Lysosomes in heart tissue

Abstract

1. 1. The acid hydrolase (β-glucuronidase, EC 3.2.1.31; cathepsin; β-galactosidase, EC 3.2.1.23; acid ribonuclease, EC 2.7.7.17 and acid deoxyribonuclease, EC 3.1.4.6) of beef-heart muscle are associated with cytoplasmic particles and are poorly reactive towards external substrates; however, their activity is enhanced or even fully displayed by injuring the particles with a variety of treatments (osmotic shock, prolonged homogenization, alternate freezing and thawing, addition of Triton X-100). 2. 2. By gradually increasing the concentration of Triton X-100 in the enzyme assays, the acid hydrolases are gradually liberated from the particles, although not in equal proportions. 3. 3. When particles in suspension are submitted to the action of various “protein” or “lipid” reagents, it appears that both types of reagent are able to increase the availability of the hydrolases for their substrates, but that a high solubilization of the enzymes is achieved only by attacking the membrane phospholipids. 4. 4. Preincubation either in hypo- or hypertonic sucrose solutions produces an increase in the availability of the hydrolases; under the same conditions, a partial solubilization, albeit to a different extent for each enzyme, is also observed. 5. 5. Each of the enzymes considered displays a peculiar intracellular distribution pattern, but none of them is strictly the same as that of succinoxidase. By isopicnic centrifugation of a mitochondrial fraction it is possible to isolate a particulate fraction ( d = 1.174) showing a high concentration of latent hydrolytic enzymes and a comparatively low concentration of cytochrome c oxidase. 6. 6. The acid hydrolases of beef heart do not seem, therefore, to belong to mitochondria but to exhibit the main features of lysosomal enzymes. A comparison between the properties of beef-heart and rat-liver lysosomes is also made and the heterogeneity of the former is discussed.