88. NONINVASIVE ASSESSMENT OF WHOLE-BODY METABOLIC RATES IN RATS, PRETERM INFANTS AND ADULTS USING RNA CATABOLITES IN URINE

Abstract

For assessing the efficiency of nutritional supply, determination of whole-body metabolic activity, exemplified by measurements of protein turnover, are becoming more and more important. However, both technique and interpretation of protein turnover measurements with ¹⁵N-amino acids are complex and time-consuming. As we have shown, RNA turnover is coupled to protein turnover (1) and can easily by estimated by measuring urinary excretion of modified RNA catabolites: N²N²-dimethylguanosine (m₂²G) or N⁶-threoninocar-bonyladenosine (t⁶A) can be used to calculate tRNA turnover, similarly pseudouridine (ψ) gives rRNA turnover and 7-methylguanine (m⁷Gua) mRNA turnover. The relation between tRNA, rRNA, mRNA and protein turnover is constant irrespective of age; preterm infants and adults displaying only minor differences (1) Thus any one of the three RNA classes can be taken as representative. We have compared tRNA turnover rates in 320-g rats (pooled urine from 10 animals; 2.45 μmol kg⁻¹ d⁻¹), 1-9-kg preterm infants (n = 21; 1.69 ± 0.39 μmol kg⁻¹ d⁻¹) and adults (n = 32; 0.50 ± 0.12 μmol kg⁻¹ d⁻¹). The relation between tRNA turnover rates in rats, preterm infants and adults (4-9 : 3-4 : 1) corresponds closely to the one observed by others for whole-body protein turnover (about 5 : 3-5 : 1 (2)) and to a lesser degree also to basal metabolic rates (about 4 : 2.5 : 1 (2)). (1) Sander G, Hülsemann J, Topp H, Heller-Schöch G, Schöch G (1986) Ann Nutr Metab 30: 137-142; 2. Waterlow JC (1984) Q J Exp Physiol 69: 409-439.

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