CHARACTERIZATION OF THE ROUTES OF METHIONINE TRANSPORT IN MOUSE MAMMARY-GLANDS

The sites of methionine uptake by mammary glands from late pregnant an d lactating mice were studied in vitro. Using the specific A system in hibitor, N-(methylamino) isobutyric acid (MeAIB) and the specific L sy stem inhibitor, 2-amino-bicyclo (2.2.1) heptane 2-carboxylic acid (BCH ), we have defined four modes of methionine entry into these tissues. (i) A sodium-dependent A system with a Vmax of 13.4 and 18.8 n mol/g c ells/min in pregnant and lactating mice, respectively, This mode of en try was completely inhibited by MeAIB and its Km value was similar (0. 45 mM) in both groups. (ii) A sodium-dependent mode with a Vmax of 6.7 and 12.4 n mol/g cells/min and a Km of 0.24 and 0.46 mM in pregnant a nd lactating mice, respectively, This mode of entry was insensitive to inhibition by MeAIB, and was similar to the ASC (alanine, serine, cys teine) system in other tissues. (iii) A sodium-independent L system wi th a Vmax of 13.8 and 30.0 n mol/g cells/min and a Km of 0.27 and 0.46 mM in pregnant and lactating mice, respectively. This mode of entry w as completely inhibited by BCH. (iv) A sodium-independent nonspecific entry amounting to 25 per cent of the total entry at O.1 mM external m ethionine which was not inhibited by high concentration of BCH. The re sults of our studies show an increase in the number of active carriers of the A, ASC and L systems of methionine uptake in mammary glands of mouse during lactation.