FREE AMINO ACIDS IN INVERTEBRATES 159 
discovered in tissues of mammals by using paper chromatography ; it is both a survey 
method and a method easily adapted to rough quantitation. Many invertebrates have 
been thus analyzed and new compounds detected as will be discussed later. The 
method has been used to establish patterns of ninhydrin-reactive substances in the 
hope to find a method for classification and speciation but the results have not been 
too satisfactory. Paper chromatography is now a well-established laboratory method 
but it has not been exploited to the limit of its potentialities in the study of amino 
acids in invertebrates. In the past, this work was in abeyance for want of a good 
analytical method. In the present, the problem is one of sampling. Invertebrates are 
collected randomly from untested environments; there is no knowledge of their 
physiological condition and often there is no precise knowledge of their feeding habits. 
In spite of these limitations the analysis of the non-protein fraction from tissues of 
invertebrates has been revealing and stimulating. Clues have been obtained that can 
be used to search for new enzymes, new metabolites and perhaps some unique metabolic 
sequences. It should be easy to maintain stocks of invertebrates in the laboratory 
where conditions could be controlled. Then a number of interesting problems could 
be solved. Invertebrates appear to contain free amino acids in extremely high con- 
centration as compared with mammals. One could explain this extremely high con- 
centration of free amino acids if one could demonstrate very active proteolysis in 
these organisms, but this could not explain the uneven distribution of some amino 
acids ; for example, it could not explain the fact that in many crustaceans over one-third 
of the amino acid nitrogen is made up of glycine. It could not explain the fact that 
in some echinoderms glycine is the only detectable amino acid next to taurine. The 
answer to this uneven distribution might be related to the rates of formation and 
utilization of these compounds; or it could be related to the ability of invertebrate 
cells to concentrate nitrogenous substances by some unknown mechanism. This is 
true in the case of the mussel W/ytilus edulis which contains taurine in a concentration 
of approx. 2% of its fresh weight. MM. edulis, however, forms taurine from its precursors 
at a very slow rate. This invertebrate must possess a mechanism to maintain this 
high concentration of taurine in the cells. 
There are few papers published related to nitrogenous compounds of invertebrates ; 
a few have been selected and will be discussed in relation to the role of amino acids 
in osmoregulation. Others have been selected as interesting reports on either new 
nitrogenous compounds or compounds which have been studied in relation to the 
metabolism of amino acids in invertebrates. 
FREE AMINO ACIDS IN INVERTEBRATES 
The available information on the content of amino acids in invertebrates has been 
obtained mostly by means of microbiological assays or by means of paper chromatog- 
graphy; in the latter case many other substances which react with ninhydrin are 
usually detected and reported under the heading “amino acids”. For the present 
discussion, “free amino acids” will mean a-amino-carboxylic amino acids and not all 
compounds detected with ninhydrin in paper chromatograms or in the effluents from 
ion-exchange resin columns. Often the composition of a whole animal is given, simply 
because the animal is too small to separate its component organs. In many papers 
this is not specified and comparisons are difficult. 
References p. 174/175 
