THREONINE IS CATABOLIZED BY L-THREONINE 3-DEHYDROGENASE AND THREONINEDEHYDRATASE IN HEPATOCYTES FROM DOMESTIC CATS (FELIS-DOMESTICA)

Isolated hepatocytes were used to study threonine catabolism in kitten s, 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). Pro duction of CO2, glucose and various metabolites from [U-C-14]threonine were measured. Inclusion of 10 mmol/L glycine, or glycine in combinat ion with 10 mmol/L acetaldehyde + ethanol, in the incubation medium de creased formation of (CO2)-C-14 and [C-14]glucose. At the same time, l arge amounts of [C-14]glycine but no [C-14]ethanol was formed. Inclusi on of 10 mmol/L 2-ketobutyrate + 2-hydroxybutyrate decreased (CO2)-C-1 4 but not [C-14]glucose production and resulted in the formation of [C -14]2-hydroxybutyrate. Under all incubation conditions, (CO2)-C-14 and [C-14]glucose production changed in response to alterations in dietar y protein but not dietary threonine. It appears that threonine dehydra tase 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.