195 
VI. VERTEBRATES 
THE FREE AMINO ACIDS OF BODY FLUIDS AND SOME 
HEREDITARY DISORDERS OF AMINO ACID METABOLISM 
kG. WESTAELE 
Medical Unit, University College Hospital Medical School, London (Great Britain) 
THE FREE AMINO ACIDS OF BODY FLUIDS 
It is intended in this article to summarize, as far as possible, our present knowledge 
of the content of the free amino acids in the body fluids. It cannot be, in the space 
allowed, a complete historical account and hence the results obtained by workers 
using modern methods of analysis take precedence over those results obtained by the 
earlier investigators who were, in many cases, the pioneers in the field. Also, from the 
terms of reference, the article should cover all the vertebrates but one is forced to 
concentrate on mammals and especially on the human, since, not unnaturally 
much of the work has been carried out on our own species. Only comparatively recently 
have reliable and accurate analyses of the amino acid content of the blood plasma 
and urine of normal healthy individuals become available although a great deal of 
work had already been done on these body fluids in various disease states. 
Methods 
The occurrence of free amino acids in blood and urine has been known for many years 
and a number of different methods have been devised to measure them quantitatively. 
The earlier methods were based upon the measurement of the amino groups by titra- 
tion in the presence of formalin!. Later, this method was modified by the substitution 
of ethanol for formalin?. Neither these methods nor that proposed by VAN SLYKE®, 
which measured volumetrically the N, gas liberated by the amino acids on treatment 
with nitrous acid were entirely satisfactory although a modified gasometric method 
devised by VAN SLYKE, MACFADYEN AND HamiLton? proved to be much more reliable. 
However, these methods only measured the total amino acids present. A number of 
methods were available for measuring individual amino acids usually based on a 
specific chemical reaction which yielded a coloured end product which could be mea- 
sured in a colorimeter. At this time (the late 1930’s) there were no practical ways of 
estimating the series of aliphatic mono-amino, mono-carboxylic acids (glycine, 
alanine etc.). This deficiency was, to a large extent, made up by the development of 
the microbiological methods using mutant strains of bacteria but these techniques 
still have their limitations and are time consuming. 
Certainly the greatest advance in our knowledge of the distribution of the amino 
acids in biological systems has arisen since the invention of the technique of paper 
chromatography®. The potential of this method was not immediately recognized by 
many workers but DENT® quickly realized its value and used it for screening urine and 
plant extracts for amino acids. Paper chromatography gives a semi-quantitative 
References p. 217/219 
