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A protein-free 90% sucrose diet was refed for several days to rats starved for 7 days. 8-Azaguanine was then injected intraperitoneally concomitant with a change to a 90% protein diet to determine if glucose-6-phosphate dehydrogenase (G6PD) mRNA was synthesized during the refeeding of an inadequate protein diet. The refeeding of the 90% sucrose diet alone resulted in an induction of G6PD to 2–3 times the prestarvation level. Glucose could be substituted for sucrose with equal effect on the induction of G6PD. After the change to the 90% protein diet a further increase in G6PD activity occurred which was insensitive to 8-azaguanine and thus presumably was independent of mRNA synthesis. This 8-azaguanine insensitive increase in G6PD was greater after a 4-day refeeding of the 90% sucrose diet than after only a 2-day refeeding of the 90% sucrose diet. This indicates that synthesis of G6PD mRNA occurred while refeeding the 90% sucrose diet. Reducing the starvation period to only 2 days resulted in no induction of G6PD by the 90% sucrose diet and the absence of an 8-azaguanine insensitive induction upon changing to the 90% protein diet.

In these experiments, two diets, containing sucrose or lard as the primary energy source, were compared as to their influence on 1) the distribution of ribosomal RNA between free and bound polysomes and 2) polysome profiles of the liver postmitochondrial supernatants. In the livers of rats which had been fed the two diets continuously for several months no differences were found between free versus membrane-bound RNA or between the polysome profiles. However, in rats first starved and then refed, marked differences were found between rats refed sucrose diet and fat diet. With respect to free ribosomes, almost none were found in the livers of rats refed a sucrose diet for 24 hours, whereas rats refed fat diet were “normal” in their free ribosomal content. With respect to polysomal patterns, livers of rats refed the sucrose diet for 24 hours showed a great increase in large aggregates as compared to essentially none in the profiles from the livers of fasted rats. The polysomal profiles from livers of rats refed the fat diet for 24 hours showed a marked increase in di-, tri- and tetrasomal aggregates, but no large aggregate accumulation. After 48 hours of refeeding the differences were less conspicuous, and with continuous feeding were nonexistent. Thus it appears that available energy whatever the source will return the profile to normal if sufficient time is allowed.

Diets with a uniform ratio of protein to nonprotein calories of 1:4, have been used to study the comparative effects of sucrose and lard on protein retention. In a nitrogen balance study on rats, the lard diet gave better N economy than did the sucrose diet. Liver glucose-6-phosphate dehydrogenase enzyme levels were found to be relatively high in the case of rats fed the sucrose diet, but when the lard diet was fed the liver levels of these enzymes were as low as those of starved rats. Short-term in vivo protein synthesis as measured by labeled amino acid incorporation into liver protein indicated increased arginine incorporation when the fat diet was fed. The observations indicate an improved protein utilization and nutritional status when the high fat carbohydrate-free diet was fed.

The level of glucose 6-phosphate dehydrogenase in liver is altered strikingly by starvation and also by refeeding, but the steps by which increased synthesis of this enzyme is induced have not been satisfactorily identified. Recent work suggests that hormones may have a general rather than a specific role. When starved rats were refed diets varying either in amount or in carbohydrate content, their liver glucose 6-phosphate dehydrogenase increased directly with their carbohydrate intakes. Insulin, and other hormones tested, given without carbohydrate, but with adequate protein and energy intakes, had no effect on liver glucose 6-phosphate dehydrogenase levels.

The previously reported dietary carbohydrate dependence of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and pyruvate kinase of rat liver has been further explored in rat liver cytoplasm under various conditions of starvation and refeeding. Similar but smaller influences of dietary carbohydrate were found on adrenal and on liver mitochondrial enzyme levels. In liver cytoplasm, glycogen accumulation appeared to be a precursor of the carbohydrate-dependent enzyme increases and, if so, could explain the increase in these enzymes following high dietary protein as well as carbohydrate. Orotate incorporation into liver nuclear RNA was seen after challenge of fasted rats with the high sucrose diet but not after challenge with the high fat diet. Actinomycin D and cycloheximide blocking of enzyme synthesis at zero time and cycloheximide but not actinomycin D blocking of enzyme increase several hours after sucrose indicates transcription level induction of synthesis following high sucrose diet feeding. The data seem consistent with regulation of these enzymes by the tissue levels of their specific substrates. The enzyme increases appear to involve fresh messenger RNA synthesis.