of the polypeptide chain. The basic variant has a glutamine residue replacing glutamate at position 89 (TTR Gln-89).

Transthyretin Leu 68 in a form of cardiac amyloidosis

A form of transthyretin (TTR)-related cardiac amyloidosis was previously described in a German patient. Electrophoretic analysis of plasma TTR showed the presence of an electrically neutral variant. We have now characterized the variant transthyretin by comparative peptide mapping, aminoacid and DNA sequencing procedures. A new mutation in TTR with a substitution of leucine for isoleucine at position 68 of the monomer is described.

Familial amyloid polyneuropathy: alanine-for-threonine substitution in the transthyretin (prealbumin) molecule

A previously reported family with amyloid polyneuropathy (FAP) was reinvestigated to determine the type of mutation in the transthyretin (prealbumin) molecule. Transthyretin was isolated from amyloid-laden myocardium and serum, and tryptic peptides were resolved by high-performance liquid chromatography. Amino acid sequencing of an anomalous peptide revealed an alanine-for-threonine substitution corresponding to position No. 60 of the transthyretin monomer. Detection of the FAP gene in asymptomatic carriers was accomplished by hybrid isoelectric focusing of transthyretin in the presence of dithiothreitol and high concentrations of urea, and by Southern blotting of Pvull-digested leukocyte deoxyribonucleic acid. This type of FAP was found to be identical to the previously described Appalachian amyloid. Patients with FAP and their asymptomatic gene-carrying offspring had significantly reduced levels of serum transthyretin and retinol-binding protein.

Pouring wide-range immobilized pH gradient gels with a window of extremely flattened slope

A gradient mixing device has been designed to pour polyacrylamide gels with a wide-range immobilized pH gradient including a "window" of extremely flattened slope. The device consists of an IBM-compatible personal computer controlling 8 step-motor-driven burettes, with four producing a density gradient from glycerol and delivering acrylamides and catalysts for gel polymerization, two delivering Immobilines for a wide-range pH gradient and the other two burettes responsible for the delivery of Immobilines for a partial pH range inside the wide range. The effect on a complex separation pattern of proteins with a wide range of pI is that resolution can be increased reproducibly to any reasonable extent at any location of the separation pattern.

Biochemical and genetic studies on rabbit hemoglobin. II. Electrophoretic polymorphism of the alpha-chain

A genetic polymorphism of rabbit (Oryctolagus cuniculus) hemoglobin alpha chain is demonstrated by means of acid starch gel electrophoresis. The polymorphism is not detected by isoelectric focusing and may be based upon neutral for neutral amino acid substitutions in accordance with previous findings by means of amino acid sequencing. Segregation analysis was performed on 15 matings with 49 offspring and confirmed the initial genetic hypothesis of three common codominant alleles at an autosomal locus. The calculated gene frequencies in a random sample of 86 unrelated individuals are HBA*1 = 0.73, HBA*2 = 0.22, and HBA*3 = 0.05.

Electrofocusing of acid phosphatases from frog liver, using an immobilized pH gradient

Isoelectric focusing on carrier ampholyte-containing immobilized pH gradient gels was applied (i) to gels submerged in silicone oil on a Peltier cooled apparatus, (ii) to the separation of the higher molecular weight (HMW, Mr 140,000) and the lower molecular weight (LMW, Mr 38,000) acid phosphatases (AcPases) from frog livers. (i) Electrofocusing was conducted on gels submerged under silicone oil cooled and stirred on a Peltier-thermoregulated horizontal gel support plate. This procedure aimed at a) improving the temperature control of the gel by direct contact of coolant with the gel surface, and thus at being able to focus at the maximal field strength and consequently highest resolution; b) preventing evaporation from the gel and c) excluding atmospheric carbon dioxide. Silicone oil submersion did not abolish water loss from the gel into the electrolyte strips during isoelectric focusing, or a rippled gel surface. Absence of water exudation on the ripples noted previously by Atland [1] was observed. (ii) The electrofocusing of AcPases on immobilized pH gradients yielded patterns which remained stationary as a function of time, by contrast to previous analyses on carrier ampholyte generated pH gradients. The total number of enzymatically active components found in the enzyme preparations from different stages of purification and in the isolated HMW and LMW AcPases was 18. The HMW and LMW AcPases focused in characteristic pH ranges and exhibited qualitative and quantitative pattern differences. Their band patterns add up to that of a crude preparation containing both enzymes. Neither polyacrylamide gel electrophoresis (PAGE) at any nondenaturing pH, nor isoelectric focusing in carrier ampholytes with pattern changes due to the pH gradient drift were able to yield that result.

Transthyretin microheterogeneity and thyroxine binding are influenced by non-amino acid components and glutathione constituents

Two non-amino acid components as well as the glutathione constituents in labile associations with transthyretin (TTR) have been detected by preparative polyacrylamide gel electrophoresis from preparations isolated by affinity chromatography on Sepharose-bound retinol-binding protein (RBP). Incubation of native or reduced TTR with these novel components influenced the quaternary structure and caused reactions with reduced TTR in particular. Reduction of isolated TTR monomers released cysteine from the quantitatively major monomer, but non-amino-acid components from another dominating monomer. The reaction patterns also influence thyroxine (T4) binding. These relationships indicate that interactions in serum of TTR with constituents of glutathione and components different from T4 and retinol-RBP are important for the metabolism and function of TTR.

Familial amyloidotic polyneuropathy: transthyretin (prealbumin) variants in kindreds of Italian origin

As part of an epidemiological study that aims to characterize chemically the mutation(s) in transthyretin (TTR) related to familial amyloidotic polyneuropathy (FAP) of different ethnic origins, studies were carried out on TTR from two FAP kindreds of Italian origin. Two different criteria were employed in the characterization of TTR from these kindreds: (1) immunoblotting of cyanogen bromide fragments for screening of TTR(Met30) and (2) isoelectric focusing. TTR(Met30) was not detected but other substitutions were demonstrated using isoelectric focusing techniques. One of the variants found is a basic TTR variant. The substitutions occurring in the variant TTRs of these two kindreds are not known and are presently under study.

Isoelectric focusing in immobilized pH gradients with carrier ampholytes added for high-resolution phenotyping of bovine beta-lactoglobulins: characterization of a new genetic variant

The genetic variants of bovine beta-lactoglobulin (beta-lg) from the "Murnau-Werdenfelser" breed were analyzed in three different isoelectric focusing (IEF) systems. While carrier ampholyte IEF with a pH gradient of 0.2 pH/cm did not resolve the new variant W from the B variant and IEF in immobobilized pH gradients (IPG) with 0.1 pH/cm only partially resolved it, adequate separation was achieved with IPG-IEF in a pH 5.25-pH 5.7 gradient, in presence of 0.8 % w/v carrier ampholytes, both over a 10 and 17 cm separation distance. Apparent isoelectric points (pI's) and genetic frequencies (f) were as follows: beta-lg A, pI = 5.370, f = 0.364; beta-lg B, pI = 5.485, f = 0.480; beta-lg W, pI = 5.492, f = 0.076; and beta-lg D, pI = 5.610, f = 0.080. The small difference of delta pI = 0.007 between beta-lg B and beta-lg W respectively, seems to originate from a "silent" substitution of neutral amino acid residues as compared to the larger delta pI's of the other genetic variants of beta-lg, which result from substitution of charged amino acids.

Isoelectric focusing of basic proteins: the problem of oxidation of cysteines

Isoelectric focusing of human globin chains in polyacrylamide gels dried in the ambient atmosphere and rehydrated in the presence of 8 mol/L urea produces artefactual doublets of zones as a result of oxidation by the gel. This oxidation can be avoided in separations of short duration by adding a reducing agent (e.g. 2-mercaptoethanol or dithiothreitol to the rehydration solution (Altland, K. and Rossmann, U., Electrophoresis 1985, 6, 314-325). We now demonstrate that the observed zone doublets can be explained by assuming neutralization of the contribution of dissociated sulfhydryl group of cysteine to pI by partial and reversible formation of globin dimers held together by disulfide bridges. Long time separations, requiring e.g. more than 4 h at greater than or equal to 500 V/cm, in pH gradients exceeding pH 7.5, are accompanied by artefactual oxidation from both the atmosphere and the gel matrix. Oxidation from the atmosphere as well as the effect of carbon dioxide can be eliminated by overlayering the gel with paraffin oil. Oxidation from the gel matrix can only partially be inhibited by rehydration of gels in the presence of 2-mercaptoethanol or dithiothreitol. Nearly complete protection against oxidation by the gel matrix was achieved by adding a permanent supply of 2-ME to the gel or by adding DTT to the cathodic wick towards the end of the experiment. Alkylation with iodoacetamide or iodoacetic acid resulted in stable globin patterns, which, however, displayed additional artefactual zones. Our experimental data indicate that the polyacrylamide gels function as an electron acceptor for dissociated sulfhydryl groups in proteins, even after pretreatment with strong reducing agents for proteins.

A new sample applicator for isoelectric focusing in horizontal polyacrylamide gels

A new sample applicator for horizontal flat gels is described. The applicator is practically safe against contamination from adjacent samples and can be used for all types of electrophoretic separations including a concentration step for either the sample (i.e. disc electrophoresis) or the separated zones (i.e. isoelectric focusing). The applicator is a piece of flat glass with 26 or 51 parallel 2 mm wide grooves, drilled at distances of 9 or 4.5 mm. Samples, maximally 25 or 50, are applied to the areas between the grooves. By inverting the applicator, the samples are brought into close vicinity to the gel surface and the pendant droplets expand by capillary attraction into the slits between the glass and gel with resultant even distribution across the lanes of 2.5 or 7 mm width. The applicator can be used for separations with and without protection of the electrophoretic setup by paraffin oil and allows for fast multiple handling of samples by means of appropriate syringes and microtiter plates.

Immunoblotting from immobilized pH gradients. The case of alpha 2-macroglobulin

Guidelines for effective blotting of proteins from immobilized pH gradients with a soft polyacrylamide matrix (e.g. T% = 3) include: thick (1 mm) IPG slabs, electrotransfer in a buffer tank in the presence of 0.1% SDS, nitrocellulose of the sturdiest type, thorough removal of all IPG fragments before further processing of the membrane. For alpha 2-M, IPG on a 4-6.5 gradient followed by enzyme-linked immunodetection allows the recognition of a complex pattern with several bands centered around pI 5.1. The procedure may also reveal the desialylated forms of alpha 2-M (microheterogeneity reduced to 2-3 bands), the native subunits (after reduction with thiols) and the denatured half molecules (in the presence of 8 M urea).