Untersuchungen zum Threoninstoffwechsel bei Laborratten und Küken in Abhängigkeit von der Protein- und Threoninversorgung

Abstract

Aim of this research was to determine the effects of different supply of crude protein (CP), threonine (Thr) and glycine (Gly) on threonine dehydrogenase activity (TDG activity) in the liver of chickens and laboratory rats under conditions of different threonine supply. A total of 7 experiments with feather-sexed male Cobb chickens and white Wistar rats were done in different age periods: Experiment 1: Chicken day 15-25, different CP contents. The calculated CP contents were 5.5%, 11.0%, 16.5%, 22.0%, 27.5% and 33.0%. Experiment 2: Chicken day 17-30, 18.5% and 22.5% CP and 0.45% and 0.60% true fecal digestible threonine (dThr). Experiment 3: Chicken day 10-20, 16.5% and 22.0% CP and different threonine levels. Experiment 4: Chicken day 1-49, standard feeding program. Determination of liver threonine dehydrogenase at days 7, 21, 35 and 49. Experiment 5: Chicken day 5-15, 22.0% CP content, different glycine and threonine levels. Experiment 6: White Wistar rats in live weight range from 106-140 g with CP contents of 0%, 6.0%, 12.0%, 18.0% and 24.0%. Experiment 7: White Wistar rats in live weight range from 149-167 g and CP contents of 12.0% and 18.0% with different threonine levels. At the end of the experiments the livers were taken from 6 animals for the in vitro TDG assay. The threonine efficiency was calculated from results of N-balance trials with an exponential N-utilization model. Summary of the Results: 1. The elevation in CP content caused a significant increase in TDG activity in the livers of chickens starting at 22.0% CP in the feed mixtures despite a limited threonine supply. Threonine efficiency remained unchanged up to an CP content of 27.5%, dropping off significantly at 33.0% CP. This means that an intensified TDG-induced breakdown of threonine caused a reduction in threonine efficiency in the feed mixtures with a high CP content. 2. There was no effect on the TDG activity in the chick liver in the presence of an CP content of 18.5% and an increase in threonine concentration from 0.45% to 0.60 % dThr. However, TDG activity in the chick liver was elevated at an CP content of 22.5% and a 0.60% content of dThr. These findings might delineate the range in which threonine no longer has a limiting effect. 3. At a content of 16.5% CP and an increase in the Thr concentration from 0.65% to 0.79%, no influence on TDG activity was determined in the chick liver. By contrast, there was a significant increase in TDG activity when the CP content was 22.0% and the Thr content increased from 0.86% to 1.05%. 4. During the course of the phase-feeding experiment, there proved to be age-dependent changes in TDG activity that can be explained as phases in which a specifically high metabolic need for glycine existed. 5. At an CP content of 22.0% (Gly + Ser content 1.55%), the elevation in Thr content from 0.45% to 0.60% dThr led to an increased accumulation of Gly in the liver mitochondria, but not to a significant increase in TDG activity. After adding Thr at 1.90% Gly+Ser and 22.0% CP, TDG activity increased significantly. This finding would indicate the end of the Thr limit range. 6. In laboratory rats fed on a protein-free diet, TDG activity was at its lowest, increased during an CP rise to 12.0% CP, decreased slightly down to 18.0% CP and showed a trend to increase from 18.0% to 24.0% CP. Overall, the CP level only had a random effect on TDG activity. 7. The increase in Thr content from 0.28% to 0.72% in the presence of 12.0% CP caused a gradual increase in TDG activity in rat liver mitochondria. This similarly applied to the feed admixed with 18.0% CP, but at a higher level. The TDG activity was almost exclusively modulated by amino acetone accumulation. TDG activity and Thr efficiency would indicate the end of the Thr limit range. Accordingly, the in vitro TDG activity in the livers of chicks and laboratory rats is not solely affected by the Thr concentration in the feed, but also by the CP content and, hence, by the supply of other amino acids and by the animal's age. Interpretation of the TDG changes (in vitro) is difficult since there are apparent connections to the non-specific catabolism rate of other amino acids. There is obvious, albeit limited evidence of connections to the parameter of Thr efficiency. Further research is required before quantitative conclusions can be drawn.