Studies on human osteopontin : isolation and characterization of osteopontin in milk and serum.

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Osteopontin (hereafter called as OPN), a CaィイD12+ィエD1 binding protein, has been studied as a secreted highly acidic phophoglycoprotein with diverse functions in various tissue and species. It was firstly isolated from mineralized bone matrix and studied as a protein remodeling mineralized tissue. Since OPN is present in human milk, we considered that the OPN plays an important role in the osteogenesis in the developing infants. The present studies aimed to isolate and characterize OPN in human milk, serum and blood cells.We have isolated osteopontin from human milk, prepared antibodies specific for it, and tried to establish a sandwich type EIA system determining osteopontin in serum. It was found that serum osteopontin but not milk osteopontin showed paradoxical phenomena in reaction with the antibodies, making no detectable precipitation in immunoelectrophoresis and no positive reaction in EIA, but showing positive reaction in Western blot and in an inhibition tests. Taking these phenomena and other circumstantial evidences into account, it is concluded that the serum osteopontin has a high molecular fragility, fragmenting into small peptides with few epitopes during incubation time. This fragility, though much lower extent, was also seen in milk osteopontin. Although the exact casual meaning of this fragility was not defined yet, it is conceivable that the fragility of OPN itself has some meaning in its physiological functions. Such possibility is being investigated in our laboratory.