Do immunoassays differentially detect different acidity glycoforms of FSH?

OBJECTIVE

The possibility of the carbohydrate residues of glycoproteins affecting their recognition in immunoassays is an important and unresolved issue. This study looked for evidence of differential recognition of FSH glycoform preparations, of variable isoelectric point (pI) and known molarity, using three routine assays employing different antibody configurations.

DESIGN

Seven glycoform preparations with differing pI bands (between 3.8 and 5.5) were produced by isoelectric focusing of recombinant human FSH and the molecular weights determined by mass spectroscopy. Three concentrations of each glycoform were assayed and the results expressed relative to unfractionated material. From the relative responses, recognition differences between the assay methods and between the glycoform preparations were investigated.

MEASUREMENTS

Three routine assays were employed: the commercially available Amerlite(R) enzyme immunoassay and Delfia(R) immunofluorometric assay, together with an in-house competitive two-site radioimmunoassay (RIA).

RESULTS

Overall, the three assays gave the same relative responses for equivalent glycoforms, with the only exceptions involving small differences between some assay pairs for the fractions at the extremes of the pI range investigated. Within each assay type, differences (P < 0.05) of up to 33% existed between glycoforms of different pI, however, these differences showed no patterns or trends across the entire acidity range examined.

CONCLUSIONS

Between the assay methods investigated in this study, few differences exist in the recognition of individual pI bands of FSH when expressed relative to a common unfractionated standard. Differences were apparent in the recognition of the different acidity glycoforms within each assay method, however, these were small and unlikely to be of clinical significance.

Isoforms of human recombinant follicle-stimulating hormone: comparison of effects on murine follicle development in vitro

The effects of three isoforms derived from recombinant human FSH on ovarian follicle development in vitro were characterized for the first time. The three subfractions comprised discrete pI ranges of 3. 6-4.6 (acid), 4.5-5.0 (mid), and 5.0-5.6 (least acidic). Follicular growth, estradiol secretion, and antral formation were assessed for each fraction of isoforms in a range of concentrations over a 5-day culture period. Least acidic FSH produced, at and above 1.5 ng/ml, a high percentage of follicles growing above the size threshold necessary for antral formation, whereas mid and acid FSH induced similar growth only at higher concentrations (7.5 ng/ml and 50 ng/ml, respectively). Least acidic FSH specifically induced the most rapid growth of follicles during preantral development. Acid FSH at all concentrations stimulated estradiol-17ss secretion later during culture and antral formation in a lower proportion of follicles than did least acidic and mid FSH. It can be concluded 1) that the least acidic isoform induced fastest preantral growth, producing the largest antral follicles at the lowest dose of all three fractions and 2) that the less and mid acidic isoforms had more impact on stimulation of estradiol production and antral formation than the acid isoform.

Partially deglycosylated human choriogonadotropin, stabilized by intersubunit disulfide bonds, shows full bioactivity

Several studies indicate that in human choriogonadotropin the N-linked oligosaccharide at position 52 of the alpha-subunit is important for bioactivity. We have generated choriogonadotropin mutants in which the alpha52 glycosylation site is removed and the alpha and beta subunits are covalently linked by intersubunit disulfide bonds. These mutants display wild-type receptor binding and bioactivity. Furthermore, we show that removal of the alpha52 sugar leads to instability of heterodimeric choriogonadotropin. Therefore, we conclude that the alpha52 oligosaccharide of choriogonadotropin is not involved in signal transduction, but in the stability of the heterodimer.

Prediction of the in vivo biological activity of human recombinant follicle stimulating hormone using quantitative isoelectric focusing

Currently, the in vivo biopotency of commercial preparations containing the glycoprotein follicle-stimulating hormone (FSH) is declared on the basis of the ovarian weight augmentation assay as described in the British, European and United States Pharmacopoeias. Human FSH contains approximately 35% (W/W) carbohydrate which introduces considerable microheterogeneity (isohormones). Analysis of isohormones of recombinant FSH has revealed a relation between the isoelectric point (pl) and the in vivo bioactivity. Isohormones in the range of pl 3.5 are 100- to 200-fold more potent in the in vivo bioassay than isohormones with a pl of 5.5-6.0. These data suggest that quantification of the isohormone profile should enable us to predict the in vivo bioactivity. Thus, isohormones of recFSH samples were separated by isoelectric focusing (IEF), visualized by Coomassie brilliant blue G250 staining and quantified using densitometry. The data from 21 samples were compared with the in vivo bioassay data using partial least square (PLS) regression. A close correlation was found using a model with 2 PLS factors (correlation coefficient (r)=0.95, standard error of estimation SD 1. 02 lU/microg protein). In addition, ordinary least square (OLS) regression revealed a similar correlation between the fraction of isohormones between pl 3.9 and 4.9 and the in vivo bioactivity: r=0. 95, sd=1.4 lU/microg protein. Thus, an increase in the acidic isohormone fraction results in an increase in the in vivo bioactivity. The reverse is true for the amount of isohormones focusing between pl 5.1 and 5.7. An increase of this fraction results in a decrease of the in vivo bioactivity. These data are consistent with what might be expected from the in vivo bioassay data of the isohormones. The OLS model was subsequently validated using the guidelines of the European Centre for Validation of Alternative Methods (ECVAM) using 10 samples of recFSH that had not been used for the calibration. The relative standard deviation (RSD) of the mean difference between experimental and predicted in vivo bioactivity was approximately 6%. A Student's t-test performed on the experimental and predicted bioactivity data indicated that the predicted bioactivities do not deviate significantly from the experimental in vivo bioactivity data (P<0.05). These results demonstrate that the IEF scanning data can be used to predict the in vivo bioactivity with reasonable accuracy. This may be the first step towards replacing the in vivo bioassay for highly purified FSH by a physicochemical alternative. In general, quantitative charge-based separation methods like chromatofocusing, high performance capillary electrophoresis and ion exchange chromatography may also be considered as alternatives. Finally, quantitative charge profiling may prove to be as important for the estimation of the potency of other therapeutic glycoproteins like luteinizing hormone (LH), chorionic gonadotropin (CG), thyroid-stimulating hormone (TSH) and erythropoietin (EPO).

X-ray structure of antistasin at 1.9 A resolution and its modelled complex with blood coagulation factor Xa

The three-dimensional structure of antistasin, a potent inhibitor of blood coagulation factor Xa, from the Mexican leech Haementeria officinalis was determined at 1.9 A resolution by X-ray crystallography. The structure reveals a novel protein fold composed of two homologous domains, each resembling the structure of hirustasin, a related 55-residue protease inhibitor. However, hirustasin has a different overall shape than the individual antistasin domains, it contains four rather than two beta-strands, and does not inhibit factor Xa. The two antistasin domains can be subdivided into two similarly sized subdomains with different relative orientations. Consequently, the domain shapes are different, the N-terminal domain being wedge-shaped and the C-terminal domain flat. Docking studies suggest that differences in domain shape enable the N-terminal, but not C-terminal, domain of antistasin to bind and inhibit factor Xa, even though both have a very similar reactive site. Furthermore, a putative exosite binding region could be defined in the N-terminal domain of antistasin, comprising residues 15-17, which is likely to interact with a cluster of positively charged residues on the factor Xa surface (Arg222/Lys223/Lys224). This exosite binding region explains the specificity and inhibitory potency of antistasin towards factor Xa. In the C-terminal domain of antistasin, these exosite interactions are prevented due to the different overall shape of this domain.

Structure-based design and protein engineering of intersubunit disulfide bonds in gonadotropins

Pairs of cystine residues were introduced in the alpha- and beta-subunits of human choriogonadotropin at positions with optimal geometries for the formation of disulfide bonds. Using the homology with luteinizing hormone and follicle stimulating hormone, similar mutations were carried out in these glycoprotein hormones. In nearly all mutants the corresponding disulfide bonds were formed leading to a non-natural, covalent linkage between the alpha- and beta-subunits. The mutants typically display wild-type receptor binding and bioactivity. The mutants with non-natural intersubunit disulfide bonds display enhanced thermostabilities relative to the corresponding heterodimeric glycoprotein hormones, rendering them candidates for long acting gonadotropins with enhanced shelf lives.

Molecular biology and biochemistry of human recombinant follicle stimulating hormone (Puregon)

Follicle stimulating hormone (FSH) is a heterodimeric glycoprotein hormone produced in the anterior pituitary gland. The hormone is essential in the regulation of reproductive processes, such as follicular development and ovulation. It is clinically used for treatment of anovulation and in assisted reproduction technologies such as in-vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI). Until recently, the only source for human FSH has been the urine from post-menopausal women. Such a natural source implies limited availability and potential product variability. Thus, we have cloned the genes encoding the alpha- and beta-subunits of human FSH and transfected these into Chinese hamster ovary (CHO) cells. A CHO-clone was isolated capable of secreting intact glycosylated FSH with identical amino acid sequences to natural FSH. This cell line was grown in perfusion culture and enabled us to isolate highly pure FSH (> 99%). The complexity of the charge distribution of human recombinant FSH was demonstrated by Isoelectric focusing. The observed microheterogeneity is caused by the large number of carbohydrate chain structures which are added to the four potential glycosylation sites in the alpha beta-dimer. Furthermore, the carbohydrates show a variation in their degree of sialylation which reflects the different pl values of the individual isohormones. Despite the complexity of post-translational modification, the isoform distribution of recombinant FSH produced in a CHO-cell line and grown in perfusion culture is surprisingly similar to that observed with pituitary FSH and urinary FSH. In conclusion, we have shown that FSH-gene transfected CHO-cells are capable of stable serum-free production of recombinant FSH. A process has been developed which assures the consistent and reproducible production of highly-purified recombinant FSH.

Properties of nisin Z and distribution of its gene, nisZ, in Lactococcus lactis

Two natural variants of the lantibiotic nisin that are produced by Lactococcus lactis are known. They have a similar structure but differ in a single amino acid residue at position 27; histidine in nisin A and asparagine in nisin Z (J.W.M. Mulders, I.J. Boerrigter, H.S. Rollema, R.J. Siezen, and W.M. de Vos, Eur. J. Biochem, 201:581-584, 1991). The nisin variants were purified to apparent homogeneity, and their biological activities were compared. Identical MICs of nisin A and nisin Z were found with all tested indicator strains of six different species of gram-positive bacteria. However, at concentrations above the MICs, with nisin Z the inhibition zones obtained in agar diffusion assays were invariably larger than those obtained with nisin A. This was observed with all tested indicator strains. These results suggest that nisin Z has better diffusion properties than nisin A in agar. The distribution of the nisin variants in various lactococcal strains was determined by amplification of the nisin structural gene by polymerase chain reaction followed by direct sequencing of the amplification product. In this way, it was established that the nisZ gene for nisin Z production is widely distributed, having been found in 14 of the 26 L. lactis strains analyzed.

Identification and characterization of the lantibiotic nisin Z, a natural nisin variant

Lactococcus lactis strain NIZO 22186 produces an extracellular, lanthionine-containing 3.5-kDa polypeptide with antimicrobial activity. Its retention time on reversed-phase (RP) HPLC and its amino acid composition showed high similarities but no complete identity to nisin. The gene for this lantibiotic, designated nisZ, has been cloned and its nucleotide sequence was found to be identical to that of the precursor nisin gene apart from a single mutation resulting in the substitution His27Asn in the mature polypeptide. NMR studies of the natural nisin variant, which has been designated nisin Z, confirmed the His27Asn substitution and indicated that it has a similar structure to nisin.

Evolution of eye lens crystallins: the stress connection

Crystallins, the structural proteins of the eye lens, ensure the transparency and integrity of the lens throughout life. Recent sequence comparisons have shown that evolution has recruited crystallins among already existing heat-shock proteins and stress-inducible enzymes.

Distribution of MP17 in isolated lens fibre membranes

MP17 is the second most abundant integral membrane protein in the mammalian lens. It has some common features with the major intrinsic polypeptide MIP26, but amino terminal sequencing shows that MP17 is a separate gene product. Both MP17 and MIP26 are abundant in isolated lens fibre membrane vesicles and are not detectable in the fibre gap junctions.

Lambda-crystallin, a major rabbit lens protein, is related to hydroxyacyl-coenzyme A dehydrogenases

It has recently been discovered that several lens proteins in birds and lower vertebrates are active enzymes or enzyme-related proteins (Wistow, G., Mulders, J. W. M., and de Jong, W. W. (1987) Nature 326, 622-624; Wistow, G., and Piatigorsky, J. (1987) Science 236, 1554-1556). We report here a novel lens protein, designated as lambda-crystallin, that occurs in rabbit and hare. It constitutes 7-8% of the total lens protein and has a subunit molecular mass of 35 kDa. Sequencing of cDNA clones encoding rabbit lambda-crystallin revealed 30% homology (at the amino acid sequence level) with L-3-hydroxyacyl-CoA dehydrogenase from pig mitochondria and 26% homology with enoyl-CoA hydratase-3-hydroxyacyl-CoA dehydrogenase from rat peroxisomes. Also, the presence of a putative beta-alpha-beta nucleotide-binding fold and low levels of non-lens expression are indicative of some enzymatic function for lambda-crystallin (or highly related sequences) in non-lens tissues. lambda-Crystallin thus represents the first example of an enzyme-related crystallin in lenses from mammalian species. The recruitment of enzymes as lens structural proteins apparently is an evolutionary strategy which has been applied independently in different lineages.

Duck lens epsilon-crystallin and lactate dehydrogenase B4 are identical: a single-copy gene product with two distinct functions

To investigate whether or not duck lens epsilon-crystallin and duck heart lactate dehydrogenase (LDH) B4 are the product of the same gene, we have isolated and sequenced cDNA clones of duck epsilon-crystallin. By using these clones we demonstrate that there is a single-copy Ldh-B gene in duck and in chicken. In the duck lens this gene is overexpressed, and its product is subject to posttranslational modification. Reconstruction of the evolutionary history of the LDH protein family reveals that the mammalian Ldh-C gene most probably originated from an ancestral Ldh-A gene and that the amino acid replacement rate in LDH-C is approximately 4 times the rate in LDH-A. Molecular modeling of LDH-B sequences shows that the increased thermostability of the avian tetramer might be explained by mutations that increase the number of ion pairs. Furthermore, the replacement of bulky side chains by glycines on the corners of the duck protein suggests an adaptation to facilitate close packing in the lens.

MP17, a fiber-specific intrinsic membrane protein from mammalian eye lens

A major protein with a molecular weight of 17,000, designated as MP17, has been identified in mammalian eye lens plasma membranes. Hydrophobic photolabeling experiments revealed that MP17 is a genuine intrinsic membrane protein. By using monoclonal antibodies we demonstrated that MP17 is not detectable in liver, heart, muscle, spleen and kidney, and thus can be considered, like MP26, as a lens-specific membrane protein. Furthermore, we showed that MP17 is a substrate for cAMP-dependent protein kinase and that it is a calmodulin-binding protein.

Beta B1 crystallin is an amine-donor substrate for tissue transglutaminase

The effects of tissue transglutaminase on the water-soluble proteins in bovine lens homogenates are described. Addition of liver transglutaminase and Ca2+ to calf lens homogenates resulted not only in the appearance of 50- and 57-kDa dimers, but also in a decrease in the amount of beta B1 crystallin and the almost complete disappearance of beta B3 and beta A3. This is not the result of Ca2+-induced proteolysis, since histamine completely inhibits this phenomenon. It may be concluded that these polypeptides are involved in beta-crystallin crosslinking by transglutaminase. This notion was confirmed by using beta B1- and beta Bp-specific antisera. Both sera reacted with the 57-kDa dimer; the beta Bp-specific antiserum also reacted with the 50-kDa dimer. No reaction in the region 50-57 kDa was detectable when EDTA was used instead of Ca2+. Using reconstituted mixtures of beta B1- and beta Bp-crystallin chains, and N-terminally truncated derivatives thereof, it was shown that in the beta B1/beta Bp dimer, glutamine residue -9 of beta Bp crosslinks to one of the lysine residues in the N-terminal extension of beta B1.

The enzyme lactate dehydrogenase as a structural protein in avian and crocodilian lenses

The major components of mammalian lenses are tissue-specific, soluble proteins, the alpha-, beta- and gamma-crystallins. The lenses of other vertebrate classes often contain other major proteins, notably delta-crystallin in birds and reptiles. A fourth distinct type, described as epsilon-crystallin, is prominent in many bird and crocodile lenses. Here we show that epsilon-crystallin is an active glycolytic enzyme, lactate dehydrogenase (LDH) (EC 1.1.1.27) and that duck epsilon-crystallin appears to be identical to duck LDH-B4. LDH is a normal metabolic component in other lenses, but in duck is present in amounts far exceeding the requirements of any likely catalytic role. It appears that an active enzyme has been recruited, unchanged, to an extra role as a structural protein in the lens without gene duplication and sequence divergence. This surprising discovery raises the possibility that other crystallins may similarly be enzymes expressed at high levels in lens as structural proteins.

Some aspects of the phosphorylation of alpha-crystallin A

The cAMP-dependent phosphorylation of alpha-crystallin was investigated. The major products of in vitro phosphorylation of total bovine lens homogenate are the alpha A1 and alpha B1 polypeptides, but in addition a minor labeled spot is present which might correspond with a double phosphorylated alpha B chain. It is demonstrated that the A1 and B1 subunits of alpha-crystallin from bovine eye lenses are solely the result of phosphorylation of the primary gene products alpha A2 and alpha B2, respectively, as judged from the stoichiometry of the phosphate content of these polypeptides. Both the in vitro and in vivo phosphorylation sites of the A chain of bovine alpha-crystallin were determined and found to be the same. After in vitro incubation the majority of the 32P label was found in the tryptic peptides T17a and T16-17a, the latter being the result of incomplete tryptic cleavage between T16 and T17a. The in vivo phosphorylation site is also located in T17a, as could be concluded from the retention times on reversed-phase HPLC of T16-17a and T17a from alpha A1 as compared to those from alpha A2, and from the differences in their mobilities on high-voltage paper electrophoresis at pH 6.5. Furthermore, both T17a and T16-17a of alpha A1 contain approximately 1 mol phosphate/mol peptide. Thermolytic digestion of T16-17a of both alpha A2 and alpha A1, followed by separation on RP-HPLC, demonstrated that Ser-122 is the phosphorylation site of the A chain of bovine lens alpha-crystallin. The replacement of this phosphorylation site or the lack of basic amino acids at the N-terminal side of Ser-122 in some vertebrate species apparently results in the absence of phosphorylation of alpha-crystallin A both in vitro and in vivo.

Heat shock response of the rat lens

The sequence relationship between the small heat shock proteins and the eye lens protein alpha-crystallin (Ingolia, T. D., and E. E. Craig, 1982, Proc. Natl. Acad. Sci. USA, 79: 2360-2364) prompted us to subject rat lenses in organ culture to heat shock and other forms of stress. The effects on protein synthesis were followed by labeling with [35S]methionine and analysis by one- and two-dimensional gel electrophoresis and fluorography. Heat shock gave a pronounced induction of a protein that could be characterized as the stress protein SP71. This protein probably corresponds to the major mammalian heat shock protein hsp70. Also two minor proteins of 16 and 85 kD were induced, while the synthesis of a constitutive heat shock-related protein, P73, was considerably increased. The synthesis of SP71 started between 30 and 60 min after heat shock, reached its highest level after 3 h, and had stopped again after 8 h. In rat lenses that were preconditioned by an initial mild heat shock, a subsequent shock did not cause renewed synthesis of SP71. This effect resembles the thermotolerance phenomenon observed in cultured cells. The proline analogue azetidine-2-carboxylic acid, zinc chloride, ethanol, and calcium chloride did not, under the conditions used, induce stress proteins in the rat lens. Sodium arsenite, however, had very much the same effects as heat shock. Calcium ionophore A23187 specifically and effectively induced the synthesis of the glucose-regulated protein GRP78. No special response to stress on crystallin synthesis was noticed.

Interaction of alpha-crystallin with lens plasma membranes. Affinity for MP26

The binding of the major water-soluble lens protein alpha-crystallin to the lens plasma membrane has been investigated by reassociating purified alpha-crystallin with alpha-crystallin-depleted membranes and with phospholipid vesicles in which the lens membrane protein MP26 had been reconstituted. alpha-Crystallin reassociates at high affinity (Kd = 13 X 10(-8)M) with alkali-washed lens plasma membranes but not with lens plasma membranes treated with guanidine/HCl, nor with phospholipid vesicles or erythrocyte membranes. Binding to lens plasma membranes is dependent on salt, temperature and pH and occurs in a saturable manner. Reconstitution of MP26 into phospholipid vesicles and subsequent analysis of alpha-crystallin binding suggests the involvement of this transmembrane protein. Binding ist not influenced by pretreatment of membranes with proteases, suggesting that the 4-kDa cytoplasmic fragment of MP26 is not necessary for alpha-crystallin binding. Labeling experiments using (trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine as a probe for intrinsic membrane proteins further showed that alpha-crystallin contains hydrophobic regions on its surface which might enable this protein to make contact with the lipid bilayer. Newly synthesized alpha-crystallin, in lens culture, is not associated with the plasma membrane, suggesting that the assembly of alpha-crystallin aggregates does not take place in a membrane-bound mode.

Bacillus flavothermus, a newly isolated facultative thermophile

A sample from a hot spring on the northern island of New Zealand contained five different thermophilic bacterial strains. One strain with peculiar properties, i.e. the formation of dark yellow colonies at 30 degrees C as well as at 70 degrees C, was further characterized. It was found to be a gram-positive, facultatively aerobic, motile Bacillus species, with terminal endospores. According to the physiologic properties the strain closely resembled B. coagulans. However, two typical characteristics were contradictory to this conclusion, namely the intense yellow pigmentation of the colonies and the range of growth temperature. The latter was found to reach from 40 to 70 degrees C, with an optimum at 60 degrees C under aerobic and at 65 degrees C under anaerobic conditions. Growth at moderate temperatures was slower than at 60 degrees C, but the final cell yields were almost equal. The strain can therefore be considered as facultatively thermophilic. The pigment, which was found to be located in the cytoplasmic membrane, was spectroscopically identified as a carotenoid. Because the characteristics of this strain did not correspond with any of the Bacillus species described thus far, we concluded, that we had isolated a novel strain, for which the name Bacillus flavothermus is proposed.