Endogenous IGFBP-3 is protected from inducible IGFBP-3 protease activity in normal adult serum

We have recently demonstrated that the presence or absence of IGFBP-3 prote ase activity in physiological fluids may not be determined simply by the pr esence or absence of specific enzymes but rather the presence of inhibitors . In addition, it appears that these inhibitors may not only be associated with the protease(s) but with the IGFBP-3 itself, protecting it from proteo lytic cleavage. To provide further evidence for this mechanism of regulatio n we investigated whether IGFBP-3 protease activity could be unveiled in no rmal adult serum (NS) and whether the endogenous IGFBP-3 was protected from this activity. The addition of a range of concentrations of heparin, induc ed IGFBP-3 protease activity in NS. This was comparable to that seen in pre gnancy serum (PS) by virtue of the fragmentation pattern and inhibitor prof ile. While the addition of zinc also revealed IGFBP-3 protease activity in NS the pattern of fragments differed to that seen in PS. Under both conditi ons, however, the endogenous IGFBP-3 was not proteolytically modified. Thes e results demonstrate that IGFBP-3 protease activity is present in NS and c an be activated, although the endogenous IGFBP-3 is relatively protected fr om such activity. (C) 2000 Harcourt Publishers Ltd.