BINDING OF CATHEPSIN-D TO THE MANNOSE RECEPTOR ON RAT PERITONEAL-MACROPHAGES

Adherent cultures of rat peritoneal macrophages secrete lysozyme and the lysosomal marker enzymes beta-glucuronidase, beta-N-acetylglucosaminidase and acid phosphatase; the levels of secreted lysosomal cathepsin D, however, were found to be insignificant. Incubation of the cells at 4-degrees-C for 15 min with yeast mannan or with 50 mM mannose, methyl alpha-glucopyranoside, or N-acetylglucosamine caused the concentration of cathepsin D in the culture medium to increase 30-40-fold; mannose-6-phosphate had no effect. I-125-labeled cathepsin D was prepared and the binding constant to the macrophage cell surface was determined to be K(D) = 27 nM. The data suggest that cathepsin D binds to the mannose receptor of macrophages and that binding to this receptor is not in equilibrium with the bulk medium.