Mediated Na(+)-independent transport of L-glutamate and L-cystine in 1- and 2-cell mouse conceptuses

L-Glutamate and L-cystine appeared to compete for transport via a mediated Na(+)-independent transport process in 1- and 2-cell conceptuses. Not only did these substances competitively inhibit each others' uptake by conceptuses, but their Ki values for inhibition were about equal to their Km values for transport in 1-cell conceptuses. Moreover, the transport process interacted strongly with L-amino acids that had 3-6 atoms in a chain between their negatively charged groups, whereas it interacted weakly or not at all with amino acids that did not have these characteristics or that were N-methylated. Transport of anionic amino acids was not altered greatly by pH in the range 4.5-8.0, but transport of L-cystine was much faster at higher pH values. The slower cystine transport at lower pH values was due primarily to protonation of its second amino group. A small increase in the degree of deprotonation of cystine's carboxyl groups also probably contributed slightly to its faster transport at higher pH values. By all of these criteria, the transport process in conceptuses appears to be a form of amino acid transport system xc-. System xc- activity was several-fold higher in 1- than in 2-cell conceptuses. Similarly, L-glutamate uptake by unfertilized eggs was relatively rapid, whereas it was much slower in immature, fully-grown oocytes. System xc- activity in 1-cell conceptuses also appeared to increase in response to the oxidative stress of culture, whereas no such increase was observed for 2-cell conceptuses. We suggest that this transient increase in the activity of system xc- activity during development of 2-cell conceptuses from immature, fully-grown oocytes could help protect unfertilized and fertilized eggs from oxidative stresses in situ.