[The effect of nutrition on the metabolism of glycine]

Dietary protein and amino acid levels alter threonine dehydrogenase activity in hepatic mitochondria of Gallus domesticus

Experiments were conducted to determine if hepatic threonine dehydrogenase (TDH) activity is influenced by dietary protein or specific amino acid concentrations. In an initial experiment, young chicks were deprived of feed for 60 h or had access for 72 h to a 22% protein basal diet, a protein-free diet or a 51% high protein diet. TDH activity was determined as aminoacetone and glycine accumulation during incubation of liver mitochondria. TDH activity was significantly (P < 0.01) lower in chicks fed the protein-free diet and significantly greater in chicks fed the high protein diet compared with chicks fed the basal diet. Food deprivation had no effect on TDH activity. A second experiment was conducted using the 22 and 51% protein diets, the 22% protein diet plus 1.14 g/100 g diet threonine (equivalent to the free plus protein-bound threonine content of the high protein diet), and the 51% protein diet containing 0.15 g/100 g diet less threonine. TDH was increased in chicks fed either high protein diet (P < 0.05). There were no significant differences in TDH activity, however, between chicks fed the basal diet and the threonine-supplemented diet or between chicks fed the two high protein diets. In two other experiments, the activity of TDH was investigated in chicks fed for 9 d dietary supplements of either serine or glycine (5.5 or 4 g/100 g basal diet, respectively). The supplements were added to the basal diet or the basal diet imbalanced by the addition of 6% branched-chain amino acids. Neither the serine nor the glycine supplement significantly altered TDH activity or the increased activity associated with a branched-chain amino acid-induced threonine imbalance. The results suggest that hepatic TDH activity is influenced by protein level or other amino acids more than by threonine itself.