Measurements of myofibrillar protein breakdown in newborn human infants.

1. Myofibrillar protein breakdown was calculated from the urinary excretion ratio of NT- methylhistidine (3-methylhistidine) to creatinine in newborn premature and full-term infants. Representative values were obtained from single voidings provided that the infant's metabolic status was stable. 2. NT- Methylhistidine in infant urine was measured by a rapid Auto Analyser method and shown to give similar values to those obtained by ion-exchange separation techniques. 3. The molar excretion ratio of NR- methylhistidine to creatinine averaged 0.0159 in urine samples obtained within 12 h after birth. A similar ratio was found in amniotic fluid collected at birth. It is argued that this ratio does not reflect a low rate of myofibrillar protein breakdown in the foetus, but rather a more effective transplacental passage of NT- methylhistidine than of creatinine. 4. The urinary ratio increased during the first 2 days after birth to a plateau at 0.0372. This represents a myofibrillar protein degradation rate of 3.40% day-1 in full-term infants. 5. The molar excretion ratio during the period 40-120 h after birth increased in premature infants and reflects a fractional degradation rate of 5.34% day-1 in those infants weighting less than 1 kg at birth. 6. Lower excretion ratios were found in some infants of diabetic mothers and in athyroid infants. 7. The urinary excretion ratio of NT-methylhistidine to creatinine is presented as a useful method for evaluating the breakdown rate of myofibrillar protein in neonates and can be applied to a number of abnormal nutritional or hormonal states.