Protoelytic Activity in North American Squid and Its Relation to Quality

Abstract

A study was made of autolytic proteolysis in several species of squid (Illex illecebrosus, Loligo pealei, and Loligo opalescens) caught off the coast of North America. Activity was measured as the increase in TCA soluble, Folin reagent-positive material over time of a crude salt extract of mantle muscle; specific activity was defined as μg tyrosine/20h/mg protein at 25 °C. With respect to pH, optima were observed at 3.0, 5.8 and 6.6 for Illex and activity was seen over a pH range of 2.6 to 7.4, indicating a broad spectrum of both acid and alkaline proteases. Specific activities in the order of 5–10 were obtained, about 10–20 times higher than for flounder or hake, indicating the presence of powerful proteases in squid muscle. Activities in frozen tissue were inversely relative to quality opinions for the three species (Illex <L. peali < L. opalescens). Quantifying preformed TCA soluble material is suggested as a method for assessing quality in fresh and frozen squid. The presence of streptomyocin and toluene decreased specific activity by about 30% at pH 6.6 in L. pealei but not Illex, indicating the possible role of bacterial proteases in this species. The use of PMSF was effective in reducing L. pealei activity while aprotinin inhibited Illex proteolysis. Electrophoretic separation of the proteins in the crude extract and DSC of frozen muscle both identified actin as being the component least susceptible to breakdown; increasing the duration of frozen storage produced more extensive degradation. Soaking strips of squid mantle muscle in crude extract significantly (P ≤ .05) reduced the force required to shear these samples relative to a control. These results demonstrate an extremely active family of proteases in squid muscle, the presence of which seems responsible for the rapid deterioration of quality in post-mortem tissue and which also are of importance during frozen storage.