On the heterogeneity of serum albumin

Effect of glycation on the heterogeneity of human serum albumin analysed by reversed-phase high-performance liquid chromatography in a solvent containing formic acid

Non-enzymic glycation of human serum albumin (HSA) induces a change in its charge heterogeneity that may account for its particular renal clearance in patients with early diabetic nephropathy. A new high-performance liquid chromatographic analysis for the study of HSA heterogeneity is described based on a high content of formic acid in the mobile phase combined with a concave gradient of isopropanol. Under these conditions, native HSA was separated into three individual components (I, II and III). When glycated HSA was analysed, it was found that although the present method is not suitable for the separation of glycated from non-glycated HSA, it shows the effect of glycation in producing changes in HSA heterogeneity that are different from those reported on surface change. This finding suggests an additional factor (probably conformational changes) that is contributing to the heterogeneity of glycated HSA.

B700, an albumin-like melanoma-specific antigen, is a vitamin D binding protein

B700, a murine melanoma-specific antigen, is a member of the serum albumin protein family. Other members include serum albumin and vitamin D binding protein. The primary structure and biochemical functions of B700, as well as its in vivo metabolic fate, are largely unknown. We compared murine albumin, vitamin D binding protein and B700 for their ability to specifically bind [3H]-1,25-dihydroxy-vitamin D3. Scatchard analysis revealed a single binding site for B700 with a Ka of 51,000 mol/l and a Bmax of 4.51 x 10(-7) mol/l. There was no significant difference in the Ka and Bmax among the albuminoid proteins. However, differences in the binding sites could be distinguished by competition experiments where vitamin D3, vitamin D2 or 7-dehydrocholesterol competed for the specific binding of 1.25-dihydroxyvitamin D3 to a greater extent by B700 than by vitamin D binding protein. The albumin binding site more closely resembles vitamin D binding protein than B700, but the data indicate that the binding function of the albuminoid proteins is conserved in B700.

The meiotic response of cumulus cell-enclosed mouse oocytes to follicle-stimulating hormone in the presence of different macromolecules

Experiments were carried out to determine the effect of different macromolecules on the follicle-stimulating hormone (FSH)-induced maturation of mouse oocytes in culture. Cumulus cell-enclosed oocytes (CEO) were isolated from gonadotropin-primed mice and maintained in meiotic arrest for 17-18 h with the cAMP analogue, dibutyryl cAMP (dbcAMP). Germinal vesicle breakdown (GVB) was stimulated by the addition of FSH. Medium was supplemented with either no macromolecule or with varying concentrations of polyvinylpyrrolidone (PVP), polyvinylalcohol (PVA), crystallized bovine serum albumin (BSA), or fetal bovine serum (FBS). Oocyte maturation in all FSH-free cultures occurred at a frequency of about 30% or below. High frequencies of maturation were achieved when FSH was added to macromolecule-free medium or to cultures containing PVP, PVA, or BSA. Crystallized BSA was the most effective of these in supporting stimulation of maturation (94% GVB at 3 mg/ml, compared with 72-74% with synthetic polymer-supplemented or macromolecule-free media). The BSA effect was not due to contaminating fatty acids, and a less pure fraction V BSA was not as effective in supporting FSH-induced maturation. FBS suppressed FSH stimulation of maturation in a dose-dependent fashion. Sera from pigs, goats, horses, and rats were also inhibitory, but bovine calf serum (BCS) permitted a high maturation frequency (80% GVB). When added to medium containing either FBS or BCS, crystallized BSA had no effect on FSH-stimulated maturation, but fraction V BSA suppressed maturation in both serum-supplemented media. Under no conditions did FSH stimulate maturation in cumulus cell-free oocytes. These results demonstrate that hormone-induced oocyte maturation is supported in vitro by nonprotein polymers as well as BSA and that the behavior of the oocyte-cumulus cell complex depends on the purity of the BSA sample. In addition, serum contains inhibitory factors that suppress the positive response to FSH. Thus, the choice of macromolecular supplement is of critical importance when testing the hormone responsiveness of isolated cumulus cell-enclosed oocytes in culture.

Chromatofocusing profile of purified human alpha-fetoprotein and albumin differs from those of crude samples: effect of protein concentration of the elution of the sample

Chromatofocusing was utilized to characterize charge microheterogeneity of purified human alpha-fetoprotein (AFP) and human serum albumin (HSA). Crude cord blood samples yielded three isoforms: AFP-IA, IB, and II, with pIs 4.57 (52%), 4.27 (43%), and less than 4.00 (5%), respectively. In contrast, 10 micrograms of purified AFP or 250,000 cpm of 125I-AFP eluted entirely as isoform AFP-II. 125I-AFP focused in the presence of crude cord blood, amniotic fluid, adult male serum, or 25 mg purified HSA resulted in elution profiles similar to those of crude cord blood. Pure AFP focused along with 0.1, 1.0, 5.0, or 10 mg HSA showed a gradual shift from AFP-II to AFP-I. With greater than or equal to 5 mg HSA, isoform I was further resolve into AFP-IA and IB. Similarly, 250,000 cpm of 125I-HSA, which also eluted entirely as isoform II, showed a gradual shift to isoform I when increasing concentrations of unlabeled HSA were added. The resolution of isoform HSA-I in HSA-IA, IB, and IC was again improved with greater than or equal to 5 mg unlabeled HSA. When carrier proteins of varying pI values were chromatofocused along with purified AFP, it was observed that only those proteins with pIs in the range of AFP caused significant alteration in the relative distribution of AFP. We conclude that sample protein concentration and composition must be carefully considered when chromatofocusing is being used for purified samples and when the elution profiles of samples from different origins and varying protein concentrations are being compared.

Dementias: the role of magnesium deficiency and an hypothesis concerning the pathogenesis of Alzheimer's disease

Evidence is presented indicating that dementias are associated with a relative insufficiency of Magnesium (Mg) in the brain. Such insufficiency may be attributable to low intake or retention of Mg; high intake of a neurotoxic metal, such as aluminum (Al), which inhibits activity of Mg-requiring enzymes; or impaired transport of Mg and/or enhanced transport of the neurotoxic metal into brain tissue. It is proposed that Alzheimer's disease (AD) involves a defective transport process, characterized by both an abnormally high incorporation of Al and an abnormally low incorporation of Mg into brain neurons. The hypothesis is advanced that an altered serum protein contributes to the progression of AD by having a greater affinity for Al than for Mg, in contrast to the normal protein, which binds Mg better than Al. The altered protein crosses the blood-brain barrier more efficiently than the normal protein and competes with the normal protein in binding to brain neurons. Binding of the altered protein to the target neurons would both facilitate Al uptake and impede Mg uptake. Evidence suggests that albumin is the serum protein that is altered.

B700, a melanoma-specific antigen, catalyzes metabolism of prostaglandin E2

B700 is an albumin-like mouse-melanoma-specific antigen of unknown primary structure and biochemical function. The ability of mouse serum albumin to catalyze weak degradation of prostaglandin E2 has been utilized to compare functional similarities between B700 and mouse serum albumin. Both proteins catalyze the degradation of prostaglandin E2 to prostaglandin A2 and prostaglandin B2. This catalytic ability is related to the amino acid composition of the two proteins within the functional region rather than the 3-dimensional configuration, the activity is not altered upon boiling. The primary prostaglandin E2 metabolite in the presence of mouse serum albumin is prostaglandin B2, while prostaglandin A2 predominates in B700 catalyzed degradations. An additional product, presently unidentified, is produced during B700 catalyzed degradation of prostaglandin E2. Our studies indicate that the B700 protein has weak enzymatic activity for prostaglandin E2 similar to that of albumin. To our knowledge, B700 is the only melanoma antigen for which enzymatic activity has been demonstrated.

A naturally occurring molecular form of human plasma cholinesterase is an albumin conjugate

Human plasma cholinesterase (acylcholine acylhydrolase, EC 3.1.1.8) consists of four main molecular forms designated as C1, C2, C3 and C4 according to their electrophoretic mobility on gels. The major component, C4, is the tetrameric form; C1 and C3 are the monomeric and dimeric forms, respectively. The C2 form, which has an apparent free electrophoretic mobility higher than that of the three size isomers, and, moreover, a higher isoelectric point, was found to be a covalent conjugate between the cholinesterase monomer and serum albumin. This result is supported by the following arguments: the non-catalytic subunit of C2 was found to be a carbohydrate-free protein of apparent molecular mass 65 kDa that could not be labelled by diisopropylfluorophosphonate in the labelling conditions of esterases. It possesses a high affinity for a long-chain aliphatic ligand (a substituted octadecylamine) and for Cibacron blue F3 GA, and could be adsorbed on an immunoadsorbent for albumin. The two subunits of C2 are disulfide bridge linked; the active center of the cholinesterase subunit is partly masked by the albumin molecule. The conjugation reaction very likely occurs in the hepatic cell and not in plasma.

Albuminoid molecules: a novel, variability-generating cell-surface receptor system?

The mechanisms by which lymphoid cells produce infinitely variable molecules of the immunoglobulin protein superfamily have been recently elucidated. These molecules serve, in part, as the mediators of cell:cell recognition and interaction among lymphoid cells. However, the generality of those molecular mechanisms to occur in non-lymphoid cell types has not yet been established. In this paper, we propose that the serum albumin superfamily of proteins has the necessary characteristics to serve analogous functions in epithelioid cells, and we critique recent evidence which leads to this hypothesis.

Albumin, a biologically active protein acting on Leydig cells

The objective of the present study was to characterize further the albumin fraction of rat testicular fluid (rTF), which can enhance luteinizing hormone (LH)-stimulated pregnenolone production by immature Leydig cells in vitro. Testicular fluid, obtained from 300 rat testes was fractionated by sequential ammonium sulfate precipitation, dye-affinity, anion-exchange chromatography, chromatofocussing and an additional heat treatment. The final fraction showed a single band when analyzed on a silver-stained sodium dodecyl sulfate-polyacrylamide gel and isoelectric focussing gel. The protein had a molecular weight of 67 kDa, an isoelectric point of 5.0 and was identified as albumin after Western blotting using an antibody against rat serum albumin. Albumin in this fraction gave a dose-dependent (0.1-2% protein, w/v) increase in LH-induced pregnenolone production, up to 4-fold, and the increase in specific bioactivity when compared to rTF was 1.4-fold. Selective depletion of albumin from testicular fluid was used as another approach to confirm that albumin itself is the main biologically active component in rTF. rTF from mutant analbuminemic rats (albumin content 0.02 mg/ml) did not stimulate LH-induced steroid production in our assay, in contrast to rTF from normal rats (albumin content 40 mg/ml). Albumin fractions obtained from rat, bovine and human sera were also effective in stimulation of steroid production in the presence of LH, in contrast to chicken serum albumin which gave no stimulation. The stimulatory effect of albumin is not caused by bound fatty acids, nor by the presence of modified forms of albumin such as testibumin or the albumin-bilirubin complex. Our results indicate that Leydig cells are more active in steroid production when surrounded by high but physiological concentrations of albumin.

Protein folding and aggregation studied by isoelectric focusing across a urea gradient and isoelectric focusing in two dimensions

Isoelectric focusing across a concentration gradient of urea was used to study the folding-unfolding and association-dissociation processes of proteins. Myoglobulin, albumin, RNase, papain, beta L- and alpha-crystallin were analyzed with this technique, and examples are given of visualized dissociation steps and of equilibrium-unfolding intermediates. Furthermore, a two-dimensional isoelectric focusing technique is presented that is useful to deduce whether a transition of a protein aggregate observed upon urea-gradient isoelectric focusing must be attributed to a change in the protein's tertiary or quaternary structure.

The albumin fraction of rat testicular fluid stimulates steroid production by isolated Leydig cells

Rat testicular fluid (rTF), but not rat serum (rS) or plasma (rP) can further increase within 4 h maximally luteinizing hormone (LH)-stimulated or 22 R-hydroxycholesterol-supported pregnenolone production by immature rat Leydig cells in vitro. This effect of rTF is dose dependent (0.05-1.2% protein, w/v) with an increase up to 4-fold. The objective of the present study was to isolate and characterize the bioactive factor(s) in rTF. After sequential ammonium sulfate fractionation, gel filtration chromatography on Superose 12 and affinity chromatography on concanavalin A-Sepharose it was shown that the albumin fraction was a major biologically active fraction in rTF. The relative specific activity in these fractions was never greater than 1.3-1.4, which is in agreement with the purification factor required to obtain pure albumin from rTF. Commercially obtained albumin fractions from human, bovine and rat sera, up to 99% purity, also increased Leydig cell steroid production more than 3-fold when added in concentrations between 0.1 and 1% w/v in combination with LH or 22R-hydroxycholesterol. Other proteins such as hemoglobin and ovalbumin were not effective in stimulation of steroid production. Bovine serum albumin (bSA, fraction V) at concentrations of 0.25 and 1.0% (w/v), had no or minor effects on LH-stimulated steroid production by rat granulosa cells or adrenocorticotrophic hormone (ACTH)-stimulated steroid production by rat adrenal cells. These findings indicate that albumin itself or minor compounds copurified with albumin represent the main biologically active component in rTF for short-term stimulation of Leydig cell steroid production. Since bioactivity could not be demonstrated in serum which contains similar amounts of albumin as rTF, inhibitory compounds may be present in rat serum.

A protein-free blocking system for the covalently binding matrix cyanuric chloride-activated paper in immunological procedures

A new protein-free blocking system containing 10% (w/v) sulfanilic acid/10% (v/v) triethanolamine/0.05% (v/v) Tween 20 has been used to block free binding sites of the covalently binding matrix cyanuric chloride-activated paper (CCA-paper). This method allows a reversible staining of protein blots with Coomassie brilliant blue after each step of the immunological procedure and reuse of the blots for a repeated antibody probing. Non-radioactive and radioactive detection procedures have been compared with blots on CCA-paper and nitrocellulose. The best combination is a Coomassie brilliant blue staining and immunological detection with 125I-protein A.

Albumin in rabbit skeletal muscle. Origin, distribution and regulation by contractile activity

A 68-kDa protein is present in increasing amounts in the soluble fractions of fast, slow and cardiac muscles of the rabbit. Induced contractile activity by chronic 10-Hz stimulation caused in rabbit fast-twitch muscle a rapid increase of this protein, reaching fivefold to sixfold higher levels than normal after one week of stimulation. The 68-kDa protein was identified as albumin by its mobility in one-dimensional and two-dimensional electrophoreses and by its reactivity with a polyclonal antibody directed against rabbit serum albumin. Immunohistochemistry revealed a localisation pattern corresponding to a distribution mainly in the interstitium. The assumption that albumin is not an intrinsic protein of the muscle fiber was strongly supported by the failure to show its synthesis by in vivo labeling. Injection of [35S]methionine into normal and chronically stimulated muscles did not lead to a detectable incorporation of the precursor into muscle albumin. In addition, albumin could not be detected as a product formed by in vitro translation of RNA preparations from normal and chronically stimulated muscles.