Glycoprotein hormone isomorphism and assay discrepancy: the paradigm of luteinizing hormone (LH)

LH isoform profiles during short-term pulsatile LHRH administration in elderly men

LH isoform profiles were analyzed in sera resolved with isoelectrofocusing from 5 elderly men (age 70.6 +/- 2.95) and 5 young adult men (age 28.2 +/- 1.24), by using polyclonal antibodies (RIA), monoclonal antibodies directed against the beta-subunits (IRMA) and in vitro LH bioassay. Despite the fact that the elderly had lower testosterone levels than the young (293 +/- 38 vs 512 +/- 77 ng/dl, p < 0.05), no differences were noted in the isoforms detected by any of the assays, although each assay yielded a characteristic profile. Indeed, RIA showed most LH in the acidic range, while IRMA revealed LH profiles with a major peak in the basic range, thus resembling the profiles determined by means of the bioassay. In the elderly, the profiles were also analyzed on day 7 and day 14 of short-term pulsatile sc LHRH administration (150 ng/bw/120 min). Only the LH bioassay detected an LHRH-induced shift to more basic and bioactive forms; these changes accompanied an increased in testosterone levels on day 7 (396 +/- 83 ng/dl, p < 0.05 vs day 0) and on day 14 (320 +/- 58 ng/dl NS vs day 0). Our data suggest that: 1) the profiles obtained in young and elderly subjects are similar, irrespective of the antisera used; 2) as a result of treatment with LHRH in the elderly an increase in T levels occurs, possibly due to the observed changes in LH bioactivity; 3) the in vitro LH bioassay appears to be the most sensitive assay in detecting such changes, which consisted of an enrichment in more basic and bioactive glycoforms.

Establishment of an in vitro bioassay and radio receptor assay for LH/CG in human sera using immortalized granulosa cells transfected with LH/CG receptor

Levels of gonadotropic hormones in human sera or urine are routinely measured by radioimmunoassay or by enzyme-linked immunoassay (ELISA), which determine the immunoactivity of the hormone, but not its biological activity. We have utilized immortalized stable steroidogenic granulosa cells, which express 5-10 times more of the luteinizing hormone/chorionic gonadotropin (LH/CG) receptors than the primary cells, to develop a biological assay and radioreceptor assay for this hormone. We found that stimulation of cells expressing LH/CG receptor with increasing doses of human LH or human CG resulted in a dose-dependent increase of cAMP and progesterone with an ED(50) of 30 and 57 mlU/mL, respectively. These dose-response data served as calibration curves for measuring the gonadotropin bioactivity in human serum samples at concentrations as low as 1-5 mlU/mL. We found a close correlation between LH levels measured by enzyme immunoassay (EIA) and the in vitro bioassay in normal cycling and menopausal women, as well as in normal adult men. Also, a close correlation was found between the EIA and the in vitro biological assay of hCG in pregnant women. In addition, we have developed a radioreceptor assay (RRA) for this hormone using enriched cell membranes of the appropriate cell line, which corresponds well to both the EIA and the bioassay in human sera. Deglycosylated hCG was fully active in RRA, but failed to activated cAMP response in these cells, demonstrating the importance of the bioassay in the biologically inactive form of gonadotropins. We believe this novel in vitro bioassay of gonadotropic hormones will serve as a useful tool for a more comprehensive set of assays that will determine not only the amount, but also the possible modulation in bioactivity of the gonadotropin associated with gonadal failure and miscarriage.

Undetectable luteinizing hormone levels using a monoclonal immunometric assay

Previous studies have shown wide discrepancies among the results obtained with different immunometric assays. We present five cases (out of 4000 women) whose plasma luteinizing hormone was not detected using a LH immunometric assay (LH Stratus Baxter) but was recognized by other kits. These cases concerned one 28-year-old woman presenting with infertility and four postmenopausal women. The LH Amerlite kit gave detectable but low results. The results obtained with the other kits were > 7 IU/l. FSH levels were > 7 IU/l. In one case, sera were taken before and after the menopause; differences between the LH results increased. Discrepancies among LH assay kits have been attributed to variation both in standard curve calibration and in epitope specificity of the kit monoclonal antibodies. The Baxter kit might misrecognize some isoforms present in postmenopausal women. The present data illustrate the potential false results with such immunoassays in routine clinical laboratory testing. When undetectable LH results are not clinically explained or when disparities between LH and FSH are observed, we suggest using a second methodology or a bioassay if necessary. Improvement in LH assays and standardization might resolve the problem of discrepancies between the LH results.

European collaborative study of luteinizing hormone assay: 1. Epitope specificity of luteinizing hormone monoclonal antibodies and surface mapping of pituitary and urinary luteinizing hormone

This report describes the results of the first part of the collaborative study organized by a working group sponsored by the Community Bureau of Reference of the European Community Commission. The whole study was designed to understand the causes of discrepancy among LH immunoassay methods. In the parent work, we studied the characteristics of 55 monoclonal antibodies to LH which allowed us to establish a detailed map of the antigenic surface of the hormone. In the present report we used this information to interpret the discrepancy in LH concentrations assayed with 12 different methods in 300 sera from subjects with various clinical conditions. The 55 monoclonal antibodies provided by 11 commercial companies were tested in various experiments: the apparent affinity of the antibodies was, generally but not always, higher for LH presented by a second antibody coated to plastic than for LH directly coated to plastic; 26% of the antibodies recognized the alpha subunit, 26% the beta subunit and 48% reacted only with the holomolecule (anti-alpha beta); only 28% of the antibodies were strictly specific for LH. Criss-cross experiments allowed us to distinguish 13 antigenic regions on the surface of LH: 6 were located on the alpha subunit, 3 on the beta subunit and 4 on the holomolecule. The monoclonal antibodies to the alpha beta regions further separated into 12 clusters of reactivity. Accordingly, LH appeared to exhibit at least 21 epitopes. Comparison of the immunoreactivity of various LH preparations indicated that highly purified pituitary LH and immunoaffinity purified urinary LH reacted similarly with the monoclonal antibodies and strongly differed from crude urinary LH. These data indicated that the immunoreactivity of an LH preparation depends mainly upon the degree of purification and not that much upon the origin of the preparation. The epitope specificity of the monoclonal antibodies used in 11 commercially available LH assay kits was also determined: 10 kits used at least one anti-alpha beta monoclonal antibody associated with an anti-beta monoclonal antibody in 7 cases or another anti-alpha beta monoclonal antibody in 3 cases; one kit used an anti-alpha monoclonal antibody associated with an anti-beta monoclonal antibody. None of the kits were strictly identical with regards to the epitope specificity of the monoclonal antibodies used.

European collaborative study on luteinizing hormone assay: 2. Discrepancy among assay kits is related to variation both in standard curve calibration and epitope specificity of kit monoclonal antibodies

This report describes the results of the second part of the collaborative study organized by a working group sponsored by the Community Bureau of Reference of the European Community Commission. The whole study was designed to understand the causes of discrepancy among LH immunoassay methods. In the parent report we described the characteristics of LH monoclonal antibodies. In the present work we focused on the comparison of 11 commercially available monoclonal antibody based kits and one polyclonal antibody based RIA. Recovery experiments of the second IS 80/552 and of a related LH preparation showed that curve calibration differed among kits. The ratio between the LH recovery of the two most different kits was 2.30. LH concentrations were determined, using the 12 assay methods in the sera of prepubertal children (n = 46), normal women (n = 26) and men (n = 39) before and after LHRH stimulation, post-menopausal women (n = 29) and patients with renal failure (n = 71) or polycystic ovaries (n = 28). In children LH was detected in 0 to 80% of the sera depending on the kit. In adults, the mean LH concentration provided by the 12 kits varied from 6.0 to 13.55 IU/L showing a ratio of 2.26 between the two most different kits. A thorough statistical analysis allowed to distinguish two groups of kits. The first groups consisting of 6 kits provided results close to those obtained by the RIA. The 5 other kits misrecognized circulating LH in subjects with various clinical status. These 5 kits were the only ones to use, as labelled probes, monoclonal antibodies specific for the holohormone (anti-alpha beta) whereas the 6 other kits used, as labelled probes, monoclonal antibodies directed to the alpha (1 kit) or the beta-subunit (5 kits). In both groups, the coated monoclonal antibodies are directed to either the beta-subunit (1/6 and 2/5 kits) or the holohormone (5/6 and 3/5 kits). Taken together these data suggest that discrepancy among LH assay kits is related to variation in standard curve calibration and in epitope specificity of monoclonal antibodies used in the kits. These findings may prove to be instrumental to alleviate differences among LH assay methods.