ONTOGENY OF L-ALANINE UPTAKE IN PLASMA-MEMBRANE VESICLES FROM RAT-LIVER

Alanine uptake into liver plasma membrane vesicles was studied at different stages of postnatal rat development. Before weaning, alanine hepatic uptake showed lower values for the global K-M than after weaning (0.34, 0.77, 1.45, and 1.61 mM for 1-, 15-, and 28-d-old and adult rats, respectively). Alanine uptake capacity increased progressively until reaching maximum values in the adult state (values for V-max: 0.078, 0.199, 0.317, and 0.613 nmo1 alanine/mg protein/3 s for 1-, 15-, and 28-d-old and adult rats, respectively). These results seem to point to a prevalence of a high affinity, low capacity alanine transport component (traditionally assumed to be attributable to system A) in newborn and suckling rats, in agreement with our previous results on isolated hepatocytes (Martinez-Mas JV, Casado J, Felipe A, Marin JJG, Pastor-Anglada M: Biochem J 293: 819-824, 1993). The suckling-weaning developmental transition seems to play a role in establishing the pattern of adult hepatic alanine transport characterized by a higher capacity but a lower affinity (because most alanine is taken up by system ASC) inasmuch as K-M values show a 100% increase after weaning, although V-max values continue to increase steadily until the adult age.