SOLUBLE AND MEMBRANE‐ASSOCIATED FORMS OF ACID PHOSPHATASE ASSOCIATED WITH THE LYSOSOMAL FRACTION OF RAT LIVER

As initially formulated (de Duve, 1959), the lysosome concept held that the hydrolytic enzymes associated with lysosomes are contained within the particles and that constraint from hydrolytic action is provided by the particle membrane, which acts as a barrier to enzyme-substrate interaction. Rupture of the lysosomal membrane, within this context, should result in the simultaneous release of all internal enzymes in a soluble and fully active form, rendering them accessible to substrates of the cytoplasm. A disparity between the total acid phosphatase activity of the intact lysosomal fraction and the unsedimentable activity of the released enzyme was noted, however, as early as 1951 (Berthet et al., 1951). This discrepancy was accounted for on the basis of secondary adsorption of enzyme to lysosomal structure (Berthet et al., 1951; Appelmans & de Duve, 1958; de Duve, 1959). Recently, morphological and biochemical data have become available suggesting that this view of the association between the lysosomal hydrolases and lysosoma1 structure may have to be revised. Electron microscopic observations show that the lysosome may possess a complex internal structure (Novikoff et al., 1956; Van Lancker & Sempoux, 1958; Daems & Van Rijssel, 1961; Koenig, 1962), and biochemical determinations suggest that lysosomes may possess structure-bound enzymes that are not released by simple physical disruption. An inability to release all acid pliosphatase from rat liver lysosomes by physical disruption was reported by Shibko and Tappel (1963). Similarly, Allen and Gockerman (1964) were unable to effect total release of acid phosphatase from rat liver lysosomes by physical disruption, but did achieve total solubilization in the presence of Triton X-100. These authors further demonstrated the presnce of two electrophoretically distinct acid phosphatases in their mitochondrial-lysosomal fraction. These forms exhibited differential release properties, one being released by physical disruption of the particles and the other only after treatment of the fraction with Triton X-100. In addition to these instances, other lysosomal enzymes reported to be bound in part to lysosomal structure include aryl sulfatase A (Ugazio & Pani, 1963), an esterase (Shibko & Tappel, 1964), p-acetyl glucosaminidase (Weissman et al., 1967), and p-glucosidase (Beck & Tappel, 1968). This paper deals with a more definitive localization of a bound form of acid phosphatase to the lysosomal fraction of rat liver, with some physical and biochemical properties of the bound and soluble acid phosphatase of this fraction, and with the behavior of these enzymes in the lysosomes of regenerating rat liver.