Disposition of the acid-extractable amino acid pool in mammalian cells

Experimental evidence is presented which eliminates the acid-extractable (acid-soluble) pool of amino acids within cells from being the source of precursor molecules for protein synthesis. This is equally valid for those amino acids initially showing quasi-exponential incorporation into protein, viz, glycine and proline, contrary to the reports of others (e.g. Robinson, 1977). Reexamination of the design of some of the experiments showing the pool acting as the precursor supply has revealed technical defects which may have been responsible for this erroneous conclusion. The acid-extractable pool can be more clearly understood as the accumulation of complexed amino acids which have passed the selection mechanism of the intracellular cyclical perfusion system (Wheatley and Inglis, 1980), but are no longer directly available for protein synthesis.