Threonine is catabolized by L-threonine 3-dehydrogenase and threonine dehydratase in hepatocytes from domestic cats (Felis domestica)

Isolated hepatocytes were used to study threonine catabolism in kittens, and dietary threonine and crude protein were varied to study enzyme adaptation. Cells were isolated from 21-wk-old kittens which had been fed diets containing threonine at 4 or 8 g/kg of diet with either 200 or 500 g crude protein/kg of diet (2 x 2 factorial, n = 4/group). Production of CO₂, glucose and various metabolites from [U-¹⁴C]threonine were measured. Inclusion of 10 mmol/L glycine, or glycine in combination with 10 mmol/L acetaldehyde + ethanol, in the incubation medium decreased formation of ¹⁴CO₂ and [¹⁴C]glucose. At the same time, large amounts of [¹⁴C]glycine but no [¹⁴C]ethanol was formed. Inclusion of 10 mmol/L 2- ketobutyrate + 2-hydroxybutyrate decreased ¹⁴C0₂ but not [¹⁴C]glucose production and resulted in the formation of [¹⁴C]2-hydroxybutyrate. Under all incubation conditions, ¹⁴CO₂ and [¹²C]glucose production changed in response to alterations in dietary protein but not dietary threonine. It appears that threonine dehydratase and L-threonine 3-dehydrogenase, but not threonine aldolase, are active pathways for threonine metabolism in cats, and both enzymes are sensitive to levels of dietary protein.