Influence of Protein Supply on Threonine Efficiency and Threonine Catabolism in Hepatic Mitochondria of Chicks and Rats

This research investigated the relationship between efficiency of threonine (Thr) utilization and Thr oxidation in hepatic mitochondria in chicks and rats fed with graded levels (5.5-33.0% CP for chicks, 6-24% CP for rats) of protein. Calculation of efficiency of Thr utilization was based on N-balance data and an exponential N-utilization model, and Thr dehydrogenase (TDG) activity was determined. According to the results, no significant effect on TDG activity was observed in the liver of chicks who received diets containing from 5.5 up to 16.5% CP. However, significantly elevated TDG activities were observed, despite limited supply of Thr in protein, with diets containing from 22.0 up to 33.0% CP. At the levels of CP content from 5.5 up to 27.5%, no significant change in efficiencies of Thr utilization was observed. However, a significant decrease in efficiency was observed with diets containing from 27.5 to 33.0% CP. In chicks, the relationship between oxidation of Thr and Thr efficiency was observed with graded CP levels. In addition, elevated TDG activities in rat liver were observed with diets containing from 6.0 to 12.0% CP. At the levels of CP content from 6.0 up to 24.0%, no significant effect on efficiency of Thr utilization was observed in rats. In addition, no relationship was observed between Thr oxidation and efficiency of Thr utilization with graded CP levels in rats. Taken together, reactions for TDG appear to be animal species-dependent.