The levels of cathepsins D and E in various rat tissues during development were determined with the sensitive assay method we have developed. The level of cathepsin D increased gradually in each tissue during fetal development suggesting the gradual maturation of the lysosomal system in a cell. The level of cathepsin E differed significantly between tissues at various developmental stages. The level in liver increased rapidly from 13-day-gestation fetal stage and decreased gradually at later fetal stages. The level in other tissues such as stomach and spleen began to increase at later fetal stages or the infant stage. Cathepsin E was found in fetal hepatocytes and its gene was hypomethylated when the expression of the gene was elevated. The enzyme was found to be present mainly as a proform suggesting that, after working, an active form is rapidly inactivated.
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