Leukocytic proteases and the immunologic release of lysosomal enzymes.

Abstract

LySoSOMES WERE I-NTRODUCED into the field of inflammation and connective tissue injury as a result of studies conducted at New York Univ-ersity and at the Strangewavs Research Laboratory in Cambridge.1 Shortly after it became clear that vitamin A acted to cause degradation of cartilage matrix both in vitro and in vivo by the release of proteases from cartilage lvsosomes, it was hypothesized that lvsosomal enzvme release in various forms of acute tissue injury could accouint for hydrolvsis of connective tissue macromolecules.5 Further studies in both laboratories have suggested the plausibility of this hypothesis. Thus, lvsosomal proteases present in cartilage and in leukocvtes hav-e been shown capable of degrading cartilage matrix, isolated proteoglvcans and simple chemical substrates. Leukocv-te lvsosomes of the rabbit contain at least two sorts of proteases capable of cleaving isolated cartilage proteoglvcans: an enzmne or enz-mes active at near neutral pH, and one enzyme active in the acid pH range.7When purified, high molecular-weight (10') proteoglvcan of bovine nasal cartilage (PP-L) was exposed to extracts of leukoc-te Ivsosomes (but not to other subcellular fractions), this substrate was readilv cleaved, releasing polvanions of lower molecular weight which resembled those released bv trvpsin. This uronic acid-containing, readilv diffusable material was nondialyzable and precipitated readilv with hexamine cobaltic chloride. Thus, in vitro as in vivo, when proteoglycans are exposed to excess vitamin A, they7 could be degraded bv Iysosomal hydrolases to release anionic polysaccharides; degradation proceeded as readily at neutral as at acidic pH. Since it became clear that proteolytic activity in the acid pH range could be attributed to the cathepsin D in rabbit leukocyte granules, the identification of a neutral protease capable of cleaving extracellular materials became of considerable interest.