Lipoxygenase (LOX), a non-heme iron protein, catalyzes the dioxygenation of methylene-interrupted (Z,Z)-pentadiene fatty acids to furnish conjugated hydroperoxydiene fatty acids. In the plant kingdom, the substrate fatty acids are usually linoleic acid or linolenic acids. However, plant LOXs, like their animal counterparts, are fully capable of oxidizing animal polyunsaturated fatty acids such as arachidonic acid. Most LOXs can oxidize glyceride polyunsaturated fatty acids as well. Linoleic acid usually serves as the model substrate. Hydroperoxide products of linoleic acid are usually either 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid (13S-HPODE), 9(S)-hydroperoxy-10(E),12(Z)octadecadienoic acid (9S-HPODE), or both (Figure C4.2.1). Other isomers have been found as LOX products, but usually the minor amounts of (E,E)-diene isomers observed are racemic, indicating that they arise from the rearrangement of fatty acid peroxyl radicals formed by either enzymic or autooxidation processes. Although LOX activity is important to the plant's defense against pathogens, there are negative aspects of the enzyme in foods. LOX activity and the resulting fatty acid hydroperoxide products initiate free radical chains that modify proteins (particularly residues of Trp, His, Cys, Tyr, Met, and Lys) as well as vitamins or their precursors (e.g., carotene and tocopherol). Evidence of such free radical reactions is often visibly observed as loss of carotenoid/chlorophyll pigments in improperly blanched frozen foods. Another consequence of these free radical reactions is the development of potent off-flavors, many of which originate from decomposition of the fatty acid hydroperoxide products. As seen in Figure C4.2.1, there are two obvious methods of readily measuring LOX activity, that is, by oxygen uptake and ultraviolet (UV) absorbance of the conjugated hydroperoxy-diene product. Basic Protocol 1 describes the polarographic measurement Contributed by Harold W. Gardner