Distribution in tissue homogenates, and the nature of the linkage of injected proteins to subcellular particles

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Abstract
Intravenously injected labeled proteins were recovered mostly in particulate fractions of rat liver homogenate. Distribution showed changes depending on the time elapsed from the injection. 131I‐albumin undergoes an intraparticulate hydrolysis which shows the highest activity in the gradient fractions associated with the highest level of acid phosphatase. The labeled albumin‐bearing particles separated at 27,000 g × 10 minutes released their radioactive protein at the same rate as acid phosphatase appeared in the medium, under the effect of such agents as distilled water, salts, homogenization, sonication and pH changes. The substitution of sucrose for distilled water or salts showed that the particles behave as an osmotic system as do lysosomes. These experiments prove that secondary lysosomes involved in the hydrolysis of foreign proteins, whose existence was shown by other authors only at the histochemical level, may survive the distrupting action of conventional homogenization and maintain many properties characteristic of primary lysosomes in addition to the ability of hydrolysing “in vitro” the engulfed material.