Comparative metabolism of L-methionine, DL-methionine and DL-2-hydroxy 4-methylthiobutanoic acid by broiler chicks.

1. Metabolism, in broiler chicks, of DL-2-hydroxy 4-methylthiobutanoic acid (DL-HMB), DL-methionine and L-methionine was compared in vivo using 14C-labelled tracers. 2. The distribution of L-[1-14C]methionine and DL-[1-14C]HMB in the major body tissues was examined for a period of 120 min after administration. 3. The relative oxidation (14CO2 exhaled), excretion and incorporation into tissue protein of L-[1-14C]methionine, DL-[1-14C]methionine and DL-[1-14C]HMB were measured in fed birds. 4. Tissue distribution of L-[1-14C]methionine and DL-[1-14C]HMB differed during 60-90 min following administration. 5. The production of 14CO2 from each of the tracers was similar but excretion of 14C-labelled material was very different with the greatest excretion from DL-[1-14C]HMB and the least from L-[1-14C]methionine. 6. The incorporation of 14C into tissue proteins varied with the tracer given and the tissue examined. Liver and kidney had equivalent incorporation from each of the tracers while other tissues examined showed lower incorporation from DL-[1-14C]methionine and DL-[1-14C]HMB. 7. The results show that DL-HMB, D-methionine and L-methionine are metabolized differently in vivo and that they are excreted in differing proportions. There is also a difference in the ability of each to act as a precursor for protein synthesis in tissues other than liver.