Measurements of Myofibrillar Protein Breakdown in Newborn Human Infants

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Abstract
Myofibrillar protein breakdown was calculated from the urinary excretion ratio of N.tau.-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. N.tau.-Methylhistidine in infant urine was measured by a rapid auto analyzer method and shown to give similar values to those obtained by ion-exchange separation techniques. The molar excretion ratio of N.tau.-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. This ratio does not reflect a low rate of myofibrillar protein breakdown in the fetus but a more effective transplacental passage of N.tau.-methylhistidine than of creatinine. 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. 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 weighing < 1 kg at birth. Lower excretion ratios were found in some infants of diabetic mothers and in athyroid infants. The urinary excretion ratio of N.tau.-methylhistidine to creatine 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.