Abstract
Fatty acid synthesis in adipose tissue normally proceeds at a high rate when fasted animals are refed a diet containing carbohydrate, protein, and low levels of fat. This study investigated the effect of omitting protein from the refeeding diet. Rats were fasted for 48 hr and refed either a protein-free diet or a balanced diet, and the rate of fatty acid synthesis from glucose, pyruvate, lactate, and aspartate was measured. Refeeding the animals a diet devoid of protein resulted in a low rate of fatty acid synthesis from each of these substrates as well as a reduction in carbon flow over the citrate cleavage pathway. The activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, NADP-malate dehydrogenase, and ATP-citrate lyase were also reduced in epididymal fat pads from these rats. On the other hand, adipose tissue phosphoenolpyruvate carboxykinase activity was five times as great as that in tissue from animals refed a balanced diet. This difference could be eliminated if actinomycin D was injected coincident with refeeding. Refeeding rats diets high in carbohydrate is not, therefore, capable of inducing high rates of fatty acid synthesis in adipose tissue in the absence of dietary proteins. Thus, liver and adipose tissue respond differently to dietary protein.
Highlights
Rats were fasted for 48 hr and refed either a protein-free diet or a balanced diet, and the rate of fatty acid synthesis from glucose, pyruvate, lactate, and aspartate was measured
Our finding that refeeding a diet high in carbohydrate but devoid of protein to rats fasted for 48 hr will not cause an increase in fatty acid synthesis from glucose, pyruvate, lactate, or aspartate contrasts with the rise in lipogenesis from glucose and acetate reported in liver slices [10]
As noted in the present study, refeeding rats a protein-free diet does not cause the elevation of NADP-malate dehydrogenase or of ATPcitrate lyase activities that is characteristic of animals refed a balanced diet (4, 7 ) .But it seems probable that the activities of these two enzymes, normally- thought to be involved in acetyl CoA translocation from the mitochondria to the cytosol and in NADPH generation [4, 7, 17, 29], do not limit the rate of fatty acid synthesis from pyruvate
Summary
Rats were fasted for 48 hr and refed either a protein-free diet or a balanced diet, and the rate of fatty acid synthesis from glucose, pyruvate, lactate, and aspartate was measured. Adipose tissue phosphoenolpyruvate carboxykinase activity was five times as great as that in tissue from animals refed a balanced diet. This difference could be eliminated if actinomycin D was injected coincident with refeeding. Refeeding rats diets high in carbohydrate is not, capable of inducing high rates of fatty acid synthesis in adipose tissue in the absence of dietary proteins. Liver and adipose tissue respond differently to dietary protein
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