Abstract
An enzyme present in rat liver lysosomes catalyzes the conversion of neutral rabbit liver fructose 1,6-bisphosphatase (Fru-P 2ase, EC 3.1.3.11) to a form having maximum activity at pH 9.2. The converting enzyme is partly released when lysosomes are subjected to a single freeze-thaw cycle, but a significant fraction tends to remain with the lysosomal membrane fraction even after repeated freezing and thawing. After repeated freezing and thawing hexosaminidase and cathepsin D are also partly membrane-bound, but cathepsins A, B, and C are completely solubilized. The membrane-bound enzymes, unlike those in intact lysosomes, are not cryptic. The converting enzyme activity is inactivated by phenylmethanesulfonyl fluoride, and is almost completely inactive after exposure to iodoacetic acid or tosylamido-2-phenylethyl and N-α-tosyl lysyl chloromethyl ketones. Unlike cathepsin B, it is not inhibited by leupeptin. Converting enzyme is unstable above pH 6.5, and this property also serves to distinguish it from cathepsins B and D. The results suggest that the converting enzyme is not identical to any of the well-characterized cathepsins.
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