Unsuitability of Leucine Naphthylamide for the Histochemical Demonstration of Lysosomal Proteolytic Activity

Abstract

CHAYEN and his co-workers¹ have recently reported that a lysosomal enzyme shows decreased latency in synovia from rheumatoid joints as compared with normal controls. The enzyme in question was demonstrated histochemically with leucine 2-naphthylamide (the correct name of this compound appears to be leucine naphthylamide or leucine 2-naphthylamide², not, for example, leucyl-β-naphthylamide; leucine naphthylamidase would be the correct trivial name for a hydrolase acting on this substrate, according to Enzyme Commission rules³) as substrate at pH. 6.5 (ref. 4). The statement was made, however, that this staining reaction demonstrates lysosomal proteolytic activity, which is not consistent with results obtained recently in this laboratory. The point is of particular importance, because a lysosomal proteinase may participate in the degradation of articular cartilage matrix in arthritis^5,6. Sylvén⁷ has suggested that leucine naphthylamidase activity, demonstrated histochemically, may correspond to cathepsin B at pH 5.5, but not at the higher pH preferred by Chayen et al.