ABSTRACTLysosomal lipases of the dark lateral line muscle of rainbow trout (Salmo gairdneri) had pH optima of 4.2–4.5 and 7.8. The acid lipase was chosen for further study. Presence of Triton X‐100 in the reaction mixture enhanced the activity of the acid lipase. Fasting the fish for 24 and 72 hr prior to sacrifice resulted in higher acid lipase activities. Lysosomal preparations contained 60 of the acid lipase of the dark lateral line muscle. Storage on ice and fast and intermediate rates of freezing did not cause the release of acid lipase from the lysosomes. However, slow freezing and fluctuation of temperatures (−12° to −3.5° for 96 hr) of the frozen fillets resulted m appreciable release of the acid lipase from lysosomes. During frozen storage most of the release of acid lipase from the lysosomal fraction occurred within the first month of storage. Lower storage temperatures decreased the rate of acid lipase release from the lysosomal
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