A NEW STABLE-ISOTOPE METHOD ENABLES THE SIMULTANEOUS MEASUREMENT OF NUCLEIC-ACID AND PROTEIN-SYNTHESIS IN-VIVO IN MICE

We developed a method based on the incorporation of C-13(2)-units deri ved from [U-C-13]glycine that allows the simultaneous quantification o f tissue protein and RNA synthesis in vivo. Two groups of 26 mice were fed diets containing a high (HF diet) or a low quantity of fiber (LF diet). After 6 d, [U(13)G]glycine was added to the diet and groups of four mice were killed after 2, 4, 6, 8, 12 and 24 h. Hepatic and intes tinal mucosal free and RNA-bound purine nucleosides were extracted and enzymically degraded to allantoin. Allantoin was degraded to glyoxyla te, which was then reductively aminated to glycine, which contains the two C-13-atoms incorporated via de novo synthesis. Ingestion of the H F diet was associated with significantly (P < 0.05) higher rates of to tal RNA synthesis in both the liver (HF diet, 29%/d; LF diet, 21%/d) a nd mucosa (HF diet, 27%/d; LF diet, 17%/d). The mean rates of RNA synt hesis in each tissue were significantly (P < 0.01) lower than the resp ective rates of protein synthesis (liver, 67%/d; mucosa, 74%/d). The i sotopic enrichment of the free purine nucleotide pool increased rapidl y and exponentially, but the steady-state value was substantially (P < 0.001) lower than that of the RNA-bound purines. The results suggest that the free nucleotide pool consists of two kinetically distinct com partments, one of which is small and has a rapid rate of turnover. Thi s, we propose, acts as the RNA precursor pool. The other is large, has a low rate of turnover and, we believe, is the pool of adenosine trip hosphate involved in cellular energetics.