Abstract
A cathepsin D-like enzyme was isolated from erythrocyte-free Plasmodium lophurae by freezing and thawing or extraction with 0.1% Triton X-100. The enzyme, partially purified by chromatography on Sephadex G-100, had a pH optimum of 3.8 with acid denatured hemoglobin as a substrate, and an estimated molecular weight of ~44,000. The crude enzyme was stable to freezing (-70°C) and storage in the cold (4°C) for up to 10 days, but was rapidly inactivated by incubation at 60°C. Proteolytic enzyme activity was inhibited by pepstatin, p-chloromercuribenzoate, phenylmethane sulfonylfluoride, ferric chloride and lead nitrate, but proteolysis was unaffected by iodoacetamide, sodium tetrathionate, EDTA, leupeptin, TLCK and TPCK. The enzyme was capable of digesting hemoglobin only at low pH, but could degrade protein components of erythrocyte membrane vesicles at neutral or slightly alkaline pH values.
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