Interactions of Quaternary Ammonium Compounds and Proteins

A simple met.hod of titration with a cationic detergent has been developed which permits rapid estimation of the concentration of urinary protein with a minimum of manipulations and time. The method depends on the formation of an insoluble anion-cation complex between quaternary ammonium ions, hereinafter referred to as the cationic detergent, and proteins, in the present instance mainly human albumin, at alkaline pH. No protein is precipitated from solution by the addition of the cationic detergent until a slight excess of this is present, at which time the solution shows faint but definite turbidity. Addition of more cationic detergent results at first in increasing turbidity, but as the addition is continued the turbidity decreases, almost as abruptly as it appeared, to give a nearly clear solution. Further addition of the cationic detergent is not attended by reappearance of turbidity. The end-point used in the present method is the appearance of a definite turbidity. The protein-quaternary ammonium ion complex formation is sensitive to ionic strength, non-electrolyte concentration, pH, and type of protein present (see, for example, Polonovski and Macheboeuf (l), Schmidt (2), and Valko (3)). For this reason, the method is restricted for accurate results to urine protein of the order of 2 or more gm. per liter (0.4 or more mg. of protein per sample) in order that the dilutions made may be sufficient to reduce the factors of urine ionic strength, non-electrolytes, and pH to negligible proportions. The method is, however, applicable to most cases of significant proteinuria. The simplicity of the procedure is indicated by the fact that we have been able to complete nearly 200 analyses in an afternoon. If the end-point is determined by the optical density change in a photometer, the method can be used for samples containing as little as 0.02 mg. of protein.