Lysosomal Enzyme Precursors in Human Fibroblasts. Activation of Cathepsin D Precursor in vitro and Activity of beta-Hexosaminidase A Precursor towards Ganglioside GM2

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Abstract
Precursors of cathepsin D and β-hexosaminidase were isolated from secretions of human fibroblasts and their activity was studied with natural substrates. The immunoprecipitated precursor of cathepsin D, Mr 53000, was inactive with radioactive hemoglobin as substrate. At pH 3.8–4.2 an activation of the precursor took place, which was correlated by a reduction in size to Mr 51 500. The observed cleavage of cathepsin D precursor in vitro resembles the autocatalytic activation of pepsinogen. The precursor of β-hexosaminidase A is able to cleave the natural substrate GM2 ganglioside. This reaction, like that of the mature enzyme, depends on the presence of a protein activator, which interacts with the substrate and the enzyme.