Epitope mapping of nine monoclonal antibodies against osteocalcin: Combinations into two-site assays affect both assay specificity and sample stability

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Abstract
Nine monoclonal antibodies (Mabs) were raised against human recombinant osteocalcin fusion protein (rGST-hOC) or bovine osteocalcin (bOC) and selected to develop two-site immunoassays for human osteocalcin (hOC). The detection system was based on the time-resolved measurement of the fluorescence of europium chelates conjugated to the tracer Mabs. Based on the ability of the Mabs to recognize different forms of hOC (carboxypeptidase Y-digested, alkylated hOC, thermally decarboxylated hOC, recombinant forms of hOC, and tryptic peptides derived from hOC) and the information obtained from combinations of the Mabs in two-site assays, an epitope map was created. The epitope map was useful in understanding the behavior of the two-site combinations of the Mabs with serum samples. The two-site combinations could be divided into subgroups detecting either full-length hOC or full length + large NH2-terminal fragment as simulated by the carboxypeptidase Y-digested form of hOC (it lacks four COOH-terminal residues), which with intact specific assays showed cross-reactivities ranging from 7 to 14% when compared with full-length hOC. In addition, differences were observed in the ability of the combinations to detect thermally decarboxylated hOC (lacks γ-carboxylation at residues 17, 21, and 24) with cross-reactivities ranging from 8 to 85% when compared to γ-carboxylated hOC. The analysis of human serum samples showed considerable differences in the concentration and stability of serum OC. This was attributed as the varying ability of the Mabs to detect different proteolytic fragments derived from hOC and/or differences in the degree of γ-carboxylation of hOC. The in vitro generation of the large NH2-terminal fragment during incubation of the serum samples at room temperature (RT) and during prolonged storage at −20°C in an undercooled state was detectable as loss of immunoreactivity (ranging from −42 ± 17 to −50 ± 15% in 16 h at RT, n = 3) with Mab combinations detecting only full-length hOC. Two-site combinations detecting full-length + large NH2-terminal fragment showed no loss of immunoreactivity after incubation of the serum samples at RT for 16 h. With all assays, an increase of serum OC ranging from 16 to 38% was found in postmenopausal samples (n = 24) when compared with premenopausal samples (n = 17), but the degree of statistical significance varied from not significant to p < 0.01.
Funding Information
  • State Technology Development Centre of Finland
  • Turku University Foundation