Similar results have been recently reported for human blood plasma by Qureshi and Qureshi (1989). These authors demonstrated an overall loss of free amino acids when organic solvents were used to precipitate plasma proteins, but the losses reported were not as extensive as in the present study due to the use ofinternal standards, and possibly
different sample treatment. These authors hypothesized that chemical instability, hydrolysis. degradation and oxidation could account for the apparent loss of free amino acids when acetone and acetonitrile are used as deproteinizing agents. The decrease in free amino acid concentrations could be due to their attraction to the protein aggregates. We
have demonstrated that the loss of free amino acids is directly related to the concentration of protein in the sample when organic solvents are used as protein precipitants.