Purification and characterisation of cathepsin D from the digestive gland of the pelagic squid Todarodes sagittatus

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Abstract
A proteinase from the digestive gland of squid (Todarodes sagittatus) was purified from a neutral extract by ammonium sulphate fractionation, S‐Sepharose chromatography and gel filtration on Sephadex G‐75. Inhibition by pepstatin showed that the enzyme is an aspartic proteinase. The enzyme is fairly stable in the pH range 2.5‐7, and optimum pH for haemoglobin digestion is 3.7. Combined results from gel filtration and sodium dodecyl sulphate electrophoresis indicate a molecular weight of 38 kDa, and that the enzyme consists of two different subunits with molecular weights approximately 10 and 28 kDa. Analytical electrofocus‐ing showed an enzyme pi of 8.5. As judged from the physical properties and the amino acid composition, it is concluded that the enzyme is a cathepsin D. This enzyme probably counts for the major part of proteinase activity in the digestive gland.