PROTEIN-TURNOVER RESEARCH USING N-15 AND C-13 LABELED SUBSTRATES IN PEDIATRICS

Measurements in protein turnover and in metabolism of amino acids and their degradation products by means of stable isotope labelled substra tes have been increasingly applied in clinical research over the last years. In spite of numerous studies dealing with this topic, quite a f ew important insufficiently clarified methodical aspects remain. This refers, for instance, to the choice of suitable tracer substances, the difficulties in the determination of the excretion plateau and the va lidation of the oxidation rates as measured with individual-labelled a mino acids with regard to the whole body protein synthesis. Such probl ems may become of decisive importance in special subjects, such as pre term infants and critically-ill patients. Investigations into these is sues conducted by our group have revealed that the protein turnover in the very small preterm infant is by no means as intensive as previous ly claimed. The utilisation of urea nitrogen for the whole body protei n synthesis of the infant may assume substantial proportions under the conditions of marginal protein intake and of catchup-growth. Studies conducted by means of N-15-labelled bifidobacteria have pointed at the intensive substrate exchange existing between microflora and host. Pe diatric research has to be non-invasive. Consequently, methods based o n arterio-venous differences in tracer concentrations and on muscle bi opsies do not have very high priority in pediatric research. A search for references published in the last five years has shown, that N-15-g lycine is still the most frequently used tracer substance. There is a tendency towards a further increase of cell culture experiments run wi th stable isotope labelled amino acids. Clinical research groups incre asingly tum their attention to stable isotopes and mass spectrometry. This impressively demonstrates the continuing importance of tracerkine tic methods in all branches of medicine.