Yolk-sacs were removed from 17.5-day pregnant rats injected 2–5h previously with 125I-labelled bovine serum albumin. The specific activities of acid phosphatase and acid proteinase, and the specific radioactivities (trichloroacetic acid-insoluble and trichloroacetic acid-soluble) were measured in subcellular fractions prepared by homogenization and differential centrifugation. The conversion of acid-insoluble into acid-soluble radioactivity within cultured tissue was followed and the nature of the liberated products was investigated by gel chromatography. The results are consistent with the protein entering lysosomes and being digested there. The radiolabel was released chiefly as free iodotyrosine.

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