Effect of pH on the kinetics of transport by SA1 and SA2
| Km(mm) ± SEM | Vmax ± SEM | n | |
|---|---|---|---|
| SA1 | |||
| Na+, pH 7 | 52.2 ± 10.7 | 25.2 ± 12.9 pmol/3 min | 3 |
| Na+, pH 8 | 11.1 ± 4.1 | 33.2 ± 9.8 pmol/3 min | 3 |
| MeAIB, pH 7 | 0.65 ± 0.07 | 2.34 ± 0.28 nmol/3 min | 4 |
| MeAIB, pH 8 | 0.56 ± 0.07 | 3.6 ± 0.44 nmol/3 min | 4 |
| SA2 | |||
| Na+, pH 7 | >140 | >3.1 pmol/3 min | 3 |
| Na+, pH 8 | 146 ± 60 | 8.3 ± 2.3 pmol/3 min | 3 |
| MeAIB, pH 7 | 2.28 ± 0.19 | 1.95 ± 0.71 nmol/3 min | 3 |
| MeAIB, pH 8 | 2.12 ± 0.17 | 4.48 ± 1.34 nmol/3 min | 3 |
The uptake of 3H-MeAIB by transfected HeLa cells was measured in different proportions of NaCl and choline (at 4 μm MeAIB) to determine the Km for Na+ and in different amounts of MeAIB (at 120 mm Na+) to determine theKm for MeAIB. For both SA1 and SA2, theKm for Na+ is much higher at pH 7 than pH 8. The Vmax of SA1 at 4 μmMeAIB and high Na+ does not differ significantly at the two pH, and the Vmax of SA2 under these conditions is difficult to extrapolate. In contrast, the Kmof both SA1 and SA2 for MeAIB does not differ between pH 7 and 8, whereas the Vmax is higher at pH 8.