Effects of serum bilirubin on determination of uric acid by the uricase-peroxidase coupled reaction.

We examined the possible effect of different bilirubin species, including unconjugated (Bu), sugar-conjugated (Bc), and authentic delta bilirubin (Bd) isolated from serum, on the direct enzymatic measurement of uric acid by using the uricase (EC 1.7.3.3) and peroxidase (EC 1.11.1.7) coupled reaction. Bc, isolated from human bile, produced the most interference with uric acid determination. Although the presence of human serum albumin reduced interference by Bu and Bc, albumin-bound Bc complex still produced greater interference than Bu. Interference with the peroxidase reaction by Bc covalently bound to serum albumin (Bd) was less than that by Bu. We examined this phenomenon by using serum-isolated Bd obtained by ammonium sulfate precipitation and ultrafiltration and by using commercially available ditaurobilirubin (DTB) and chemically synthesized Bd (via Woodward's reagent, WBd) as surrogates for bile-isolated Bc and natural Bd, respectively. DTB produced the same amount of interference as natural Bc, but the interference produced by DTB in the presence of serum albumin was not the same as that produced by natural Bc with albumin. Our synthetic Bd appears to be a reliable surrogate for natural Bd in testing for bilirubin interference.