Histochemistry of some Lysosomal “Proteolytic” Enzymes of Human Rheumatoid Synovia

Abstract

ONE subcellular manifestation of rheumatoid arthritis is that the membranes of the lysosomes in the synovial lining cells are virtually functionless; by contrast, in non-rheumatoid synovia—even after recent mechanical trauma—the lysosomal membranes in the lining cells are functional in that they retard entry of the substrates used for the demonstration of the intra-lysosomal enzymes¹. Barrett and Poole², while not disputing this conclusion, questioned the precise nature of the lysosomal enzyme used in the study, which was visualized and measured microdensitometrically on the basis of the hydrolysis of leucine-2-naphthylamide, the liberated naphthylamine being trapped by ‘Fast Blue B’ (tetra-azotized dianisidine) to form an insoluble dye. By precise biochemical analysis, they demonstrated that human synovial tissue, taken from rheumatoid joints, contained cathepsins B and D as well as aminopeptidase activity. They showed that cathepsin D cannot hydrolyse leucine-2-naphthylamide and quoted unpublished data from other workers which suggested that cathepsin B could not hydrolyse this substrate. From this evidence they concluded that the particulate enzyme activity which was demonstrated by this substrate was a lysosomal aminopeptidase.