Cathepsin L

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Abstract
1 Cathepsin L was purified from rat liver lysosomes by cell fractionation, osmotic disruption of the lysosomes in the lysosomal mitochondrial pellet, gel filtration of the lysosomal extract and chromatography on CM-Sephadex. 2 Cathepsin L is a thiol proteinase and exists in several multiple forms visible on the disc electropherogram. By polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate, its molecular weight was found to be 23000 – 24000. The isoelectric points of the multiple forms of cathepsin L extended from pH 5.8 – 6.1 ascertained by analytical isoelectric focusing. 3 Using various protein substrates, cathepsin L was found to be the most active endopeptidase from rat liver lysosomes acting at pH 6 – 7. In contrast to cathepsin B1, its capability of hydrolyzing N-substituted derivatives of arginine is low and it does not split esters. 4 Greatest activity is obtained close to pH 5.0 with 70 – 90% of maximal activity at pH 4.0 and pH 6.0 and 30–40% at pH 7.0. 5 The enzyme is strongly inhibited by leupeptin and the chloromethyl ketone of tosyl-lysine. Leupeptin acts as a pseudo-irreversible inhibitor. 6 The enzyme is stable for several months at slightly acid pH values in the presence of thiol compounds in a deep-frozen state.