Novel delivery of antioxidant enzyme catalase to alveolar macrophages by Fc receptor-mediated endocytosis.

Excessive production of reactive oxygen species by alveolar macrophages (AMs) in response to inhaled toxic substances is a major cause of oxidative lung injury. Therapeutic approaches designed to protect the lungs from oxidative injury by administering native antioxidant enzymes such as catalase and superoxide dismutase have been suggested. However, problems associated with poor penetration of these enzymes to the intracellular target sites have limited their effective use. The present study reports a drug targeting method based on receptor-mediated endocytosis of the antioxidant enzyme catalase to the AMs. This method employs molecular conjugate consisting of a cognate moiety, in this case IgG which recognizes the macrophage Fc receptor, covalently linked to the enzyme catalase via the reversible disulfide linkage. The uptake efficiency of the enzyme conjugate and its protection against oxidative injury were evaluated microfluorometrically using the intracellular oxidative probe dichlorodihydrofluorescein BSA: anti BSA antibody complex (DCHF-IC), and the cell viability indicator propidium iodide. The DCHF-IC-stimulated macrophages exhibited a dose- and time-dependent increase in intracellular fluorescence with a half maximal response dose of approximately 120 micrograms/ml. Free catalase (50-500 U/ml) failed to inhibit the DCHF-IC-induced oxidative burst and had only a marginal protective effect on AM injury. In contrast, the catalase-IgG conjugate (50-500 U/ml) strongly inhibited both the DCHF-IC-induced oxidation and injury in a dose-dependent manner. Effective inhibition was shown to require both the antioxidant catalase moiety ant the cognate moiety for the cell surface receptor. Specific internalization of the conjugate through the Fc receptor was also investigated by competitive inhibition using free IgG.(ABSTRACT TRUNCATED AT 250 WORDS)