Matrix metalloproteinase-9 activity detected in body fluids is the result of two different enzyme forms

In vitro activation of matrix metalloproteinase-9 (MMP-9) (Gelatinase B) with MMP-3 shows the presence of two different forms: an 82 kDa, N-terminal truncated form, and a 65 kDa, N- and C-terminal truncated form. So far the presence of the 65 kDa form has not been reported in vivo. Affinity chromatography was performed to separate MMP-9 from MMP-2 and immunoprecipitation to isolate ∼65 kDa MMP-9 from 82 kDa MMP-9 in sera of healthy donors. The presence of ∼65 kDa active MMP-9 was demonstrated both with gelatin zymography and western blot analysis. The ∼65 kDa MMP-9 lacks the haemopexin domain required for the high-affinity binding of the tissue inhibitor TIMP-1, and can be evaluated by activity assay in the presence of TIMP-1. This opens the possibility to investigate the role of this form of MMP-9 that escapes physiological regulation.