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V. COMPARATIVE, DEVELOPMENTAL AND EVOLUTIONARY ASPECTS 
FREE AMINO ACIDS IN INVERTEBRATES: 
A COMPARATIVE STUDY OF THEIR DISTRIBUTION 
AND METABOLISM 
JORGE AWAPARA 
Biology Department, Rice University, Houston, Texas (U.S.A.) 
INTRODUCTION 
Tissues of both vertebrates and invertebrates contain a number of nitrogenous sub- 
stances that can be easily extracted. This fraction is relatively small as compared with 
the protein fraction of tissues. However, it contains all the free amino acids and many 
compounds which are derived from them. The free amino acids were discovered during 
the study of protein digestion in mammals and the factors which affect their concentra- 
tion in various organs were carefully studied. Free amino acids were also discovered in 
the non-protein fraction of tissues from invertebrates. Many analyses were made of the 
non-protein fraction from all sorts of animal tissues and from time to time a new 
nitrogenous substance would be found for which no metabolic role could be ascribed. 
ACKERMANN has isolated and identified a very large number of nitrogenous compounds 
from invertebrates, many of them obviously derived from amino acids. The tedious 
but rigorous methods of isolation used could not be applied to the analysis of large 
numbers of specimens and any comparative study would be almost impossible. The 
development of microbiological methods of assay for amino acids made possible the 
study of many specimens and these methods were successfully applied to the analysis 
of free amino acids in the non-protein fraction of tissues from mammals as well as in- 
vertebrates. The interest in the free amino acids in invertebrates was limited to 
their possible role as osmoregulators. It is true that the concentration of free amino acids 
in invertebrates is much higher than in mammals and in marine invertebrates higher 
than in terrestrial or fresh water invertebrates. The question has not been resolved 
and the study of amino acids as osmoregulators is being followed actively in many 
laboratories. 
Microbiological assay is a good and rapid method to measure qualitatively those 
amino acids that can be measured by this method; it will not reveal the existence of 
new amino acids or compounds such as taurine which are closely related to them. 
* Taurine is present in many invertebrates in very high concentration and failure to 
measure this compound would neglect an important factor in osmoregulation; in 
some marine invertebrates taurine comprises over one-third of the total non-protein 
nitrogen fraction. This raises the possibility that other nitrogenous compounds could 
exist which could contribute in large measure to osmoregulation. This problem is 
eliminated when the analysis of the non-protein fraction is carried out by chromatog- 
raphic methods. Paper chromatography is simple and has a high resolving power; 
it lends itself to the analysis of many samples and the quantities of tissues needed are 
relatively small. Paper chromatography has been used extensively in the analysis of 
free amino acids in mammalian tissues. We know that many new compounds were 
References p. 174/175 
