Protein and RNA Turnover in Preterm Infants and Adults: A Comparison Based on Urinary Excretion of 3-Methylhistidine and of Modified One-Way RNA Catabolites

Urinary excretion of 3-methylhistidine in preterm infants (n = 42; 1,712 ± 408 g, 4-91 days old) was 24.2 ± 6 μmol/mmol creatinine or 2.26 ± 0.56 μmol/kg body weight-day. In adults (n = 6; 66 ± 10 kg, 17-50 years), the corresponding values were 10.5 ± 1.1μmol/mmol creatinine and 2.21 ± 0.23 μmol/kg body weight-day. For both collectives, the breakdown per kg body weight of 3-methylhistidine-containing protein (i.e. actin and myosin) was similar, at approximately 0.7 g/kg-day (preterm infants 0.84, adults 0.60). Since the preterm infants studied contain ~ 21% muscle instead of the 43% found in adults, the 3-methylhistidine excretion in preterm infants probably indicates muscle (and intestinal) protein turnover to be about 3 times higher than in adults, a figure in accord with data on whole-body protein turnover in preterm infants and adults (~ 15 g/kg. day and ~ 4 g/kg.day, respectively). Urinary excretion of pseudouridine (ψ), 7-methylguanine (m⁷Gua) and N²,N²-dimethylguanosine (${\text{m}}_2^2G$) can be used to estimate the turnover of rRNA, mRNA and tRNA, respectively. The values obtained (in μmol/mmol creatinine) in preterm infants are for ψ: 164 ± 32; for m⁷ Gua: 39.1 ± 9; and for ${\text{m}}_2^2G$: 10.6 ± 2.1. In adults, the values are for ψ: 25.3 ± 3. 1; for m⁷ Gua: 4.8 ± 0.89; and for ${\text{m}}_2^2G$: 1.53 ± 0.38. This yields 3-4 times higher turnover rates in preterm infants than in adults for all 3 RNA classes: rRNA, 0.1 versus 0.038; tRNA, 1.87 versus 0.66; mRNA 2.35 versus 0.64 μmol/kg.day. Thus all data indicate that preterm infants have a 3- to 4-fold higher turnover than adults not only of whole-body protein, but also of rRNA, tRNA and mRNA.