Microestimation of tryptophan in plasma by a fluorometric procedure

Abstract

Many indoles can be assayed fluorimetrically either by measurement of their native fluorescence or by conversion to a suitable derivative, e.g., a norharman in the case of tryptamine and tryptophan (1, 2). For tryptophan the latter method has two main advantages over measurement of native fluorescense; first, separation between excitation and emission maxima is great enough for simple filter instruments to be used, and second, it possesses a much greater sensitivity. Furthermore, the excitation wavelength is in the visible region, whereas native fluorescence is measured in the ultraviolet region. It was the high sensitivity which prompted us to adapt the norhaman method for use on a small-scale to measure tryptophan in blood plasma in a clinical study which necessitated withdrawal of several blood samples in an 8-hour period. The method described, which can be applied to as little as 10 μ plasma, minimizes the inconvenience of repeated venupunctures.