Distribution of ¹²⁵I-asialofetuin among liver particles separated on colloidal silica gradients

Rat livers were cyclically perfused with ¹²⁵I-asialofetuin, and after various time intervals, mitochondrial-lysosomal fractions were layered on colloidal silica gradients. We observed a time-dependent progression of trichloroacetic acid-precipitable radioactivity from the plasma membrane through a "buoyant" region, which contained lysosomal marker and other subcellular particles, to a dense region of relatively pure lysosomes. The buoyant peak of radioactivity represents an intermediate step in the transfer of asialofetuin from the plasma membrane to lysosomes since a) it appears prior to the release of acid-soluble degradation products and b) it can be dissociated from lysosomal markers.