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The present report describes the development of and results with a modified ninhydrin colorimetric method for the determination of free plasma -amino nitrogen. Precipitation of the plasma proteins with a hydrochloric acid-sodium tungstate reagent, filtration of the deproteinized filtrate through glass wool plugs, and reduction of the pH of the reaction resulted in improved reproducibility, lower water blank readings, and lower plasma -amino nitrogen concentrations which agreed closely with values obtained using other standard technics. The stoichiometry of the modified reaction and recovery of standard amino acid solutions were investigated and found to be satisfactory. A comparative study with the ninhydringasometric technic showed that the modified colorimetric procedure described herein yielded values in very close agreement with the more laborious manometric determination. Fasting venous plasma samples obtained from 21 normal volunteers yielded a mean plasma concentration of 3.85 mg./100 ml. The report concludes from these observations that the modified colorimetric procedure provides an accurate, simple, and rapid estimation of the total free amino acid content of human blood plasma.

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