3-(4-Carboxybenzoyl)quinoline-2-carboxaldehyde, a reagent with broad dynamic range for the assay of proteins and lipoproteins in solution

This paper describes the application of 3-(4-carboxybenzoyl)quinoline-2-carboxaldehyde (CBQCA), a sensitive fluorogenic reagent for detection of amines, to the assay of proteins in solution. The sensitivity and dynamic range of CBQCA in the determination of protein concentration is modulated by the number of accessible amines present in the protein assayed. Thus, this method bears the same limitations as other reagents used in fluorogenic assays based on fluorescent amine adducts such as those obtained with fluorescamine or o-phthaldialdehyde, or of other spectrophotometric methods, where the color development is determined by the abundance in the protein of certain amino acids. However, in comparison to other fluorescence-based protein detection methods, CBQCA has proven to be an extremely sensitive reagent with a very broad, essentially linear dynamic range, capable of detecting from 10 ng to 150 micrograms of protein (in a 100- to 200-microL assay volume). The CBQCA reagent also functions well in the presence of substances, such as lipids, known to interfere in many other protein determination methods.