Solubilization of type I collagen from fish muscle connective tissue by matrix metalloproteinase-9 at chilled temperature

Post-mortem muscle tenderization during chilled storage is thought to result from loss of physical strength due to the disintegration of intramuscular connective tissue. Although metalloproteinases may be involved in the disintegration, details of the proteolytic processes occurring during chilled storage remain to be elucidated. To determine the involvement of matrix metalloproteinase-9 (MMP-9) in the post-mortem disintegration of intramuscular connective tissue, we produced recombinant Japanese flounder MMP-9 (jfMMP-9) in COS-7 cells. The recombinant jfMMP-9 showed gelatinolytic activity in the zymographic analysis at chilled temperature, but no apparent proteolytic activity against the triple helical domain of types I and type V collagens. In contrast, recombinant jfMMP-9 solubilized type I collagen from a crude connective tissue preparation at chilled temperature. This finding, together with the fact that a protein corresponding to MMP-9 was immunologically detected in the muscle extract, indicates that MMP-9 may be involved in the disintegration of the intramuscular connective tissue that induces the post-mortem tenderization of fish muscle during chilled storage.