STUDIES IN HISTOCHEMISTRY XXXI. A METHOD FOR THE DETERMINATION OF PROTEIN IN MILLIMICROGRAM QUANTITIES

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Abstract
The spectrophotometric determination of protein in millimicrogram quantities was described which is suitable for analysis of microtome sections of tissue and other samples of histochemical interest. The macro procedure of Greif was adapted to the micro scale and a modification was employed in which protein was quantitatively precipitated with an excess of bromsulfalein at a low pH and the excess dye was photometrically determined after addition of alkali. Kjeldahl nitrogen analysis of the protein was used for standardization and the results were expressed in terms of protein-nitrogen. With samples and equipment described, a standard error of 1.4% corresponding to 0.9 mµg. of protein-nitrogen was observed. With smaller pipettes and the same cuvettes the volumes employed could be rather easily reduced to about one half with a doubling of the sensitivity of the method. Further increase in sensitivity could be achieved without inconvenience by the use of cuvettes of smaller diameter, e.g. 0.7-0.8 mm., and pipettes of correspondingly reduced capacities.