Plasma alkaline ribonuclease (EC 3.1.4.22) and nitrogen retention in low-birth-weight infants.

1. Nitrogen retention was determined by classical N balance techniques in fourteen rapidly growing low-birth-weight infants receiving 3 g protein/kg body-weight and during their 3rd week of life. This was compared with plasma free alkaline ribonuclease (EC 3.I.4.22; RNase) activity and other biochemical measurements of protein nutrition. 2. Plasma RNase showed a significant positive correlation with N retention and a corresponding negative correlation with urine urea-N. These results were unexpected and suggest a different relationship between RNase and N retention in infants compared with that found by other workers in children and adults. 3. The most likely explanation of this apparent anomaly is that in all instances high activities of plasma RNase are associated with a need to conserve N. In the infants studied this may indicate some measure of 'protein economy' and they could therefore benefit from a higher protein intake.