A sensitive, direct colorimetric assay of serum copper using 5-Br-PSAA.

A direct colorimetric assay of serum copper in 0.2-ml sized samples, using a new, water-soluble reagent, 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino) aniline Na salt (5-Br-PSAA epsilon = 6.5 X 10(4) l/mol per cm at 580 nm), is presented. Protein-bound serum copper is dissociated with guanidine HCl which acts also as a protein denaturant. Metal ions, except for cobalt and palladium, do not affect the determination even when present at levels exceeding those normally found in serum. Although bilirubin does interfere with the Cu-5-Br-PSAA complex formation, the copper values in the icteric sera with total bilirubin concentrations of less than 137 mumol/l (80 mg/l) can be corrected by a correction factor which varies with the bilirubin content. Within-run and between-run precisions were in the ranges of 0.8-2.3% and 1.1-3.0% coefficient of variation (CV). Good correlation was obtained between this method and atomic absorption spectrometry.