SECRETION OF OSTEOCALCIN AND ITS PROPEPTIDE FROM HUMAN OSTEOBLASTIC CELLS - DISSOCIATION OF THE SECRETORY PATTERNS OF OSTEOCALCIN AND ITS PROPEPTIDE

Specific immunoassay systems for intact human osteocalcin (I-OC) and i ts 26-residue propeptide have been newly developed to assess their use fulness as biochemical markers of bone metabolism. Using human culture d osteoblastic periosteal cells, we monitored 24 h secretion of these molecules from the osteoblastic cells and also examined the deposition of Ca, P, and I-OC on the extracellular matrix. At day 5, both I-OC a nd its propeptide were secreted by osteoblastic cells in a concentrati on-dependent manner by treatment with 1,25-(OH)2D3. This propeptide wa s not detected in the serum of adult subjects but was detected in the serum of normal children, which confirmed this in vitro result of prop eptide secretion. The secretion of I-OC into medium transiently decrea sed at day 11, when the rapid accumulation of I-OC, Ca, and P, namely mineralization, was observed on the extracellular matrix of osteoblast ic cells, although secretion of the propeptide constantly increased th roughout the culture period. Therefore, the ratio of the amount of pro peptide to I-OC in the supernatant markedly increased when mineralizat ion started. These data demonstrate the superior specificity of propep tide as a marker of osteoblastic function in vitro compared with I-OC and that monitoring the changes in propeptide to I-OC ratios in the cu lture supernatant may be useful for predicting the timing of mineraliz ation on the extracellular matrix of osteoblastic cells.