N-15 CONSERVATION IN THE METABOLIC CONVERSION OF ISOLEUCINE TO ALLOISOLEUCINE IN THE RAT

N-15,C-13(6)-L-Isoleucine was given by stomach tube to a pair of rats and the urine excreted over the following 6 h period was collected. Th e urinary amino acid fraction showed that the majority of the L-allois oleucine produced from the labeled isoleucine was formed with the N-15 label intact. This fails to support the commonly held supposition tha t L-isoleucine and L-alloisoleucine interconversion occurs through the reversible enolization of the 2-keto-3-methylvaleric acids formed by their transamination. In contrast to N-15 label conservation in L-allo isoleucine, the majority of the N-15 in the administered L-isoleucine underwent exchange with N-14.