Elongase Reactions as Control Points in Long-Chain Polyunsaturated Fatty Acid Synthesis

Abstract

BackgroundΔ6-Desaturase (Fads2) is widely regarded as rate-limiting in the conversion of dietary α-linolenic acid (18:3n-3; ALA) to the long-chain omega-3 polyunsaturated fatty acid docosahexaenoic acid (22:6n-3; DHA). However, increasing dietary ALA or the direct Fads2 product, stearidonic acid (18:4n-3; SDA), increases tissue levels of eicosapentaenoic acid (20:5n-3; EPA) and docosapentaenoic acid (22:5n-3; DPA), but not DHA. These observations suggest that one or more control points must exist beyond ALA metabolism by Fads2. One possible control point is a second reaction involving Fads2 itself, since this enzyme catalyses desaturation of 24:5n-3 to 24:6n-3, as well as ALA to SDA. However, metabolism of EPA and DPA both require elongation reactions. This study examined the activities of two elongase enzymes as well as the second reaction of Fads2 in order to concentrate on the metabolism of EPA to DHA.Methodology/Principal FindingsThe substrate selectivities, competitive substrate interactions and dose response curves of the rat elongases, Elovl2 and Elovl5 were determined after expression of the enzymes in yeast. The competitive substrate interactions for rat Fads2 were also examined. Rat Elovl2 was active with C20 and C22 polyunsaturated fatty acids and this single enzyme catalysed the sequential elongation reactions of EPA→DPA→24:5n-3. The second reaction DPA→24:5n-3 appeared to be saturated at substrate concentrations not saturating for the first reaction EPA→DPA. ALA dose-dependently inhibited Fads2 conversion of 24:5n-3 to 24:6n-3.ConclusionsThe competition between ALA and 24:5n-3 for Fads2 may explain the decrease in DHA levels observed after certain intakes of dietary ALA have been exceeded. In addition, the apparent saturation of the second Elovl2 reaction, DPA→24:5n-3, provides further explanations for the accumulation of DPA when ALA, SDA or EPA is provided in the diet. This study suggests that Elovl2 will be critical in understanding if DHA synthesis can be increased by dietary means.

Measurement of red blood cell eicosapentaenoic acid (EPA) levels in a randomised trial of EPA in patients with colorectal cancer liver metastases

“…In particular, post-treatment % RBC EPA values were similar to those observed in an eight-week study of exactly the same EPA-FFA formulation and dose in healthy volunteers and inflammatory bowel disease patients [23]. In keeping with previous studies of pure EPA treatment, there was some evidence of elongation of EPA to DPA leading to a small, statistically insignificant increase in RBC DPA incorporation [8,[23][24]. However, there was no evidence of DHA incorporation in RBC membranes, with an actual small reduction in % DHA content in EPA users, unlike the previous study with EPA-FFA [23].…”

“…This may be explained by the shorter duration of EPA supplementation in the EMT study, which may lead to displacement of membrane DHA by excess EPA-FFA, prior to de novo synthesis of DHA from EPA. Alternatively, it is understood that excess EPA may saturate the second ELOVL2 elongase reaction necessary for DPA to DHA conversion, which could lead to a reduced relative DHA content [24].…”

Metabolism and functional effects of plant-derived omega-3 fatty acids in humans

“…24:5ω-3 is then translocated to the peroxisome where it undergoes one round of β-oxidation to form DHA [63,67,68]. Recent mechanistic studies suggest conversion of EPA to DHA may be limited by the second product of Elovl2 (24:5ω-3) [69]. This suggests that understanding the action and regulation of Elovl2 may be critical in understanding DHA synthesis.…”

Omega-3 fatty acid supplementation and cardiovascular disease

“…The products and some intermediates of the pathway are typically assimilated into complex lipids as phospholipids used in cell membranes. Both FADS1 and FADS2 are required for C [20][21][22] PUFA synthesis, and FADS2 is considered the ratelimiting enzyme in the pathway ( 44,45 ). Fatty acid desaturation requires additional factors, including i ) fatty acyl CoA substrate, ii ) NADH, iii ) cytochrome B5, and iv ) cytochrome B5 reductase.…”