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

Abstract

Urinary excretion of 3-methylhistidine in preterm infants (n = 42; 1,712 .+-. 408 g, 4-91 days old) was 24.2 .+-. 6 .mu.mol/mmol creatinine or 2.26 .+-. 0.56 .mu.mol/kg body weight .cntdot. day. In adults (n = 6; 66 .+-. 10 kg, 17-50 years), the corresponding values were 10.5 .+-. 1.1 .mu.mol/mmol creatinine and 2.21 .+-. 0.23 .mu.mol/kg body weight .cntdot. 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 .cntdot. day (preterm infants 0.84, adults 0.60). Since the preterm infants studied contain .apprx. 21% muscle instead of the 43% found in adults, th 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 (.apprx. 15 g/kg .cntdot. day and .apprx. 4 g/kg .cntdot. day, respectively). Urinary excretion of pseudouridine (.PSI.), 7-methylguanine (m7Gua) and N2,N2-dimethylguanosine (m22G) can be used to estimate the turnover of rRNA, mRNA and tRNA, respectively. The values obtained (in .mu.mol/mmol creatinine) in preterm infants are for .PSI.: 164 .+-. 32; for m7Gua: 39.1 .+-. 9; and for m22G: 10.6 .+-. 2.1. In adults, the values are for .PSI. 25.3 .+-. 3.1; for m7Gua: 4.8 .+-. 0.89; and for m22G: 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 .mu.mol/kg .cntdot. 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.