Effect of the ribozyme on the internalization of 125I-labeled IGF-II (A). Ribozyme-treated or control cells were incubated in serum-free F-10 culture medium containing 125I-labeled IGF-II with or without excess unlabeled IGF-II. Following the incubation, cell-associated radioactivity was determined by a γ counter. Determination of binding (B) and endocytosis (C) of M6P-containing protein in cells. The M6P/bearing enzyme β-glucuronidase was used as a probe. The M6P-inhibitable binding was measured by incubating saponin-peameabilized cells with β-glucuronidase in the presence or absence of M6P. The M6P-inhibitable uptake was determined by incubating cells with medium containing β-glucuronidase in the presence or absence of M6P. Following incubation, cells were washed extensively, and cell-associated β-glucuronidase was measured fluorometrically. The data are average ± SE of at least four independent experiments (*, p < 0.05 versus control).