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OCCURRENCE OF FREE AMINO ACIDS — VERTEBRATES 
FREE AMINO ACIDS OF BLOOD AND URINE IN THE HUMAN 
PIERRE SOUPART 
Department of Biochemistry and Nutrition, Faculty of Medicine, University of Brussels, 
Brussels (Belgium) 
Interest in free amino acids of blood and urine in man may arise from many sources. 
For instance, physicians and more specially pediatricians might be interested in 
amino acid excretion, which exhibits disturbances related to several pathological 
conditions in children. Kidney physiologists or obstetricians might be interested in 
urinary amino acids, as was the case at a time, when histidine excretion measure- 
ment was thought to furnish the basis of a “simple chemical test” for pregnancy 
diagnosis. Owing to kidney function, or dysfunction, free amino acids of urine are 
necessarily related to plasma free amino acid levels. More recently, many important 
studies in the field of cancer (leukemia) and also in that of radiation therapy or 
injury effects, as well as studies on free amino acids available for protein synthesis, 
have reinforced the interest of research workers in free amino acid composition 
of body fluids and its possible alterations. As our knowledge of free amino acids in 
body fluids has developed chronologically, and it has been particularly true in the 
case of urine amino acids, a striking observation has arisen, namely, that much 
information was gathered in the study of aminoaciduria in disease long before an 
exact picture of the normal situation regarding amino acid excretion became avail- 
able. Despite the number of invaluable data collected in this way, it has however 
often led to misleading interpretation of the pathological findings, this being mainly 
due to the very nature of methods used for amino acid determination. It is only 
recently that a rather exact and complete account of amino acid excretion has been 
given for normal subjects!) 1*. There was therefore a need for thorough revision 
of our knowledge of free amino acids in normal and pathological body fluids. 
The purpose of this report is threefold. First, it will describe, qualitatively and 
quantitatively, the free amino acid composition of normal human blood plasma 
and urine, both in adults and children. Secondly, as free amino acids of blood plasma 
are necessarily closely related to free amino acids of tissue cells and blood cells, 
this report will describe a tentative model of quantitative distribution of free amino 
acids between blood plasma and the various types of blood cells. Thirdly, as this 
report is also intended to be a progress report, some problems of more limited scope 
but currently under examination, will be dealt with which have arisen from obser- 
vations made during studies on intracellular free amino acid composition as well as 
studies on radiation and protein-malnutrition effects. As will be seen, radiation and 
protein-malnutrition effects seem to share some particular features, at least as regard 
to two amino acids, namely taurine and f-aminoisobutyric acid. In the light of these re- 
cent developments, a provisional classification of free amino acid excretion patterns in 
pathological disorders will also be submitted. Before closing these introductory 
remarks, it seems advisable to stress the point that what is referred to in this report 
References p. 261/262 
