144 SUBCELLULAR PARTICLES 



resembling the microbodies are also found in lysosome-rich fractions (see fig. lo). 

 It would be interesting in this respect to know whether the droplets found in 

 kidney after the injection of hemoglobin contain such granules, since iron de- 

 posits are seen histochemically under those conditions (70). Iron-containing par- 

 ticles (siderosomes) have been described in kidney by Richter (80). 



Brain. In rat brain homogenates, acid phosphatase, acid ribonuclease, acid 

 deoxyribonuclease, cathepsin and ^-glucuronidase are all present in low but sig- 

 nificant concentrations. All five enzymes are largely sedimentable and show struc- 

 ture-linked phenomena of latency and activation, as they do in liver prepara- 

 tions (7). Similar observations have been made on cat brain, which, however, 

 appears to be devoid of acid deoxyribonuclease activity. In neither preparation could 

 the lysosomal enzymes be clearly separated from cytochrome oxidase or glutamic 

 dehydrogenase, presumably owing to the cellular heterogeneity of the material. 

 Very complex diagrams were indeed given by density equilibration experiments. 

 These results make it probable that lysosome-like particles exist in brain tissue, 

 but the exact properties of these particles and their relationship with mitochondria 

 remain to be established. In a few preliminary trials, they appeared to be more 

 resistant to disruptive treatment than the hepatic lysosomes. 



Other Mammalian Tissues. A few preliminary investigations have been made 

 in our laboratory on spleen and thyroid tissues, with results which are not 

 entirely negative. In rat spleen, which is a particularly rich source of the acid 

 hydrolases which characterize lysosomes, Mrs. Deckers-Passau has found free ac- 

 tivities of the order of 60 per cent for acid phosphatase and /3-glucuronidase, and 

 unsedimentable activities of 35—40 per cent for acid phosphatase and cathepsin. 

 Thus these enzymes appear to be partly sedimentable and inactive, though to a 

 smaller extent than in liver. It is interesting that Doyle (31) has located most of 

 the acid phosphatase of spleen in the macrophages and that these cells contain an 

 exceptionally large number of particles resembling the dense peribiliary bodies in 

 several respects (including the ferritin-like granules) and described by Palade (72) 

 under the name of residual bodies. 



In a number of experiments on beef thyroid, Neil was able to isolate particulate 

 fractions containing the lysosomal enzymes in partially inactive form. The yields 

 were low and measurements made on the homogenates indicated that only 20— 

 30 per cent of the activities were present in bound form. Thyroid tissue is ex- 

 tremely difficult to homogenize and it is possible that a greater percentage of the 

 enzymes are particle-bound in the intact cells. 



It should finally be recalled that particles having some morphological features 

 in common with the dense peribiliary bodies or the kidney droplets have been 

 described in several tissues. To the siderosomes (liver, kidney) of Richter (80) 

 and the residual bodies (macrophages) of Palade (72), which have already been 

 mentioned, one may add the dense bodies of mesothelial cells (69), the osmio- 



