Effect of fasting and of methionine deficiency on L-methionine, DL-methionine and DL-2-hydroxy-4-methylthiobutanoic acid metabolism in broiler chicks.

Metabolism of L-[1-14C]methionine, DL-[1-14C]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. 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. 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. 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. 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. Methionine-deficient birds had similar levels of 14C in tissue proteins but lower values for protein bound 14C:protein-free 14C. 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.