Two pregnancy-associated serum proteins with pregnancy-specific beta1-glycoprotein determinants

Variation in the levels of pregnancy-specific beta-1-glycoprotein in maternal serum from chromosomally abnormal pregnancies

Human pregnancy-specific beta-1-glycoprotein (SP1) was assayed retrospectively in stored maternal serum (MS) samples from 82 chromosomally abnormal pregnancies and 377 matched controls. The median MSSP1 concentration in 48 Down's syndrome pregnancies was significantly elevated at 1.17 multiples of the control median (MOM), and significantly reduced (0.5 MOM) in a group of eight cases of unbalanced translocations. There was no significant difference in median SP1 concentrations in cases of trisomy 18, trisomy 13, balanced translocations, or sex chromosome abnormalities. A comparison with human chorionic gonadotrophin results in the same series of samples indicates that SP1 is a less sensitive predictor of Down's syndrome pregnancies.

Immunoturbidimetric determination of pregnancy-specific beta 1-glycoprotein (SP-1)

We describe a simple and rapid immunoturbidimetric end-point assay for measuring pregnancy-specific beta 1-glycoprotein (SP-1) in human serum with the Kone Progress clinical chemistry analyser, using a commercially available antiserum. The absorbance was measured at 340 nm 5 min after addition of antiserum. The assay range was 3-250 mg/l. Over this range the CV for between-day precision was below 6%. Bilirubin up to 160 mumol/l, haemoglobin up to 2 g/l and lipaemia up to 320 mg/l did not interfere. The reference interval in maternal serum at the 15th week of gestation, was 13 to 46 mg/l with a median of 25 mg/l (n = 78) and that of the 16th week was 6 to 60 mg/l with a median of 28 mg/l (n = 78). Comparison of results obtained by two commercially available radio-immunoassays and by the present method shows acceptable correlation. Rapid and reproducible, the immunoturbidimetric method is suitable for use in routine determination of SP-1.

Enzyme activity electrophoresis and rocket immunoelectrophoresis for the qualitative and quantitative analysis of Geotrichum candidum lipase activity

The development and application of a rocket immunoelectrophoretic and an enzyme activity electrophoretic assay for the qualitative analysis of Geotrichum candidum lipase activity is presented. The sensitivities of the four assays were (in arbitrary units): enzyme activity electrophoresis, 1-0.5; rocket immunoelectrophoresis, 0.5-0.2; radial diffusion, 1; titrimetry, 1. The electrophoretic methods made it possible to distinguish between high and low molecular weight forms of the G. candidum lipases. The enzyme activity electrophoretic methods can be combined with other electrophoretic techniques, as demonstrated here with isoelectric focusing, and produce useful information on physico-chemical differences between different molecular forms of the lipase, e.g. forms with different pI.

Molecular cloning of a cDNA for human pregnancy-specific beta 1-glycoprotein:homology with human carcinoembryonic antigen and related proteins

Human pregnancy-specific beta 1-glycoprotein (SP1) plays an essential role in normal pregnancy. It is also a well-characterized oncodevelopmental antigen, expressed aberrantly by all trophoblastic tumors and some other malignant cell types. Here we report the identification of a human placental cDNA encoding the SP1 polypeptide sequence. The coding sequence shows 95% identity at the nucleotide level with a distinct, recently published SP1 cDNA sequence (PSG16). Unexpectedly, the sequence is also highly homologous to the published sequence of human carcinoembryonic antigen (CEA). SP1, CEA and CEA-related nonspecific cross-reacting species thus belong to a group of closely related though antigenically diverse tumor-associated glycoproteins. Comparison of the deduced amino acid sequence of the SP1 cDNA with that of CEA provides insight into the modular nature of these related proteins. This may have implications for the genomic organization and evolution of the CEA gene family.

Diagnosis of early pregnancy by enzyme immunoassay of Schwangerschafts-protein 1

Serum Schwangerschafts-protein 1 (SP1) and beta subunit of human chorionic gonadotropin (beta-hCG) were measured by enzyme immunoassay and radioimmunoassay, respectively, in the follicular phase and luteal phase of 50 human menopausal gonadotropin treatment cycles. SP1 could be detected in 63% of blood samples taken in the follicular phase up to the level of 1.56 micrograms/l. Using a cutoff level of 2 micrograms/l, SP1 could be used to diagnose pregnancy 16 to 25 days after the ovulating dose of hCG. The serum SP1 level is not affected by hCG injections and it can be used for diagnosis of pregnancy even when hCG has been given recently. SP1 also may be used as a prognostic indicator of the outcome of the pregnancy.

Interference of an alpha 2 component in immunological determinations of pregnancy-specific beta 1 glycoprotein in serum

Pregnancy-specific beta 1 glycoprotein (SP1-beta) was purified from human retroplacental blood by sequential anion exchange chromatography, gel chromatography and affinity chromatography. The final preparation appeared to be electrophoretically and immunochemically pure and was in particular free from any component with alpha mobility. The preparation was used as immunogen in rabbits as well as tracer and standard for radioimmunoassay and for cross- and rocket-immunoelectrophoresis. It was shown that this radioimmunoassay procedure, allowed quantitative determination of SP1-beta glycoprotein without interference by the alpha component.

The disappearance rate of Schwangerschaftsprotein 1 in normal and pathological pregnancies

In six women with a normal vaginal delivery at term, in 12 women who had a suction curettage between 6 and 12 weeks gestation and in eight women with an ectopic pregnancy, the post-partum or the post-surgery decline in radioimmunoassayable SP1 was faster during the first 24 h after surgery or delivery than later, so that two half-lives were calculated. The first 'half-life' of about 20 h (0-24 h after delivery or surgery) was of the same order of magnitude in all groups studied and corresponded well to previously published values. The mean second 'half-life' (greater than 24 h after delivery or surgery) was significantly longer in term (72.2 h) and in ectopic pregnancies (64.1 h) than in first trimester pregnancies (45.5 h). These results might indicate that the metabolism of SP1 either changes during pregnancy or that the changing SP1 beta/SP1 alpha ratio during pregnancy markedly influences the levels of SP1 as measured by radioimmunoassay.

New placental proteins: chemistry, physiology and clinical use

In the past decade, several new placental proteins have been isolated and studied. The 'pregnancy-specific' beta 1-glycoprotein (SP1) is a major placental product with unusual physicochemical properties that has been extensively investigated, but its biological function remains uncertain. Pregnancy-associated plasma protein A (PAPP-A), a glycoprotein of mol. wt 400,000, has effects in vitro on the coagulation and complement cascades, probably by its properties of protease inhibition. Placental protein 5 (PP5) may be involved in the coagulation and fibrinolytic systems, and in follicle maturation and semen liquefaction. 'Placental protein 12' (PP12) is not a product of the placenta at all; it appears to be produced in the female genital tract under the influence of progesterone and may also be produced by proliferating liver cells. Further study may reveal new roles for these placental proteins beyond their traditional roles as tumour markers.

Oligomerization of pregnancy-specific beta 1-glycoprotein (SP1) at physiologic pH and ionic strength

Highly purified pregnancy-specific beta 1-glycoprotein (SP1) migrated in gel electrophoresis as a homogeneous species and behaved as a single species in 6 mol/l guanidinium chloride (GdmCl), both in the ultracentrifuge and HPLC. At physiologic pH and ionic strength, in the absence of GdmCl, SP1 existed in the form of oligomers of apparent molecular weights of 40 000 to greater than 300 000. The specific activity of these oligomers varied over a 5-fold range. Electrophoretic mobility also varied among SP1 oligomers, with increasing (alpha-like) mobility shown by oligomers of increasing molecular size. Oligomerization may explain some or all of the reports of SP1 heterogeneity.

Identification of an SP-1-like protein in non-pregnancy serum: isolation using a monoclonal antibody

Human SP-1, a glycoprotein synthesized by the placenta during pregnancy, was shown to exist as polymers in maternal serum by a rapid passive transfer immunoblotting technique following conventional agarose electrophoresis. Moreover, the SP-1 polymers in serum were shown to dissociate into one main component upon treatment with 8 M urea prior to electrophoresis. However, an unexpected observation was the existence of an SP-1-like immunoreactive species in male serum with the sensitive immunoblotting technique. This SP-1-like protein in male serum had similar properties to its placentally derived counterpart in pregnancy serum, namely a propensity for complex formation and a reduced electrophoretic mobility following neuraminidase treatment. The relationship between the two SP-1 proteins was demonstrated by isolating them from their respective sera using an immobilized monoclonal antibody raised to purified SP-1 from pregnancy serum. Immunoblotting after sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed the existence of two major SP-1 species in pregnancy serum. These two SP-1 species had apparent molecular weights of 68,000 and 64,000. In addition, there were two minor bands at 35,000 and 32,000. These smaller SP-1 species did not appear to be subunits of the larger entities since they were detectable in the absence of reducing conditions. SDS-PAGE and immunoblotting showed that the immunoaffinity-purified SP-1 species from male and pregnancy sera had similar, but not identical, molecular weights.

Postpartum decline in serum concentration of placental proteins in an abdominal pregnancy

The alpha and the beta forms of Schwangerschaftsprotein 1 (SP1) and chorionic gonadotrophin (hCG) were measured after delivery in an abdominal pregnancy with the placenta left in situ. All three proteins declined more slowly than after normal delivery; the half-lives ranging from 6.2-6.5 days. Traces of the proteins could still be detected in the maternal circulation 9 weeks after delivery. These findings are construed as further support for the belief that the placenta plays a major role in the synthesis of these proteins.

Periovulatory secretion of immunoactive pregnancy-specific beta 1-glycoprotein (SP1) in humans: comparison with "normal" SP1

The pregnancy-specific beta 1-glycoprotein (SP1)-immunoreacting material in serum from two patients around ovulation was compared with serum SP1 activity derived from normal pregnant women. This was done by antibody binding experiments, density gradients and binding to lentil lectin. Periovulatory SP1 behaved similarly to SP1 from pregnant patients. It is therefore suggested that the periovulatory immunoreacting material is true SP1 and that this protein, though rarely, can be synthesized outside the trophoblast in healthy women.

Determination of the stage of gestation by the assay of chorionic gonadotrophin and Schwangerschaftsprotein 1

Serial assays of Schwangerschaftsprotein 1 (SP1), SP1 alpha, SP1 beta and human chorionic gonadotrophin were performed in 12 subjects from ovulation until the pregnancies had reached 16 weeks. From these data formulae were devised for deducing the stage of gestation from the concentration of the placental protein. These formulae were then tested by assays on 34 women not included in the original study. Assays of hCG do not give reliable indications of the stage of gestation when this has progressed beyond 9-10 weeks but SP1 assays give predictions of gestation corresponding closely to that derived from the last menstrual period up to 16 weeks gestation.

The concentrations of SP1 beta and SP1 alpha in retroplacental and peripheral blood at term

The concentrations of the two components of Schwangerschaftsprotein 1 (SP1 beta and SP1 alpha), human chorionic gonadotrophin (hCG) and human placental lactogen (hPL), were measured in peripheral venous blood and in retroplacental blood at term delivery in 22 women. Like hCG and hPL, the values of SP1 alpha were higher in the retroplacental than in the peripheral blood, as might be expected with placental secretory products. On the other hand, the concentration of SP1 beta showed a reverse gradient, being higher in the peripheral than in retroplacental blood.

Pregnancy-associated plasma protein A: a possible marker in the classification and prenatal diagnosis of Cornelia de Lange syndrome

The concentration of human platelet lactogen (hPL), pregnancy specific beta-1 glycoprotein (SP-1) and pregnancy-associated plasma protein A (PAPP-A) were analysed in consecutive serum samples from a patient who gave birth to a child with Cornelia de Lange syndrome. HPL and SP-1 were present in normal concentrations from week 20 to week 35 of gestation whereas PAPP-A could not be detected in any of the samples examined. Immunohistochemical examination of two placentae from Cornelia de Lange syndrome revealed normal localization of hPL and SP-1 but the absence of PAPP-A from the syncytiotrophoblast. The significance of association between Cornelia de Lange syndrome and compromised synthesis of PAPP-A is discussed.

Diagnosis of early pregnancy by assay of placental proteins

The plasma concentrations of Schwangerschaftsprotein 1 (SP1) and human chorionic gonadotrophin (hCG) were measured in 22 women before and after conception. In these pregnancies ovulation occurred spontaneously in seven subjects, or followed treatment with either clomiphene citrate (6 subjects), or human menopausal gonadotrophin (hMG) and hCG (6 subjects). The remaining three patients became pregnant after artificial insemination with donor semen. Seventeen of these pregnancies were normal and five were abnormal (3 abortions, 1 ectopic and 1 twin). SP1 could be detected 6-14 days after ovulation in all these patients. In contrast 40 non-pregnant women gave a negative test for SP1.

Purification of pregnancy-specific beta 1-glycoprotein with a monoclonal antibody immunoadsorbent

Hybrid cell lines secreting monoclonal antibodies against pregnancy-specific beta 1-glycoprotein (SP1) were produced by fusion of a mouse myeloma cell line with spleen cells from BALB/c mice immunized with purified placental SP1. The clones were further grown intraperitoneally in mice, and the ascites fluids contained anti-SP1 antibodies in high titers. One of the monoclonal antibodies was coupled to cyanogen-bromide-activated Sepharose and used for affinity chromatography of SP1 using late pregnancy serum as starting material. The purification of SP1 by means of immunoadsorption utilizing monoclonal antibodies offers remarkable advantages compared with the purification procedures used so far.

Schwangerschaftsprotein 1 (SP1) in serum and cerebrospinal fluid of patients with gestational trophoblastic disease

The beta subunit of human chorionic gonadotrophin (beta-hCG) and Schwangerschaftsprotein 1 (SP1) were measured by radioimmunoassay in the serum and cerebrospinal fluid (CSF) of 46 postmolar and postpartum patients who developed gestational trophoblastic disease. There was a significant correlation between beta-hCG and SP1 serum levels. The mean serum SP1 level in high-risk patients was significantly higher than that in low-risk patients. There was a significant correlation between serum and CSF beta-hCG levels. The ratio of serum to CSF beta-hCG levels was low in the three patients with clinical evidence of intracranial metastasis. SP1 was present in the CSF of only one of these three patients, but it could be detected in the CSF of another four patients without clinical evidence of metastases in the central nervous system. The two low-risk patients with SP1 present in the CSF showed poor response to intramuscular methotrexate.

Complex formation of pregnancy-specific alpha 2-glycoprotein (SP1 alpha) and heparin

The Schwangerschaftsprotein 1 (SP1)alpha values in blood measured by rocket immunoelectrophoresis are affected by the addition of anticoagulants. When compared with values in serum, those in heparin plasma were much higher, while those in sequestrene ethylenediamine-tetra-acetic acid), sodium citrate and fluoride oxalate were lower. On the other hand, SP1 and SP1 beta concentrations were not significantly affected in heparin or sequestrene and were only slightly depressed in sodium citrate and fluoride oxalate. The effect of heparin on SP1 alpha in serum was dose-dependent. We surmise that SP1 alpha forms a complex with heparin and that it may be involved in the coagulation system in pregnancy.

Molecular heterogeneity of pregnancy-specific beta 1-glycoprotein (SP1)

Ion-exchange chromatography has been used to isolate three separate forms of Pregnancy-Specific beta 1-Glycoprotein (SP1) from pooled, late-pregnancy human sera. Each of the three components was immunochemically identifiable as SP1, yet distinguishable from each other on the basis of rocket morphology resulting from rocket immunoelectrophoresis (RIE). The elution profile from DEAE Sephacel indicated that each component has an isoelectric point below pH 8.6, but suggested a different charge for each molecular species. Crossed immunoelectrophoresis (XIE) of pooled, late pregnancy serum suggested the presence of each component prior to physico-chemical treatment and showed the components to be characterised by alpha, beta or gamma electrophoretic mobility. The differences in rocket morphology suggested differing affinities for the antiserum, and a difference in molecular size, which was confirmed by gel filtration.

Heterogeneity of the pregnancy-specific beta 1-glycoprotein (SP1)

The pregnancy-specific beta 1-glycoprotein (SP1) in pregnant serum fractionated by ammonium sulfate precipitation and anion exchange chromatography was investigated by various electrophoretic methods. From these investigations it appears that in addition to the major component SP1(beta) three other components are present, termed SP1(gamma), SP1 (alpha 2(2)) and SP1 (alpha 2(1) on the basis of their electrophoretic mobilities. The isoelectric point (pI), estimated by isoelectric focusing, and the relative molecular mass (Mr), estimated by Sephadex G-200 gel filtration chromatography are for SP1(gamma): pI 2.9--4.9, Mr approximately 80000; SP1(beta): pI 2.5--4.5, Mr approximately 80000; SP1(alpha 2(2)): pI less than or equal to 6, Mr greater than or equal to 200000; SP1(alpha 2(1)): pI less than 3, Mr approximately 30000. Based on the interaction with phenyl-Sepharose and concanavalin A all four species of SP1 are amphiphilic glycoproteins.

Measurement of SP1 in samples with varying SP1 alpha: SP1 beta ratios

Mixtures containing a fixed amount of SP1 alpha, increasing amounts of SP1 beta and vice versa have been measured by rocket immunoelectrophoresis, by enzyme immunoassay and by radioimmunoassay. Both proteins have immunological determinants which react with standard antisera to SP1 and the value obtained for any particular mixture is dependent on the ratio of the proteins. The effect of the ratio on measurement by immunoelectrophoresis is different from that on measurement by enzyme immunoassay or radioimmunoassay. As these two proteins exist together in the blood of pregnant women, Results obtained by the two types of assay cannot be compared. Even with the same method the results will depend on the ratio which varies from subject to subject. It is concluded that until specific measurements for SP1 alpha and SP1 beta are designed, clinical application studies must be viewed with reserve.

Comparative assays in three laboratories of serum specimens containing pregnancy-specific beta 1 glycoprotein with different proportions of the alpha variant

Following several reports that the alpha component of pregnancy-specific beta 1 glycoprotein (SP1) gives dose-response curves that are flatter than those of the beta component when assayed by radioimmunoassay of the competitive type, a collaborative study was carried out by 3 laboratories to examine whether radial immunodiffusion or rocket immunoelectrophoresis give similar result. Five sera from pregnant women at the 34-39th week of gestation with beta : alpha ratios of 0.8-4.7 were assayed by both the latter methods against either the IARC 78 610 reference preparation of the SP1 Behring standard. When preparations were assayed by rocket immunoelectrophoresis (two laboratories), the dose-response curves were parallel. When preparations were assayed by radial immunodiffusion (two laboratories), parallelism was observed (one laboratory) after the addition of polyethylene glycol, the presence of which is necessary to visualize immune precipitates in which alpha is involved. Discrepancies observed when methodologies such as competitive radioimmunoassay are used, in which there are limited amounts of antibodies, are explained, at least partially, but the fact that the beta component has a greater relative affinity for the antibody as higher concentrations of beta : alpha mixtures are assayed. However, when antibody is present in excess, as in radial immunodiffusion and rocket immunoelectrophoresis, no such difference in affinity seems to occur.

Pregnancy specific beta 1-glycoprotein (SP1) conversion of SP1 beta to SP1 alpha by "acidification": an artefact

Late pregnancy serum and plasma were examined in rocket immunoelectrophoresis (RIE) and crossed immunoelectrophoresis (XIE) before and after "acid treatment" (pH 2.5, 10 min). The analysis revealed changes in the precipitation morphology in the majority of circulating proteins extending from the albumin to the gamma-region. In RIE, the precipitation pattern of SP1 was altered, this change being independent of polyethylene glycol. In addition, XIE analysis of SP1 revealed a change in the precipitation morphology with no change in electrophoretic mobility. The results can be explained by non-specific aggregation and denaturation due to "acid treatment", and are therefore unlikely to be of any biological or immunochemical importance.

Pregnancy-specific beta 1-glycoprotein as a screening test for at-risk pregnancies

A serum assay was performed on 605 women for pregnancy-specific beta 1-glycoprotein (SP-1) between 31 and 34 weeks' gestation. SP-1 was also estimated for 255 of these women between 15 and 20 weeks' gestation, coincidental with maternal serum alpha-fetoprotein (AFP) screening. A clinical classification of pregnancy outcome was constructed for ech group. SP-1 values at 31 to 34 weeks' gestation in pregnancies that resulted in the birth of a small-for-dates (SFD) infant were statistically different from those in normal pregnancies (p less than 0.001). A value of 100 mg per liter was chosen as a suitable discriminatory level at 31 to 34 weeks, below which SP-1 had a sensitivity of 64%, a predictive value of 24%, and defined a relative risk of 5.3 for SFD infants. These parameters compare favorably with published data on the use of human placental lactogen as a screening test. No statistical relationship was found between SP-1 values at 31 to 34 weeks and birth weight, placental weight, parity, or other abnormal pregnancy outcomes. SP-1 screening at 15 to 20 weeks' gestation showed no correlation with serum AFP or with other parameters studied.

An immunochemical study of proteins with SP1 determinants in native and acidified pregnancy serum

The alpha and beta forms of the pregnancy-associated protein, SP1, have been studied in late pregnancy serum and in similar serum after acidification. In both sera only two forms of the protein, SP1 alpha and SP1 beta, could be found; both reacting with antisera against SP1. On gel chromatography these two forms could be separated, with the intermediate effluent containing a varying mixture of both proteins. Immunoelectrophoresis of the effluent fractions from gel chromatography showed rocket shaped immunoprecipitates whose morphology depended on the mixture of SP1 alpha and SP1 beta. Crossed immunoelectrophoresis confirmed that only two proteins could be defined with SP1 antisera; the front running protein having an alpha 2 electrophoretic mobility and a molecular weight of 430,000, while the slower moving component had the electrophoretic mobility of a beta 1 globulin and a molecular weight of 90,000.

Observations on the separation of SP1 alpha and SP1 beta by affinity chromatography

SP1 antigens in late pregnancy blood, in placental extracts and in acidified plasma have been purified on positive affinity chromatography columns with immobilized SP1 antiserum and on negative affinity columns with immobilized antihuman non-pregnancy serum. Both SP1 alpha and SP1 beta are bound by the positive column but only SP1 alpha is bound by the negative column. This is adduced as further support for the view that SP1 alpha is formed by the combination of SP1 beta with a protein present in non-pregnancy serum.

Investigations into the molecular heterogeneity of pregnancy-specific beta 1-glycoprotein (SP1)

The molecular heterogeneity of pregnancy-specific beta 1-glycoprotein (SP1) was examined by analytical immunoelectrophoresis and a radioimmunostaining technique of immunoelectrophoretic plates. Analytical crossed immunoelectrophoretic analysis and radioimmunostaining of these plates demonstrated the presence of normal human serum components in the alpha-mobile precipitate previously considered to be exclusively the high molecular pregnancy-specific protein, SP1 alpha. This observation suggested that two molecular populations were contributing to the alpha-mobile precipitate. Following fractionation of late-pregnancy serum by size chromatography, the radioimmunostaining technique further demonstrated the presence of normal serum components in the intermediate fraction but not in authentic SP1 (i.e., SP1 beta) or the high molecular weight form (SP1 alpha). We suggest that SP1 antigenic determinants are distributed in three different fractions of pregnancy serum, one of which (intermediate fraction) is a complex of authentic SP1 (SP1 beta) and a normal serum protein, whereas non-pregnancy serum components were not demonstrable in the remaining two (i.e., SP1 beta or SP1 alpha).

Heterogeneity of pregnancy-specific beta 1-glycoprotein following purification from maternal serum and partial characterization of a high-molecular-weight component

Pregnancy-Specific beta 1-Glycoprotein (PSB1G) has been purified from maternal serum by physico-chemical and immunochemical methods. Marked instability of the protein was noted after partial purification which correlated with hydroxylapatite chromatography of PSB1G. Three molecular heterogeneous forms were demonstrated by gel filtration on Sephacryl S-200 which were designated as PSB1G-I, II and III. Each of the latter purified materials displayed altered immunoelectrophoretic mobility, i.e., altered from beta 1 to gamma mobility. In immunodiffusion experiments PSB1G-I reacted in complete immunological identity with 'native' serum PSB1G whereas PSB1G-II and III reacted in partial identity with each of the latter, suggesting that they are dissociation and/or degradation products of the parent PSB1G. The apparent molecular weights of PSB1G-I, II and III as determined by gel filtration were 280 000, 69 000 and 23 000, respectively, PSB1G-I was selected for structural analysis on the basis of its immunological identity with 'native' antigen. SDS-PAGE studies of PSB1G-I with and without reducing agent strongly suggest that PSB1G-I and 'native' serum antigen are composed of two dissimilar subunits of equal molecular size which are bonded together by non-covalent linkage. One of the subunits appears to represent a single polypeptide chain, whereas the other subunit appears to be composed of two polypeptide chains of unequal size that are linked together by covalent bonds. Investigations are continuing towards confirmation of the proposed structural model of PSB1G-I by an alternate methodology of protein structure analysis.

Sequential fluoroimmunoassay for measurement of pregnancy specific beta 1 glycoprotein using antibody coupled to magnetisable particles

A direct, sequential fluoroimmunoassay has been developed for the determination of serum levels of pregnancy specific beta 1 glycoprotein (SP1). The method employs rabbit anti-SP1 serum coupled to magnetisable cellulose-iron oxide particles and fluorescein-labelled SP1. Serum samples or standards are incubated with magnetisable solid phase anti-SP1 for 30 min. After magnetic sedimentation of the particles, the supernate, which includes endogenous fluorophores and other interfering factors, is discarded. Fluorescein-labelled SP1 is then added and incubated for a further 45 min; the particles are again sedimented and the fluorescence of the labelled SP1 remaining in the supernate is estimated. This reading related directly to the SP1 content of the original sample. The entire procedure, including fluorometry, is performed within a single disposable polystyrene test tube and is sufficiently simple and reliable for routine application. The sensitivity, specificity and precision is very similar to that of radioimmunoassay, and the results correlate with those of the radioimmunoassay (r = 0.9887).

The demonstration of two pregnancy-associated proteins with SP1 determinants in placental extracts

The SP1 content of the placenta has been examined. It was found that, as in pregnancy serum, two variants of the protein existed in the placenta. The smaller molecule, with a beta 1 electrophoretic mobility, is readily extractable with phosphate buffered saline. The second larger molecule is more tightly bound in the placenta and requires Triton X-100 to be solubilized. The hypothesis is put forward that the larger alpha variant is the precursor form which is related more irregularly and in a smaller amount from the placenta into the maternal circulation.

Separation of two pregnancy-associated proteins with SP1 determinants and the conversion of SP1 beta to SP1 alpha

The alpha and beta forms of the pregnancy-associated protein, SP1 which have been previously identified in late pregnancy serum were found to be also present in the placenta. The alpha form was somewhat more difficult to extract, requiring the use of detergents in the extracting buffer. The two variants can be separated by ion exchange chromatography or by gel filtration and distinguished by the different shapes of the precipitation rockets on immunoelectrophoresis. The alpha variant appears to be the larger molecule on gel filtration. When solutions of SP1 are acidified to pH 2.5 the beta form is rapidly converted to the alpha form and the latter is slowly destroyed. It is suggested that SP1 alpha represents the combination of SP1 beta with some other protein present in the placenta and in the serum.

Immunochemical and biological studies on two molecular variants of pregnancy specific beta 1 glycoprotein (SP1) SP1 alpha and SP1 beta

Analytical crossed immunoelectrophoresis was performed to examine the relationship of a high molecular mass alpha 2 mobile glycoprotein (SP1 alpha) which is immunochemically identifiable with pregnancy specific beta 1 glycoprotein (SP1 beta). There was no evidence for any reaction of SP1 alpha with antisera to normal human serum protein, nor for complex formation following incubation with normal serum proteins. The half-life of SP1 alpha was apparently shorter than that of SP1 beta after delivery of the placenta.

The variable influence of an alpha component in pregnancy plasma on four different assay systems for the measurement of pregnancy-specific beta1 glycoprotein

Four methods (radioimmunoassay, electroimmunoassay, laser nephelometry and kinetic immunoturbidimetry) have been evaluated for the measurement of proteins with pregnancy-specific beta1 glycoprotein immunoreactivity. The influence of two maternal plasma proteins with different electrophoretic mobilities (alpha2 and beta1) but with similar antigenic determinants has been assessed in each assay system. The radioimmunoassay method, which utilises a homogeneous preparation of pregnancy-specific beta1 glycoprotein for the preparation of iodinated tracer, and the kinetic immunoturbidimetric assay which measures antigen-antibody complex formation over the first minute of the reaction, were both considerably more specific for the beta1 component of pregnancy specific beta1 glycoprotein immunoreactivity than the laser nephelometric or electroimmunoassay methods.

Pregnancy specific beta-1 glycoprotein (SP1) in patients with trophoblastic disease: molecular heterogeneity and a "SP1 consuming factor"

The reactivity of sera from 19 patients with trophoblastic disease was analyzed in analytic immunoelectrophoresis using rabbit and human pregnancy specific beta-1 glycoprotein (SP1) antisera. Molecular heterogeneity of SP1 was demonstrated in crossed immunoelectrophoresis and a "SP1 consuming factors" was found in serum from a patient with choriocarcinoma. The "SP1 consuming factor" was precipitated by 1.65 M (NH4)2SO4. The results are discussed in relation to quantification of circulating SP1 patients with trophoblastic disease.

The ratio between molecular variants of pregnancy-specific beta 1 glycoprotein (SP1) in the peripheral and retroplacental circulation during later pregnancy

The ratio between two molecular variants of pregnancy-specific beta 1 glycoprotein (SP1) in the peripheral and retroplacental circulation was analysed in nine women during late pregnancy. The ratio between the two molecular variants, i.e. SP1 beta and SP1 alpha, was found to be identical in the peripheral and retroplacental circulation in all women examined. Comparisons were also made between SP1 beta as measured by radioimmunoassay and a normal serum protein, alpha 2 macroglobulin. A positive concentration gradient from peripheral to retroplacental circulation was found for SP1 beta whereas the gradient was less pronounced for alpha 2 macroglobulin. These findings add weight to the concept that SP1 alpha is a unique molecular species and not a genetic variant or a product of SP1 beta.