Inactivation of phagocytosis-stimulating activity of tuftsin by polymorphonuclear neutrophils. A possible role of leucine aminopeptidase as an ecto-enzyme.

The subcellular localization of the tuftsin-inactivating activity was studied using guinea-pig polymorphonuclear neutrophils and the following results were obtained. 1. The tuftsin-inactivating activity was present in the membrane fraction but not in the cytosol and the granular fractions. 2. Intact neutrophils inactivated tuftsin rapidly. However, when neutrophils were modified chemically by a poorly permeant reagent, diazotized sulfanilic acid, the tuftsin-inactivating activity decreased significantly without any inhibition of marker enzymes of cytosol, microsome, granules and mitochondria, suggesting that the tuftsin-inactivating activity is located on the plasma membrane as an ecto-enzyme. 3. When neutrophils were modified by diazotized sulfanilic acid at different concentrations, the tuftsin-inactivating activity of neutrophils was inhibited in proportion to the degree of inhibition of the activity of leucine aminopeptidase, an ecto-enzyme. 4, Hydrolysis of L-leucyl-beta-naphthylamide, a synthetic substrate of leucine aminopeptidase, was inhibited competitively by tuftsin. 5. Treatment of neutrophils with serine protease inhibitors affected neither tuftsin-inactivating nor leucine aminopeptidase activity at all, indicating no involvement of serine proteases, which is said to be located on the cell surface membrane, in the tuftsin-inactivation activity of neutrophils. The possibility was deduced from the above results that leucine aminopeptidase may act as a tuftsin-inactivating enzyme.