Mouse L-cells that contain the cation-independent (CI) mannose 6-phosphate (Man 6-P)/insulin-like growth factor (IGF) II receptor endocytose acid hydrolases and deliver these enzymes to lysosomes. The postendocytic loss of the Man 6-P recognition marker from the cell-associated acid hydrolases was assessed by CI-Man 6-P receptor affinity chromatography. 125I-labeled acid hydrolases internalized by L-cells grown at high density were delivered to lysosomes but were not dephosphorylated. In contrast, the same 125I-labeled hydrolases internalized by L-cells maintained at low density were delivered to lysosomes and were extensively dephosphorylated. The dephosphorylation at low density required 5 h for completion suggesting that the phosphatase responsible for the dephosphorylation is located within the lysosomal compartment. Transition from the high to low density state was rapid and was not inhibited by cycloheximide. Medium substitution experiments indicated that serum factors were necessary to maintain the L-cells in the dephosphorylation-competent (low density) state, and that serum-free conditions led to a dephosphorylation-incompetent (high density) state. Addition of IGF II to cells in serum-free medium allowed acid hydrolases subsequently introduced by endocytosis to be dephosphorylated. The results indicate that the removal of the Man 6-P recognition marker from endocytosed acid hydrolases is regulated by serum factors in the growth medium, including IGF II.

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