Table I. External pH influences the dependence of glutamine transport by SN1 on the cation.
| pHo | Km (mM) | Vmax (nmol/min) | n |
|---|---|---|---|
| 8.5 | 28 ± 12 | 5.6 ± 0.7 | 3 |
| 8.0 | 73 ± 7 | 6.1 ± 0.6 | 4 |
| 7.5 | >120 (269 ± 39) | >2 (2.5 ± 0.5) | 4 |
| 7.0 | >120 (171 ± 59) | >2 (3.0 ± 0.7) | 3 |
[3H]glutamine uptake was measured as a function of Li+ concen tration in PS120 cells expressing SN1, and the results fitted to Michaelis–Menten kinetics. The increasing Km (for Li+) with little change in Vmax (for saturating Li+ in 1 mM [3H]glutamine) at progressively lower pH suggests competition between Li+ and H+ for activation of transport by SN1. Since 120 mM was the highest concentration of Li+ used, saturation considerably above this level (at pHo 7.5 in particular) makes the Km and Vmax less certain. Nonetheless, even the results at low pHo fit Michaelis–Menten kinetics well, producing the kinetic values shown in parentheses. Values represent mean ± SEM.