Osteocalcin: An Endocrine Link Between Bone and Glucose Metabolism

Bu B Yeap

Disclosures

Expert Rev Endocrinol Metab. 2011;6(2):177-185. 

In This Article

Feasibility of Measuring ucOC in Large Cohort Studies

Measurement of TOC in large cohort studies can be conveniently achieved using automated immunoassays capable of detecting both intact osteocalcin (amino acids 1–49) and the major N-terminal fragment (amino acids 1–43) (Figure 2).[35] This is preferable to an intact osteocalcin assay which may underestimate osteocalcin levels due to lability of the C-terminus and loss of amino acids 44–49. However, measurement of ucOC is less straightforward, contributing to the relative lack of ucOC data. A commercial antibody for ucOC is available (Takara Biomedical Group, Japan) but it is known to overestimate the large 1–43 and 4–43 fragments of ucOC.[20] Therefore, hydroxyapatite binding has been the standard method for measurement of ucOC. For this, aliquots of serum are treated with hydroxyapatite, which binds carboxylated osteocalcin, allowing it to be precipitated by centrifugation, leaving ucOC from the supernatant available for detection (Figure 2). This method is technically demanding and labor intensive, hence, it is not widely available.[20] Recently, Ferron et al. reported an ELISA-based assay for detection of osteocalcin carboxylation in mice.[36] Murine osteocalcin has 46 amino acids and the glutamic acid residues (GLU) that can be γ-carboxylated (GLA) are at positions 13, 17 and 20.[19,36] Using specific antibodies, Ferron et al. demonstrated in cultured osteoblasts and in mice that inhibition of γ-carboxylation by warfarin therapy resulted in a decrease in serum 13-GLA osteocalcin and a corresponding increase in serum 13-GLU osteocalcin.[36] Therefore, 13-GLU osteocalcin, which comprised the large majority of ucOC, may be the biologically active form corresponding to ucOC measured by the hydroxyapatite-binding assay. However, assay properties of 13-GLU and 13-GLA osteocalcin-specific antibodies have not been validated in human sera. Establishment of a robust assay for human ucOC, either by standardization of the hydroxyapatite-binding assay or creation of immunoassays based on 17-GLU and 17-GLA osteocalcin-specific antibodies, would greatly improve the ability of researchers to assess the relevance of ucOC to human metabolism and health. Important questions to address are whether 17-GLU osteocalcin is the main form of ucOC in humans, and the degree to which carboxylation status of GLU at positions 21 and 24 modulate any metabolic effects of ucOC.

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