Rats fed a purified nonlipid diet containing vitamins A and D exhibit a reduced growth rate and reproductive deficiency which may be cured by the addition of linoleic, a-linolenic, and arachidonic acids to the diet. These fatty acids are found in high concentrations in vegetable oils (Table 14-2) and in small amounts in animal carcasses. These essential fatty acids are required for prostaglandin, thromboxane, leukotriene, and lipoxin formation (see below), and they also have various other functions which are less well defined. Essential fatty acids are found in the structural lipids of the cell, often in the 2 position of phospholipids, and are concerned with the structural integrity of the mitochondrial membrane.
Arachidonic acid is present in membranes and accounts for 5-15% of the fatty acids in phospholipids. Docosahexaenoic acid (DHA; ffl3, 22:6), which is syn-
m 9 Family
m 6 Family
Accumulates in essential fatty acid deficiency
m 3 Family
Figure 23-3. Biosynthesis of the m9, m6, and m3 families of polyunsaturated fatty acids. Each step is catalyzed by the microsomal chain elongation or desaturase system: 1, elongase; 2, A6 desaturase; 3, A5 desaturase; 4, A4 desaturase. (©, Inhibition.)
2H2O + NAD
MICROSOMAL CHAIN ELONGATION SYSTEM (ELONGASE)
14 11 B
2H2O + NAD
Arachidonoyl-CoA (A5,8, -eicosatetraenoyl-CoA)
Figure 23-4. Conversion of linoleate to arachido-nate. Cats cannot carry out this conversion owing to absence of A6 desaturase and must obtain arachidonate in their diet.
thesized from a-linolenic acid or obtained directly from fish oils, is present in high concentrations in retina, cerebral cortex, testis, and sperm. DHA is particularly needed for development of the brain and retina and is supplied via the placenta and milk. Patients with retinitis pigmentosa are reported to have low blood levels of DHA. In essential fatty acid deficiency, nonessential polyenoic acids of the 0)9 family replace the essential fatty acids in phospholipids, other complex lipids, and membranes, particularly with A5,8,11-eicosatrienoic acid (o9 20:3) (Figure 23-3). The triene:tetraene ratio in plasma lipids can be used to diagnose the extent of essential fatty acid deficiency.
Trans Fatty Acids Are Implicated in Various Disorders
Small amounts of trans-unsaturated fatty acids are found in ruminant fat (eg, butter fat has 2-7%), where they arise from the action of microorganisms in the rumen, but the main source in the human diet is from partially hydrogenated vegetable oils (eg, margarine). Trans fatty acids compete with essential fatty acids and may exacerbate essential fatty acid deficiency. Moreover, they are structurally similar to saturated fatty acids (Chapter 14) and have comparable effects in the promotion of hyper-cholesterolemia and atherosclerosis (Chapter 26).
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