ABSTRACT
There are two groups of essential fatty acids (EFAs), (n-6) and (n-3) EFAs. Linoleic acid (LA, 18:2 n-6)) and α-linolenic acid (ALA, 18:3 n-3) are the parent fatty acids of the (n-6) and (n-3) EFAs, respectively. Other members of the (n-6) and (n-3) families can be derived (through desaturation and elongation) from their parent EFAs and thus are called derived EFAs. It is commonly believed that both pathways involving (n-6) and (n-3) EFA metabolism share the same enzymes (Fig 9.1). In the (n-6) family, LA can be converted to γ-linolenic acid (GLA) by action of ∆6-desaturase and further to dihomo-γ-linolenic acid (DGLA), the precursor of 1-series prostaglandins (PGs) and 3-series leukotrienes (LTs), by elongase. DGLA can be also converted by ∆5-desaturase to arachidonic acid (AA) which is the precursor of 2-series PGs and 4-series LTs. ∆6-Desaturase is the rate-limiting factor in the (n-6) EFA metabolism cascade (1,2), and it is influenced by changes in metabolic and endocrine regulation as well as by the progression of certain diseases. For example, ∆6 desaturase activity has been demonstrated to be increased by insulin and inhibited by epinephrine, cortisol, thyroxin, glucagon, saturated fat and aging (3-5). Decreased levels of GLA are reported in patients with inflammatory diseases (6-8). GLA supplementation has been used to by-pass ∆6-desaturase and to increase levels of GLA and other derived EFAs, in particular 20-carbon precursors of eicosanoids in phospholipids. GLA in substantial amounts is found naturally in plant oils such as evening primrose oil (9%), borage oil (23%) and black currant seed oil (18%). These oils are commonly used in most studies as the sources of GLA.
