Effects of Dietary Threonine Levels on the Tissue Amino Acid Concentration and Threonine Metabolic Enzyme Activity in Laying Hens

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  • 産卵鶏の組織中の遊離アミノ酸濃度とトレオニン代謝酵素活性に対する飼料中のトレオニン含量の影響
  • Effects of Dietary Threonine Levels on

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Abstract

In order to clarify the metabolism of threonine (Thr) in laying hens, the effects of dietary Thr levels on tissue amino acid (AA) concentrations and Thr metabolic enzyme activities; Thr aldolase (Thr to glycine (Gly)), Thr dehydratase (Thr to α-ketobutyric acid), Thr dehydrogenase, serine (Ser) hydroxymethyltransferase, Ser dehydratase, were studied. Laying hens at 38 weeks of age were fed diets with five graded Thr, from deficiency to excess, 0.32 to 0.67%, for 28 days. After then, the concentrations of the three AA, Thr, Ser and Gly, and the enzyme activities of Thr and Ser degradation and Gly formation in homogenates of liver, kidney and duodenum were determined. When the dietary Thr was increased from 0.32 to 0.42%, the concentrations of Thr in the three tissues remained constant, and then increased linearly with increasing dietary Thr levels. The break point of both lines of Thr concentrations which was estimated to be 0.46%, agreed with the requirement determined from the performance and plasma concentration of Thr. The concentrations of Ser remained relatively constant until the dietary Thr was less than the requirement level and then decreased, and those of Gly remained constant in all the dietary Thr levels in the three tissues. However, the concentration of each AA differed significantly among tissues. The activities of all enzymes investigated in the liver were not affected by the changes of dietary Thr levels. In the kidney, only the activity for the formation of Gly from Thr was enhanced by increasing the dietary Thr levels. In the duodenum, all enzyme activities except for Gly to Ser responded to increase of dietary Thr levels. The activities of all enzymes in the duodenum were two or three times higher than those in other tissues. From these results, the increment of conversion from Ser to Gly might be one of the reasons why the tissue Ser concentration decreased when dietary Thr levels were higher than the Thr requirement levels.

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