Dietary Protein and Amino Acid Levels Alter Threonine Dehydrogenase Activity in Hepatic Mitochondria of Gallus domesticus

Abstract

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.

Structure and function of the l-threonine dehydrogenase (TkTDH) from the hyperthermophilic archaeon Thermococcus kodakaraensis

“…1), an intermediate in the metabolism of threonine to glycine (Potter et al, 1977). Many studies show that TDH is responsible for the bulk of threonine breakdown in the majority of species (Aronson et al, 1989;Ballevre et al, 1991;Bird and Nunn, 1983;Boylan and Dekker, 1981;Dale, 1978;Davis and Austic, 1997;Hammer et al, 1996;Ray and Ray, 1984) although in animals the threonine dehydratase pathway can become dominant during periods of fasting or high protein diet to mobilise threonine and serine for gluconeogenesis. Hence, it is interesting that in the majority of species there are essentially two enzymes in different cellular compartments that are each capable of assuming a dominant role in threonine catabolism in different metabolic states (Bird and Nunn, 1983).…”

“…Overall, this two-step metabolic pathway accounts for 80-100% of threonine catabolised in animal liver (Aoyama and Motokawa, 1981;Ballevre et al, 1990;Bird and Nunn, 1983). It has been estimated from studies of hepatic mitochondria that in the fed state 65% of threonine metabolised by TDH is converted to glycine by KBL (Bird and Nunn, 1983;Davis and Austic, 1997). It is likely that the level of CoA determines the flux through the KBL-catalysed reaction and dictates whether threonine is converted to glycine or to the alternative breakdown product aminoacetone ( Fig.…”

Dieta aminoacídica na determinação da perda endógena ileal de frangos de corte: uma proposta metodológica

“…Segundo Hodgkinson et al (2000), o uso da ultrafiltração da digesta, necessária quando se utiliza a caseína hidrolisada enzimaticamente, subestimava o fluxo de aminoácidos endógenos devido à perda de aminoácidos e peptídeos de baixo peso molecular. De acordo com Gabert et al (2001), a DLP não estimulava a síntese de enzimas proteolíticas, o que subestimaria a perda endógena no íleo terminal de suínos. Nesta mesma linha de pensamento, Butts et al (1991), confirmaram que o uso da caseína enzimaticamente hidrolisada é melhor que o da dieta livre de proteína, porém, o processo da ultrafiltração provoca grandes diferenças na perda endógena de valina, isoleucina, leucina, serina e ácido glutâmico.…”

“…Davis & Austic (1997), confirmam que frangos de corte alimentados com dietas com elevado teor de proteína têm elevada atividade de treonina dehidrogenase no figado, responsável pela conversão da treonina em glicina, podendo dar origem à serina.…”

“…Entretanto, Zanella et al (1999) observaram que enzimas digestivas podem ou não ser estimuladas pela fonte exógena de proteína. Nesta mesma linha de pensamento, Gabert et al (2001) comentaram que a falta de aminoácidos dietéticos compromete a produção de enzimas digestivas e pancreáticas, de modo que o uso de DLP não estimula a secreção normal das enzimas proteolíticas, portanto, subestimando a perda endógena de aminoácidos. Chung & Baker (1992) (Moughan & Schuttert, 1991;Moughan et al, 2005).…”