Molar Absorption Coefficients for Bilirubins in Adult and Infant Serum with Determination of an Isosbestic Point

The molar absorption coefficient curve (ϵ-curve) for bilirubin in human sera is determined in a tetraborate buffer at pH=9.3 in the wavelength interval 430 to 600 nm by enrichment of adult and infant human sera with bilirubin in a solution of potassium cyanide in formamide (15). The curve in adults does not differ significantly (P> 0.1) from that obtained with sodium hydroxide as the main primary solvent for bilirubin; adult and infant sera (from the umbilical cord) give different ϵ-values (P< 0.01). The ϵ-curve for conjugated bilirubin is obtained by subtracting the absorbance of hemoglobin and turbidity from native sera with a high concentration of conjugated bilirubin. The coordinates for an isosbestic point is given (470 nm, ϵ=47.7 × 102m2mol−1). The ϵ-curve of oxyhemoglobin and the rules for turbidity as used in the calculations for conjugated bilirubin are given.