The temporal response of chicken hepatic threonine dehydrogenase activity and plasma threonine concentration to threonine imbalance, and characterization of chicken threonine dehydrogenase

Experiments were conducted to determine whether the change in threonine dehydrogenase (TDH) activity occurs rapidly enough after consumption of an imbalanced diet to be considered a possible primary metabolic response. Chicks were given free access to a basal diet marginally limiting in threonine or the same diet but supplemented with a mixture of indispensable amino acids (IAA). Body weight gains, feed consumption, and feed utilization were lower and TDH activities were higher in chicks receiving 11.1% IAA than those receiving 5.5% IAA. Subsequently, TDH activities and plasma amino acid profiles of control and experimental groups were determined at 1.5, 3, 6, 12, and 24 h after offering of diets. Increased TDH activity and decreased plasma threonine concentration were detected in the experimental group at 3 h. The results indicate that metabolic changes in threonine metabolism occur within 3 h after consumption of a threonine-imbalanced diet and suggest the possibility that altered TDH-activity may contribute to the increased threonine requirement associated with threonine imbalance. A diet containing a supplement of 5% or 10% isolated soybean protein and the 11.1% IAA supplemented diet with adequate threonine also caused an increase of TDH activity but did not depress growth and feed consumption. It is likely that increased TDH activity is triggered by total protein concentrations whether they are in the form of protein or free amino acids. Glucagon is a possible mediator for an increase of amino acid catabolism and TDH activity. However, no temporal change in plasma glucagon concentration after offering of experimental diets was detected.; Purification of TDH was achieved by sequential chromatography on DEAE anion exchange, hydrophobic interaction, AM-Gel blue affinity, and Matrex Gel Red A affinity columns. The subunit molecular weight was determined to be 36 kD by SDS-PAGE electrophoresis, while an apparent native molecular weight of 62 to 74 kD was estimated by gel filtration chromatography, suggesting a dimeric structure. The isoelectric point of TDH was determined to be approximately 5.3. The N-terminal amino acid sequence was determined to residue 27. The Km values for L-threonine and NAD+ are 5.38 and 0.19 mM, respectively. TDH is inhibited by methylglyoxal and D-3-hydroxybutyrate.