The biuret and Lowry procedures are used for protein determination. The former is widely used for clinical assays. The latter, though more sensitive, is used for investigative work and is limited by poor stability of combined reagents, non-reproducibility of color, especially at low protein concentration, and nonlinear chromogenic response with protein concentration. Ohnishi and Barr modified the biuret reagent for the Lowry procedure, thereby simplifying it while improving the stability of the combined reagent.
Colorimetric, Endpoint.assay. Ohnishi and Barr′s modification of micro Lowry method. According to procedure, dilute biuret reagent reacts with peptide bonds to yield a purple-blue complex, the color of which is intensified by the addition of phenol reagent. Absorbance, read at 550-750 nm, is used to determine results from a standard curve.