Reaction of sodium aminohippurate with Folin's reagent.

ITIS WELL RECOGNIZED that the Folin phenol method (1) for the determination of protein is not specific. This method was in fact originally standardized with protein or tyrosine, and the Folin phenol reagent was shown to react with numerous mono-, di-, and trihydroxyphenols in addition to other substances (s). Although most of these substances are not present in sufficient amounts to interfere with this method when it is applied to serum and tissues, we recently discovered the reaction of the Folin phenol reagent with aminohippurate to be quantitatively significant. Aminohippurate is often used for in vivo and in vitro studies of the kidney. Apparent protein was determined by the method of Lowry et al. (1), with crystalline human serum albumin (Scientific Products, Evanston, Ill.) as reference material. Aminohippurate solutions of 2, 20, 50, and 100 g/m1 were prepared from a 20 g/100 ml solution of sodium aminohippurate (Merck, Sharp, and Dohme, West Point, Pa.). All samples were read at 750 tim in a Beckman DU 2 spectrophotometer. The relationship between the actual concentration of aminohippurate in the samples and the concentration of protein, as estimated by the amount of color developed in these samples, is linear. As can be calculated from the slope of the graph (Figure 1), 1 mg of aminohippurate is equivalent to 0.6 mg of protein by this method. If a patient has received p-aminohippurate, the Lowry method should not be used for the determination of fluid proteins unless a correction is made for the color contributed by the aminohippurate in its reaction with Folin’s phenol reagent.