Comparative study of various extraction methods for the quantitative determination of free amino acids from brain tissue

Seven published extraction methods for the quantitative determination of the amino acids of biological fluids and tissues were applied to brain tissue and the experimental data analyzed statistically. All procedures employing organic solvents, i.e., 75% ethanol, acetone-HCl, and chloroform-methanol, yielded low values of the basic amino acids as compared to perchloric acid extraction. Of the acid extraction methods, trichloroacetic, sulfosalicylic, and perchloric acids gave essentially similar values for all but a few of the amino acids. Picric acid extracts demonstrated similarities to the organic solvents in giving low basic amino acid values as well as showing significant differences in a number of other amino acid values as compared to perchloric acid extracts. On the basis of these findings, perchloric acid extraction was judged to be the most suitable extraction method for free amino acid analysis of body tissues or fluids. When used in conjunction with a single-column ion-exchange resin automated technique and lithium citrate buffer elution, it makes possible the separation and quantitative analysis of all the amino acids, including glutamine and asparagine, usually found in biological fluids and tissues.