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-[l-14C]methionine, DL-[l-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.
1. The rate of elimination of administered Nτ-[14CH3]methyl histidine was used to assess the validity of Nτ-methyl histidine excretion as an index of muscle protein breakdown in poultry.2. Broiler chicks (2–3 and 4–5 weeks old), laying hens, adult quail (Coturnix coturnix japonica), adult cockerels and turkey poults (2–4 weeks old) were tested.3. All except the turkey poults showed quantitative recoveries of Nτ-[14CH3]methyl histidine within 1 week.4. Turkeys showed a different pattern of Nτ-[14CH3]methyl histidine output; the mean total recovery after 14 d was less than 50% of the injected dose. The majority of the label remaining after this time was found in breast muscle.5. All birds tested excreted Nτ-methyl histidine unchanged, although a small amount sometimes appeared as another metabolite.6. No significant oxidation of Nτ-[14CH3]methyl histidine by broiler chicks, turkey poults or adult quail was found.7. The results show that excretion of Nτ-methyl histidine is a useful measure of muscle protein breakdown in the domestic fowl and quail but not in turkeys.
1. Metabolism of L-[1-14C]methionine, DL-[l-14C]methionine and DL-[ 1-14C]2-hydroxy-4-methylthiobutanoic acid (DL-HMB) by broiler chicks which had been fasted overnight or given a methionine-deficient diet was compared with fed (control) birds.2. The excretion of 14C-labelled material, total 14CO2 exhaled, 14C incorporation into tissue proteins and the 14C-labelled material in perchloric-acid-soluble tissue fractions were measured 6 h after injection of the 14C-labelled materials.3. The incorporation of 14C into tissue proteins and the relative rates of conversion of D-methionine and DL-HMB to L-methionine in tissues under different nutritional regimens were compared using protein-bound 14C:protein-free 14C values.4. Fasted birds exhaled more 14CO2 than control birds but excreted less 14C, while methionine-deficient birds behaved very similarly to the control animals in these respects.5. Fasted birds incorporated much less 14C into proteins of tissues other than liver and kidney from all three labelled tracers. The values for protein-bound 14C: protein-free 14C were lower in all tissues.6. Methionine-deficient birds had similar levels of 14C in tissue proteins but lower values for protein bound 14C: protein-free 14C.7. Examination of the values for protein-bound 14C:protein-free 14C suggest that brain and probably liver tissues from fasted and methionine-deficient birds showed improved rates of conversion of D-methionine and DL-HMB to L-methionine compared with control animals.
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