It is suspected that the proteolytic breakdown of extracellular matrix proteins is responsible for the postmortem tenderization of fish muscle during chilled storage. In order to identify the type (s) of proteinases involved in this phenomenon, the effect of proteinase inhibitors, EDTA (ethylenediamine tetraacetic acid), 1, 10-phenanthroline, ρ-APMSF [(ρ-amidinophenyl) methanesulfonyl fluoride hydrochloride] and E-64 [L-trans-epoxysuccinyl-leucylamido-(4-guanidinobutane)] on tenderization was investigated by using Japanese flounder. Proteinase inhibitor solution was injected into a blood vessel in a caudal portion of live flounder and the firmness of muscle was then evaluated as a shear force value at 0 h and 6 h after death. Metalloproteinase inhibitors, EDTA and 1, 10-phenanthroline, significantly suppressed postmortem tenderization. These findings suggest that metalloproteinases are candidates for proteinases involved in the postmortem tenderization of fish muscles. Although not significantly, ρ-APMSF, a serine proteinase inhibitor, partially suppressed muscle tenderization, which suggests that serine proteinases are also implicated in postmortem tenderization. A cysteine proteinase inhibitor, E-64, showed no effect, suggesting that cysteine proteinases are not involved.

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