ó-Linolenic acid or GLA (INN: gamolenic acid) is an nâÂÂ6, or omega-6, fatty acid found primarily in seed oils. When acting on GLA, arachidonate 5-lipoxygenase produces no leukotrienes and the conversion by the enzyme of arachidonic acid to leukotrienes is inhibited.
GLA is categorized as an nâÂÂ6 (also called ÃÂâÂÂ6 or omega-6) fatty acid, meaning that the first double bond on the methyl end (designated with n or ÃÂ) is the sixth bond. In physiological literature, GLA is designated as 18:3 (nâÂÂ6). GLA is a carboxylic acid with an 18-carbon chain and three cis double bonds. It is a regioisomer of ñ-linolenic acid, which is a polyunsaturated nâÂÂ3 (omega-3) fatty acid, found in rapeseed canola oil, soybeans, walnuts, flax seed (linseed oil), perilla, chia, and hemp seed.
GLA was first isolated from the seed oil of evening primrose. This herbal plant was grown by Native Americans to treat swelling in the body. In the 17th century, it was introduced to Europe and became a popular folk remedy, earning the name king's cure-all. In 1919, Heiduschka and Lüft extracted the oil from evening primrose seeds and described an unusual linolenic acid, which they name ó-. Later, the exact chemical structure was characterized by Riley.
Although there are ñ- and ó- forms of linolenic acid, there is no ò- form. One was once identified, but it turned out to be an artifact of the original analytical process.
GLA is relatively rate in the human diet. It is present in small amounts in oats and barley. According to the USDA Standard Release, it is present in hemp seeds (1.34% by mass), some vegetable oil products (0.59% in soft margarine), and some meats (0.1% in chicken skin). USDA lists it as not found in canola, soybean, corn, coconut, or flaxseed oil, which is surprising given the reported presence in margarine and salad dressings.
Spirulina lipid contains 18-21% GLA. However, only 7.8% of Spirulina is lipid by dry mass, so the overall content by dry mass is closer to 1.23%.
The human body produces GLA from linoleic acid (LA). This reaction is catalyzed by ÃÂ<sup>6</sup>-desaturase (D6D), an enzyme that allows the creation of a double bond on the sixth carbon counting from the carboxyl terminus. LA is consumed sufficiently in most diets, from such abundant sources as cooking oils and meats. However, a lack of GLA can occur when there is a reduction of the efficiency of the D6D conversion (for instance, as people grow older or when there are specific dietary deficiencies) or in disease states wherein there is excessive consumption of GLA metabolites.
Significant amounts of GLA are found in specialty seed oils such as evening primrose (Oenothera biennis) oil (EPO, 8–14%), blackcurrant seed oil (15âÂÂ20%), borage seed oil (17-25%), Lithospermum purpurocaeruleum oil (18%), Onosmodium molle oil (20%), Symphaticum officinale oil (27%), and hemp seed oil (0.5–4.5%). Normal safflower (Carthamus tinctorius) oil does not contain GLA, but a genetically modified GLA safflower oil available in commercial quantities since 2011 contains 40% GLA. These concentrated sources are useful in the production of GLA dietary supplements.
Fungi are also prolific producers of GLA. Some fungi produce biomass with lipid levels comparable to seeds (up to 70% by dry mass). These fungi can therefore be grown industrially to obtain large amounts of GLA-rich (13.2–49%) fungal lipids. The fungi Mucor circinelloides and Mucor plumbeus are prolific producers of GLA, efficient enough for industrial use. Other useful fungi include Mortierella vinacea, Mortierella ramanniana, Mortierella isabellina, Rhizopus stolonifera, Rhizopus nigricans, Rhizopus oryzae, and Cunninghamella elegans.
From GLA, the body forms dihomo-ó-linolenic acid (DGLA). This is one of the body's three sources of eicosanoids (along with AA and EPA.) DGLA is the precursor of the prostaglandin PGH<sub>1</sub>, which in turn forms PGE<sub>1</sub> and the thromboxane TXA<sub>1</sub>. Both PGE1<sub>1</sub> and TXA<sub>1</sub> are anti-inflammatory; thromboxane TXA<sub>1</sub>, unlike its series-2 variant, induces vasodilation, and inhibits platelet consequently, TXA<sub>1</sub> modulates (reduces) the pro-inflammatory properties of the thromboxane TXA<sub>2</sub>. PGE<sub>1</sub> has a role in regulation of immune system function and is used as the medicine alprostadil.
Unlike AA and EPA, DGLA cannot yield leukotrienes. However, it can inhibit the formation of pro-inflammatory leukotrienes from AA.
Although GLA is an nâÂÂ6 fatty acid, a type of acid that is, in general, pro-inflammatory, it has anti-inflammatory properties. (See discussion at Essential fatty acid interactions: The paradox of dietary GLA.)
In 2002, the UK Medicines and Healthcare products Regulatory Agency withdrew marketing authorisations for evening primrose oil as an eczema remedy. The BMJ commented in 2003 that it had taken 20 years to demonstrate that the substance was of no use in atopic dermatitis, and called for more transparency in the research on which drug licensing decisions were taken.