290 E. ROBERTS AND D. G. SIMONSEN 
It has not yet been possible to relate the extractability of the small molecular- 
weight ninhydrin-reactive constituents of tissues by the above procedures to their 
state in living tissue. For this reason they have been defined as “free or easily ex- 
tractable”. Insofar as virtually identical quantities of these constituents have been 
obtained from the same tissues by the different procedures employed, it appears 
unlikely that they arise from larger molecules by the rupture of covalent bonds. 
The amino acids may exist in the free form in the cytoplasm and nucleoplasm or 
may be held by adsorptive forces at interphases and surfaces. Loose bonds may 
exist between these constituents and proteins, nucleic acids and lipids or complexes 
of these materials; or sequestration may take place in intracellular structures. It 
is also possible that the amino acids may exist in the cell in the form of extremely 
labile derivatives or complexes which are dissociated by the extraction procedures 
26 

25 28 
Figs. 23-28. Free amino acids of whole rat liver homogenates (75 mg) Figs. 23, 26: prepared 
in 10% polyvinylpyrrolidone and 20% sucrose. Figs. 24, 27: extracts of residues from homo- 
genates of 75 mg of fresh weight of tissue prepared at o° and 25°, respectively. Figs. 25, 28: 
residues washed once with original volumes of suspending medium at o° and 25°, respectively 
and centrifuged at these temperatures. 
References p. 348/349 
