Temporal response of hepatic threonine dehydrogenase in chickens to the initial consumption of a threonine-imbalanced diet

Amino acid imbalances contribute to higher requirements of amino acids than would occur if the dietary profile of amino acids perfectly matched the re quirements. The mechanisms of imbalances have not been fully elucidated. Be cause threonine dehydrogenase (TDH) activity in liver mitochondria increase s in chicks and rats subjected to threonine imbalance, the current study wa s carried out to determine whether the change in TDH activity occurs rapidl y enough after the consumption of an imbalanced diet to be considered a pos sible primary metabolic response. In a series of experiments, Leghorn chick s were allowed free access to a semipurified basal diet marginally limited in threonine or the same diet containing a mixture of indispensable amino a cids (IAA) lacking threonine to cause a threonine imbalance. In the first e xperiment, dietary supplements of 5.5 and 11.1% IAA were used to determine a level of supplement that would cause a robust response in the specific ac tivity of TDH. Feed intake, body weight gains and efficiency of feed utiliz ation were lower and specific activities of TDH were higher in chicks fed 1 1.1% IAA than in those fed 5.5% IAA. In subsequent experiments, hepatic TDH activities and plasma amino acid profiles of the control and experimental groups were determined at 1.5, 3, 6, 12 and 24 h after the first offering o f the diet containing 11.1% IAA. The specific activities of TDH in chicks f ed the IAA supplement were 40-150% higher (P < 0.05) and plasma threonine c oncentrations were 42-53% lower(P < 0.05) than in chicks fed the basal diet at ail times except 1.5 h. These results indicate that changes in the capa city for threonine degradation via TDH may occur in the liver within a few hours after the consumption of a threonine-imbalanced diet and suggest the possibility that altered TDH activity may contribute to the increased threo nine requirement associated with threonine imbalance.