I.U.B.: 1.11.1.7

Lactoperoxidase (LPO) is a glycoprotein with a hemin prosthetic group which may occur as a mixture of two isozymes. It has a molecular weight of 77,500 daltons. LPO catalyzes the hydrogen peroxide oxidation of iodide according to the following reaction:

2I^- + H₂O₂ + 2H⁺ → I₂ + 2H₂O.

Iodide reacts directly with the heme group; upon addition of H₂O₂ the complex iodinates the substrate. LPO is inhibited by hydrazines. The assay procedure has been updated from that of Morrison to an ABTS^®/H₂O₂ based method with increased sensitivity and reproducibility.

Unit Definition: One Unit reduces one micromole of hydrogen peroxide per minute at 25°C, pH 6.0. Note: This unit definition is based upon a new assay method. The previous method yielded approximately 2.8 times higher results.