Properties of uptake and exchange of L-methionine associated with the compartments of the blastocyst were examined in the mouse embryo. The inner cell mass (ICM) was derived from the 102 h blastocyst by immunosurgical procedures and studied in isolation. The ICM possessed strong exchange and efflux transport character. The Vmax of transport was about one-fifth that of the intact blastocyst, but the Km (660 µM) was about 30 times as high as that of the intact blastocyst. Consequently, at a concentration of about 1OO µM-methionine, the velocity of uptake into the ICM is about one-thirtieth of that into the intact blastocyst. Uptake of amino acid into the intact blastocyst was resolved into cellular and cavity components by mechanically collapsing the blastocyst following uptake of radiolabelled methionine. By this method, it was found that about 70% of the label accumulated by the blastocyst was in the cavity.

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