**2.5 Measurement of OAT activity**

OAT activity was measured in whole kidneys. Samples were mixed at 4°C with a Turrax homogenizer in a buffer (0.33 M sucrose, 5 mM HEPES, 1 mM EGTA, 1 mM DTT, and 0.5% Triton X 100). Homogenates were centrifuged 21,000 x *g* for 10 min at 4°C. The protein concentration was determined on the supernatant using the Bradford protein assay. OAT activity was determined using the enzyme assay previously described (Herzfeld & Knox, 1968; Peraino & Pitot, 1963). Briefly, the supernatant was incubated with a buffer composed of 75 mM potassium phosphate pH 8.0, 20 mM L-ornithine, 0.45 mM pyridoxal phosphate, 5 mM α-aminobenzaldehyde, and 3.75 mM α-keto-glutarate for 15 or 30 min at 37°C. Blanks did not contain α-keto-glutarate. The reaction was stopped by adding trichloroacetic acid 40%. Samples were centrifuged 21,000 x *g* for 3 min at 4°C and absorbance was measured on the clear supernatant at 440 nm on a Hitachi U-1100 spectophotometer (Meylan, France). Duplicate or triplicate samples and blanks were performed for all experiments.
