The Catabolism of valine in the malnourished rat. Studies in vivio and in vitro with different labelled forms of valine

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Abstract
1. The catabolism of valine was estimated in vivo by measurement of the production of labelled CO2for 2 h after the oral administration of either [U-14C]valine or [I-14C] valine. It was also estimated in vitro in homogenates of liver and muscle incubated with labelled valine. Experiments were performed in rats given diets providing either 215 g (HP) or 25 g (LP) protein per kg diet.2. The proportion of [U-14C]-valine excreted as14CO2was not reduced in rats given the LP diet for 16 d but the excretion of14CO2from [I-14C]valine was reduced by 40% in these animals. When rats were transferred from the HP diet to the LP diet there was a reduction in the excretion of14CO2from [I-14C]valine; when the diet was changed from LP to HP output of14CO2increased to control values.3. Homogenates of muscle and liver catabolized valine to CO2. Both liver and muscle from rats fed on the LP diet catabolized less [I-14C]valine than tissues from control animals.4. Valine aminotransferase activity was higher in muscle than in liver, and did not change in tissues from rats fed on the LP diet. In these animals 2-ketoisovaleric acid dehydrogenase activity was reduced in both liver and muscle.5. The production of14CO2was lower with [U-14C]valine as the substrate than with [I-14C]valine and there was no difference between tissues from rats fed on the HP and LP diets.6. The results with [I-14C]valine suggest that both liver and muscle from protein-depleted rats catabolize valine at a reduced rate. The reason for the discrepancy between these results and those with [U-14C]valine is not clear. It is concluded that the results with [U-14C]valine in vitro are affected by dilution of the label before the formation of14CO2, but that this does not hold in vivo.