A one-step method for protein estimation in biological samples: nitration of tyrosine in nitric acid.

A number of methods are commonly employed for the determination of protein in biological samples. Unfortunately, several compounds that are constituents of biological buffers interfere with these methods, limiting their application. Previous studies have demonstrated that tyrosine rapidly undergoes nitration in nitric acid to yield 3-nitrotyrosine, which has a lambdamax of 358 nm. Utilizing this reaction, we have developed a one-step method for the assessment of protein content in biological samples. Common interfering substances, including SDS, urea, glycerol, ammonium sulfate, and beta-mercaptoethanol, do not interfere with this method. Because of its simplicity, this reaction might be useful for estimating protein content in a variety of biological samples.