B. Medwadowski

Linolenic acid deficiency.

Linolenic acid deficiency has not been demonstrated clearly in warm blooded animals, yet circumstantial evidence suggests that n−3 fatty acids may have functions in these animals. The fact that several species of fish definitely require dietary n−3 fatty acids indicates that n−3 fatty acids have important and specific functions in these animals and suggests that such functions may also be present in warm blooded animals. It is also true that n−3 fatty acid distribution in tissues of birds and mammals appears to be under strict metabolic control, and that this complex metabolic control mechanism apparently has survived evolutionary pressure for a very long time. So far, attempts to produce linolenic acid deficiency in mammals have not revealed an absolute requirement for n−3 fatty acids. If functions for n−3 fatty acids do exist in warm blooded animals, it seems probable that they may be located in the cerebral cortex or in the retina, because these tissues normally contain high concentrations of n−3 fatty acids.

Linolenic acid deficiency: Changes in fatty acid patterns in female and male rats raised on a linolenic acid-deficient diet for two generations

Rats were fed for two generations a purified, linolenic acid-deficient diet in which the only source of lipid was purified methyl linoleate. This diet contained about 38 mg linolenic acid/kg diet. Control rats were given the same diet supplemented with methyl linolenate (2,500 mg/kg diet). Male and female rats ranged in age from weanling pups to adults. Lipids were extracted from liver, brain, kidney, spleen, heart, muscle, gastrointestinal tract, lung, ovary, testis, adrenal, plasma, erythrocytes, retina, and adipose tissue. Fatty acids of major phospholipid classes (choline phosphoglycerides, ethanolamine phosphoglycerides, and mixed serine phosphoglycerides plus inositol phosphoglycerides) or of total lipid extracts were measured by gas liquid chromatography. Growth rates and organ weights were similar in control and linolenic acid-deficient rats. The major effect of the deficiency was to lower the proportions of n−3 fatty acids, especially 22∶6 n−3, in all the organs analyzed. Docosahexaenoic acid (22∶6 n−3) was mainly replaced by 22∶5 n−6 in deficient rats. The greatest changes in composition were found in brain, heart, muscle, retina, and liver.

Depletion of docosahexaenoic acid in retinal lipids of rats fed a linolenic acid-deficient, linoleic acid-containing diet

Rats were raised for 2 generations on a diet in which 1.25% methyl linoleate was the only source of fat. Control rats were given 1.0% methyl linoleate plus 0.25% methyl linolenate. Lipids were extracted from retinas and their fatty acids were analyzed by gas-liquid chromatography. Docosahexaenoic acid accounted for 33.8% of total fatty acids in control retinas, for 13% of fatty acids in first-generation deficient retinas, and for 2.7% of fatty acids in second-generation deficient retinas.

The lipids of krill (Euphausia species) and red crab (Pleuroncodes planipes)

The composition of the lipids of two samples of krill and one of “red crab” was determined by thin layer, column and gas chromatographic procedures. A large number of unusual fatty acids were present.

Effect of low methionine, choline deficient diets upon major unsaturated phosphatidyl choline fractions of rat liver and plasma

To see how the metabolism of specific phosphatidyl choline fractions might be affected when only a limited source of methyl groups was available, rats were fed for 7 days a low methionine, cholinedeficient diet or one supplemented with either choline or methionine. Prior to killing, they were injected with14C-methyl methionine and liver and plasma phosphatidyl choline isolated and separated by argentation chromatography into 3 major unsaturated fractions. Fatty acid composition and radioactivity of the fractions were determined. Deficient rats had reduced total liver phosphatidyl choline when compared with the supplemented groups, but the proportions of 20∶4 and 22∶6 fatty acids in the total phosphatidyl choline were unchanged. Plasma phosphatidyl choline also was reduced sharply by the deficiency, as was its proportion of 20∶4 fatty acid. Specific activities of the liver 22∶6, 20∶4, and 18∶2 phosphatidyl choline fractions showed that deficient rats had less radioactivity in their 20∶4 and 18∶2 phosphatidyl choline than did the supplemented animals. Plasma phosphatidyl choline fractions presented a similar pattern. Feeding methionine or choline nearly doubled radioactive methyl group incorporation into the 20∶4 phosphatidyl choline fraction of liver and plasma, while incorporation into the 22∶6 phosphatidyl choline was reduced or unchanged. The results suggested that, in the rat, limited availability of methyl groups altered the metabolism of liver and plasma phosphatidyl choline fractions. Methionine, as a source of labile methyl groups, appears necessary for the normal synthesis of certain unsaturated phosphatidyl choline fractions (particularly 20∶4 phosphatidyl choline). Transmethylation of phosphatidyl ethanolamine molecular species to the corresponding phosphatidyl choline species may be an important reaction in normal lipid metabolism and transport. Relative affinities for incorporation of the labeled methyl groups into the phosphatidyl choline fractions of either deficient or supplemented rats were: 22∶6>20∶4>18∶2.

Ethanolamine kinase activity and compositions of diacylglycerols, phosphatidylcholines and phosphatidylethanolamines in livers of choline-deficient rats.

These experiments were performed to find the reasons for the increased concentrations of docosahexaenoyl phosphatidylethanolamines (PE) in livers of choline-deficient rats. We measured the activity of ethanolamine kinase, which catalyzes the first step in PE formation. We also measured the compositions of PE and phosphatidylcholines (PC) and concentrations and fatty acid compositions of diacylglycerols (DG), which are precursors of PE. Young male rats were fed for one week a low-methionine, choline-deficient diet, or the same diet supplemented with choline. Ethanolamine kinase activity was measured in liver cytosol (100,000 g supernatant). Fatty acids were measured in total liver diacylglycerols and in microsomal PE and PC. Ethanolamine kinase activities were equal in choline-deficient and choline-supplemented rats. Concentrations of DG were elevated 6-fold by choline deficiency. The percentage of docosahexaenoic acid (22∶6n−3) in microsomal PE was nearly doubled by choline deficiency. Although the increased concentrations of PE in choline-deficient livers cannot be attributed to increased activity of ethanolamine kinase, the rate of PE formation probably was increased by increases in concentrations of its precursors, including DG. The disproportionate increase in 22∶6n−3 PE probably was caused by a selective formation of PE from DG that contain 22∶6n−3.

Effect of storage on lipids of fish protein concentrate

Fish protein concentrates (FPC) made from pout, alewife and Gulf menhaden were stored for 6 months at temperatures of 37 C and 50 C with no attempt to control humidity. When the amount of extractable lipid was 0.1% or less there were small changes in the lipid pattern: a small decrease in the amount of neutral lipid-free fatty acid fraction and larger decreases in the long chain, unsaturated fatty acids (C20:5, C22:6 for pout and C20:5 for alewife). In the FPC sample containing residual lipid of about 0.5% there were decreases in the amount of lipid extractable after 6 months, appreciable in the 37 C sample, and greater in the sample stored at 50 C. These samples also showed decreases in the neutral lipid plus free fatty acid fractions and decreases in the long chain, unsaturated fatty acids C20:5 and C22:6.

Incorporation of 14C-Docosahexaenoic Acid into Lipids of Rats

Summary 14C-docosahexaenoic acid, prepared by administration of 14C1-linolenic acid to essential fatty acid-deficient rats, was isolated from liver phospholipids and converted to methyl docosahexaenoate, sp act 8 X 105 dpm/mg, purity about 90%. This was converted to the free acid and injected into rats. 14C-docosahexaenoic acid was incorporated mainly into carcass and liver. In liver, radioactivity appeared most rapidly in PE and later in PC. This behavior is unlike the incorporations of 16:0, 18:0, 18:1n9, 18:2n6, or 20:4n6, which are all incorporated most quickly into PC. Most of the radioactivity was recovered as 22:6n3, although considerable activity was present in C-16 and C-18 chains.