Macromolecules of vitellogenin were sequestered by Xenopus laevis oocytes 20-50 times (on a molar basis) more rapidly than other proteins tested. Selectivity for vitellogenin did not appear to involve molecular size or charge. The Km for vitellogenin incorporation was at least several orders of magnitude less than that for bovine serum albumin (BSA). At concentrations less than 10 mg-ml-1, BSA did not measurably compete with vitellogenin; a slight, apparent competition observed above a BSA concentration of 10 mg-ml-1 was probably spurious. Above a concentration of 2 mg-ml-1, vitellogenin promoted BSA incorporation by about 40%. These results are consistent with the notion that vitellogenin binds to specific receptor sites on the oocyte membrane and is subsequently internalized by micropinocytosis. Other proteins, such as BSA, which do not compete with vitellogenin are most likely to be incorporated by adventitious engulfment during micropinocytosis.

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