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1. Eight distinct acid-hydrolase activities present in cytoplasmic extracts from bone tissue occur in latent form to the extent of 50-70% of their total activity, depending on the enzyme. 2. This latency can be decreased or suppressed by exposure to Triton X-100 or to media of low osmotic pressure, by treatment in the Waring Blendor, and by freezing and thawing, but not by increasing the substrate concentration in the assay medium up to 10-fold the Michaelis constant of the enzymes. 3. Latency is the property of the particle-bound enzymes, and treatments that suppress latency simultaneously cause solubilization of the enzymes. Most enzymes show an excess of free over soluble activity; the magnitude of this excess seems to depend largely on the nature of the enzyme, and sometimes also on the kind of treatment suffered by the preparations; it is attributed mainly to adsorption artifacts. 4. In preparations subjected to graded activating treatments, seven of the eight acid hydrolases studied are released in closely parallel fashion, suggesting that they are associated with particles possessing similar properties. Acid phenylphosphatase is released less readily than the other enzymes by Triton X-100 and by exposure to media of low osmotic pressure. 5. It is concluded from these and previous published fractionation experiments that, with the possible exception of part of the acid-phenylphosphatase activity, the eight acid hydrolases studied belong to lysosome-like particles. Bone lysosomes exhibit a relatively high degree of biochemical and physical heterogeneity. Their possible functions are discussed. Part of the acid-phenylphosphatase activity could be linked to another group of particles. 6. Catalase is also partly (30%) latent in cytoplasmic extracts of bone. Latent catalase can be released by some of the treatments that suppress the latency of the lysosomal enzymes, but differs from the latter by a greater resistance to Triton X-100, and, especially, by a complete insensitivity to exposure to media of low osmotic pressure. It is concluded from these results that the catalase-containing particles are probably different from lysosomes, as they are in liver. 7. Cytochrome oxidase, which is presumably associated with the mitochondria, and alkaline phenylphosphatase, an enzyme occurring predominantly in the microsomal fraction, exhibited no latency under the conditions of the present experiments.

Studies on bone enzymes. The activation and release of latent acid hydrolases and catalase in bone-tissue homogenates