VALINE OXIDATION IN THE RAT MEDULLARY THICK ASCENDING LIMB

In the kidney, a branched-chain amino acid transferase (BCAAT) activit y has been localized mainly in the medullary thick ascending limb (MTA L) of the rat.nephron. BCAAT is the first enzyme involved in the metab olic pathway of the three branched-chain amino acids (BCAA): leucine, isoleucine and valine. The present work has been designed to study val ine catabolism. Valine and leucine oxidation in MTAL were compared by measuring the rate of (CO2)-C-14 produced when these substrates were i ncubated as sole substrates at a final concentration of 1 mM. Since gl ucose is also metabolized in MTAL, valine and leucine oxidation were q uantified also in the presence of glucose (1 mM). The results show tha t: (1) valine oxidation was greater than that of leucine (63.0 +/- 4.7 vs 39.7 +/- 5.2 pmol . h-1 . mug-1 protein, respectively; P < 0.001). As previously shown, leucine oxidation was found to be increased in t he presence of glucose whereas glucose oxidation decreased. In contras t, the presence of glucose strongly diminished valine oxidation (19.2 +/- 1.9 vs 63.1 - 4.7 pmol . h-1 . mug 1 protein; P < 0.001) whereas g lucose oxidation was increased in the presence of valine (268.2 +/- 14 .9 vs 229.6 +/- 16.2 pmol . h-1 . mug-1 protein; P < 0.05). We conclud e that in rat MTAL, under near physiological conditions (in the presen ce of glucose, as in vivo), leucine is a preferential respiratory fuel as compared to valine. However, valine supports energetic salt transp ort and facilitates glucose oxidation.