ONTOGENY AND PLASMA-MEMBRANE DOMAIN LOCALIZATION OF AMINO-ACID-TRANSPORT SYSTEM-L IN RAT-LIVER

Na+-independent hepatic transport of branched-chain amino acids occurs via at least two distinct transport processes. System L(1). character ized by micromolar K-m values, predominates in hepatoma and fetal hepa tocytes, whereas System L(2), distinguished by K-m values in the milli molar range and sensitivity to inhibition by N-ethylmaleimide (NEM), p redominates in adult hepatocytes. To determine the plasma-membrane dom ain localization and ontogeny of System L activity in the rat, we prep ared membrane vesicles from the livers of suckling (10 days old) and a dult rats enriched for either basolateral (BLMV) or canalicular (CMV) domains. The initial rate of [H-3]leucine uptake into BLMV and CMV der ived from adult liver was significantly inhibited by the addition of 5 mM NEM; transport into BLMV and CMV derived from 10-day-old rat liver was not affected. Michaelis-Menten kinetic parameters estimated in BL MV derived from adult liver were consistent with System L(2) (K-m = 2. 16+/-0.62 mM, V-max = 781+/-109 pmol/5 s per mg of protein), as were t hose estimated in adult CMV (K-m = 0.83+/-0.21 mM, V-max = 385+/-38 pm ol/5 s per mg of protein). Conversely, kinetic parameters estimated in BLMV derived from livers of suckling rats were consistent with System L(1) (K-m = 0.041+/-0.024 mM, V-max = 8.8+/-1.5 pmol/5 s per mg of pr otein), as were those from CMV of suckling rats (K-m = 0.023+/-0.09 mM , V-max = 28.1+/-2.1 pmol/5 s per mg of protein). We conclude that NEM -inhibitable Na+-independent leucine transport activity consistent wit h System L(2) is present in both BLMV and CMV derived from adult rat l iver, whereas System L(1) predominates in 10-day-old rat liver tissue.