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Threonine Is Catabolized by L-Threonine 3-Dehydrogenase and Threonine Dehydratase in Hepatocytes from Domestic Cats (Felis domestica)^1,2,

Department of Molecular Biosciences, University of California, Davis, Davis, CA 95616

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 ¹⁴CO₂ but not [¹⁴C]glucose production and resulted in the formation of [¹⁴C]2-hydroxybutyrate. Under all incubation conditions, ¹⁴CO₂ and [¹⁴C]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.

KEY WORDS: • feline • L-threonine 3-dehydrogenase • threonine dehydratase • threonine aldolase

¹ Supported by the Companion Animal Research Laboratory, School of Veterinary Medicine, University of California, Davis. The amino acids were generously donated by Ajinomoto USA Inc., Raleigh NC, and the vitamin mixture was a generous gift from Hoffman La Roche Inc., Nutley, NJ.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ Current address: Nutrition Department, 226 Henderson Building, The Pennsylvania State University, University Park, PA 16802-2311.

⁴ To whom correspondence should be addressed.

Manuscript received 22 September 1995. Revision accepted 3 June 1996.

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