Specific anti-(rabbit cathepsin D) serum, previously shown to inhibit cathepsin D, arrested the intracellular digestion of sheep IgG and radiochemically labeled hemoglobin and proteoglycan in rabbit alveolar macrophages. In the presence of antiserum, cells remained viable, but became very vacuolated. Both sheep IgG and hemoglobin were demonstrated immunocytochemically in vacuoles most of which could also be shown to contain cathepsin D. When the antiserum was removed, cells regained their normal morphology, and digestion of endocytosed proteins returned to normal. These results indicate that cathepsin D can be inhibited within lysosomes of viable cells, in which it plays a major role in the intracellular digestion of certain proteins.

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