Unique biochemical and biological features of cathepsin D in rodent lymphoid tissues

Cathepsin D, an enzyme consistently found to be lysosomal in many cells, has an unusual localization in rat thoracic duct lymphocytes (TDL). After fractionation of homogenates of rat TDL, most of the enzyme activity, as measured at pH 3.6 on denatured bovine hemoglobin, is distributed differently from the other lysosomal enzymes. The enzyme also has some unique properties: it is not inhibited by an antiserum inhibitory for rat liver cathepsin D; it exists in two molecular weight forms (approximately 45,000 and approximately 95,000) both of which have a higher specific activity than rat liver cathepsin D, as determined by studies using the irreversible inhibitor, sodium pepstatin; the high molecular weight form converts to the low molecular weight form after treatment with beta-mercaptoethanol without any loss in activity. These enzymes appear to be restricted to rodent lymphoid tissues. Reasons for considering them to be a type of cathepsin D are given in the text.