"Oxylipin" has been introduced as a collective term for oxygenated compounds biosynthesized from fatty acids by sequences involving at least one step of mono- or dioxygenase-catalyzed oxygenation. A large part of oxylipins in animals or plants is biosynthesized from polyunsaturated fatty acids by action of enzyme(s) of the dioxygenase type. Further conversion of the resulting unstable peroxide or hydroperoxide derivatives results in a range of compounds often exerting potent biological effects.
    Oxylipins formed in the animal and plant kingdoms differ significantly in terms of chemical structure. This can be ascribed to the facts that animals and plants display different fatty acid profiles and possess partly different sets of enzymes catalyzing biosynthesis and metabolism of oxylipins. Despite these differences, the logic of the processes leading to oxylipin formation in animals, plants and fungi show striking similarities.

In the animal kingdom, the C₂₀ fatty acid arachidonic acid serves as the most important precursor of oxygenated derivatives, compounds commonly referred to as "eicosanoids" (gr. eicosa, twenty) because of their carbon chain length. Primary oxygenation of arachidonic acid and other fatty acids in animal tissue is mainly catalyzed by prostaglandin endoperoxide synthases¹ and lipoxygenases² leading to a number of oxygenated derivatives such as prostaglandin H₂, leukotriene A₄, and various fatty acid hydroperoxides. These intermediates are further modified by secondary enzymes including prostaglandin E, D, and F synthases, thromboxane A synthase, prostacyclin synthase, leukotriene A₄ hydrolase, and leukotriene C₄ synthase to generate members of the prostaglandin, leukotriene and thromboxane families³. Another pathway involves primary oxygenation by enzymes of the monooxygenase type leading to the formation of epoxy, hydroxy, and dihydroxy derivatives³. Non-enzymatical oxygenation of arachidonic acid and other polyunsaturated fatty acids can also take place resulting in the isoprostane group of compounds⁴.
    Many oxylipins formed in animal tissue exert potent biological effects, and some of them have been found to play major roles in physiological processes such as aggregation of blood platelets in man and luteolysis in many species. Others participate in pathological processes such as inflammatory reactions and asthma.
    In higher plants, the two C₁₈ fatty acids linoleic and linolenic acids serve as the most important precursors of oxylipins. Primary oxygenation is catalyzed by lipoxygenases, α-dioxygenases and monooxygenases belonging to the peroxygenase or cytochrome P-450 families^5-7. Higher plants lack prostaglandin endoperoxide synthase activity, however, it is noteworthy that α-dioxygenase, present in higher plants and certain algae, as well as linoleate diol synthase, present in certain fungi, are heme proteins which are structurally related to the animal prostaglandin endoperoxide synthases 1 and 2.
    Enzymes involved in the further conversion of lipoxygenase-generated hydroperoxides in plant tissue include allene oxide synthase, allene oxide cyclase, divinyl ether synthase, hydroperoxide lyase, peroxygense, and epoxy alcohol synthase^5-8. The resulting cascade of oxylipins consists of compounds incorporating oxo, hydroxy, ether, epoxy, and aldehyde functionalities. As in animals, non-enzymatical oxygenation can also take place leading to a dinor-isoprostane group of compounds⁹.
    Oxylipins appear to participate in the multitude of defense reactions taking place in plants against insects, fungi, bacteria and other pathogens. One specific lipoxygenase product, i.e., the 13(S)-hydroperoxide derivative of linolenic acid, is of particular importance because of its role as precursor in the biosynthesis of 12-oxo-10,15(Z)-phytodienoic acid, which is further converted to the plant hormone jasmonic acid.
    The attached schemes summarize the pathways of oxylipin biosynthesis from linoleic and linolenic acids in higher plants.

Intense research is devoted to the chemistry, biochemistry, and molecular biology of oxylipins. Participation of these compounds in inflammation, hemostasis and possibly certain malignancies such as colon cancer has led to the development of several new drugs including COX-2 inhibitors and antileukotrienes.
    Research in the plant area has stressed the biological importance of oxylipins in various defense reactions counteracting infections caused by pathogens. Current research aims at engineering of crops having an increased resistance to pests, thus requiring less use of toxic agrochemicals.