10 acid hydrolases known to be lysosomal in other tissues were detected in rat thoracic duct lymphocytes (TDL). Except for cathepsin D, all these enzymes occurred in lower levels in TDL than in rat spleen.

Fractionation by means of differential centrifugation revealed two types of distribution patterns. Most of the total cathepsin D activity appeared in a high-speed supernatant (S), whereas the other acid hydrolases associated mainly with a high-speed pellet (P). Further studies by isopycnic centrifugation in a sucrose density gradient showed that all the enzymes, including a small portion of cathepsin D, sedimented around a modal density of 1.18. In contrast, most cathepsin D activity was recovered in a soluble, unsedimentable form. These results suggest the presence of two populations of lysosomes. Because the possibility could be excluded that other cell types contaminated the lymphocyte preparations, it is concluded that both lysosomal populations arise from TDL.

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