HIGH FETAL PLASMA OSTEOCALCIN (OC) AND POSITIVE RELATIONSHIP OF OC TO PLASMA PHOSPHATE (P) IN FETAL (F) AND MATERNAL (M) SHEEP USING A HOMOLOGOUS RIA

Abstract

Plasma concentrations of the bone-specific protein OC correlate with bone formation rate. OC may regulate the mineralization process. We isolated OC from ovine bone and used an homologous RIA to investigate plasma OC in chronically catheterized M and F sheep during the last month of gestation. OC was purified from adult cortical bone by sequential gel filtration, anion exchange chromatography and SDS PAGE. Characterization was on the basis of amino acid analysis, including presence of γ-carboxyglutamic acid and its conversion to glutamic acid upon decarboxylation. N-terminal aminoacid sequencing (20 residues) revealed hydroxyproline at amino acid position 9. The protein demonstrated a typical stoichiometry of binding to hydroxylapatite (HTP) of 64mg protein/mg HTP and retained this property when radioiodinated. The RIA has a sensitivity of 1 ng/tube and intra and inter assay CV of 7% and 12.4%. Mean (±SEM) plasma OC in 51 samples from 10 pregnant sheep of 6.3±0.8ng/ml was not different from 23 non-pregnant sheep (5.8±1.1ng/ml) but was markedly lower than the corresponding F value of 141±8 ng/ml (p<0.0001). There was no correlation between F and M vaTues and neither F nor M values correlated with gestation, total plasma Ca or ionized Ca. However, both F and M plasma OC was directly correlated with plasma P (F:r= 0.45, p<0.01; M r=0.54, p<0.001). We suggest there is 1) lack of placental transfer of OC, 2) positive regulation of bone turnover by P during pregnancy, 3) plasma OC is useful in the study of fetal mineral metabolism.