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 mumol/mmol creatinine or 2.26 +/- 0.56 mumol/kg body weight X day. In adults (n = 6; 66 +/- 10 kg, 17-50 years), the corresponding values were 10.5 +/- 1.1 mumol/mmol creatinine and 2.21 +/- 0.23 mumol/kg body weight X 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 X day (preterm infants 0.84, adults 0.60). Since the preterm infants studied contain approximately 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 (approximately 15 g/kg X day and approximately 4 g/kg X day, respectively). Urinary excretion of pseudouridine (psi), 7-methylguanine (m7Gua) and N2, N2-dimethylguanosine (m2(2)G) can be used to estimate the turnover of rRNA, mRNA and tRNA, respectively. The values obtained (in mumol/mmol creatinine) in preterm infants are for psi: 164 +/- 32; for m7Gua: 39.1 +/- 9; and for m2(2)G: 10.6 +/- 2.1. In adults, the values are for psi: 25.3 +/- 3.1; for m7Gua: 4.8 +/- 0.89; and for m2(2)G: 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 mumol/kg X day.