Hydrolysis of an exogenous 125I-labelled protein by rat yolk sacs. Evidence for intracellular degradation within lysosomes.

When added to the serum-free medium in which 17.5-day rat yolk sacs were incubated, formaldehyde-denatured 125I-labelled bovine serum albumin was rapidly degraded. More than 80% of the radiolabelled digestion products appearing in the incubation medium consisted of [125I]iodo-L-tyrosine; larger digestion products were found only in association with the yolk-sac tissue. In the early stages of an incubation, low-molecular-weight digestion products began to appear in the incubation medium only after they could be detected within the tissue, and progressive association of trichloroacetic acid-insoluble radioactivity with the tissue preceded both these events. None of the observed proteolysis could be attributed to proteinases released into the incubation medium. Tissue-associated acid-insoluble radioactivity showed a lysosomal distribution on sub-cellular fractionation, and cell-free homogenates of yolk sacs degraded albumin only at acid pH values. Progressively decreasing the rat of pinosome formation (either by progressively lowering the incubation temperature or by the use of increasing concentrations of the metabolic inhibitor rotenone) caused a corresponding decrease in the rate of degradation of albumin. These findings indicate that, in vitro, formaldehyde-denatured 125I-labelled bovine serum albumin is digested by rat yolk sacs exclusively intracellularly, within lysosomes.