Influence of glucogenic amino acids on the hepatic metabolism of threonine.

The supplementation of a low-protein diet with L-threonine leads to a marked accumulation of threonine in plasma and liver, whereas increasing dietary protein generally leads to an induction of threonine dehydratase in the liver, hence depressed availability for extrasplanchnic tissues. The aim of the present study was, thus, to further investigate the factors which control the utilization of threonine by the liver. Increasing the dietary supply of threonine led to parallel increases in the afferent and hepatic concentrations and in the rate of utilization by the liver; however, the fractional extraction tended to decrease. It appears that the addition of a mixture of glucogenic amino acids to the diet prevented the accumulation of threonine in plasma induced by exogenous threonine. The glucogenic amino acids increased the fractional hepatic uptake of threonine, and counteracted its accumulation in the liver. These effects reflect the fact that the glucogenic amino acids elicited a potent induction of the threonine dehydratase, whereas threonine alone was uneffective. Our results suggest that, besides the well-established effect of glucogenic conditions, the availability of some glucogenic amino acids is an important factor in the control of threonine catabolism.