Govind A. Dhopeshwarkar

Uptake and transport of fatty acids into the brain and the role of the blood-brain barrier system.

Publisher Summary This chapter discusses the uptake and transport of fatty acids into the brain and describes the role of the blood–brain barrier system. In addition to the usual saturated and unsaturated fatty acids such as palmitic, stearic, and oleic acids, the brain contains a rather large amount of polyunsaturated fatty acids with four, five, and six double bonds. Another class of fatty acids found largely in the brain is that of the α-hydroxy long-chain fatty acids. The brain has the necessary enzymes for synthesis of the required fatty acids, thus, reducing the demand on transport of fatty acids synthesized elsewhere or ingested in the diet. The essential fatty acids, which cannot be synthesized by the mammalian systems, must come from the diet and transported into the brain by the circulating blood. Both of these fatty acids are found in trace amounts in the rat brain; however, their polyunsaturated products occur abundantly. Dietary fatty acids can influence the composition of brain lipids.

Role of oleic acid in the metabolism of essential fatty acids

Groups of young male guinea pigs were fed diets containing corn oil, coconut oil, coconut oil plus elaidic acid, and coconut oil plus oleic acid. The oleic acid-fed group showed signs of essential fatty acid deficiency after four weeks and severe signs after eight weeks. The elaidic acid-fed group did not show these symptoms. It is proposed that oleic acid competes competitively with linoleic acid as a substrate for the enzymes involved in linoleate transformations when only a very limited supply of linoleic acid is available to the animals and oleic acid is made available in relatively large amounts.A detailed analysis of the serum, liver, and adipose tissue lipid and a study of the incorporation of acetate-1-C14 into different lipids is presented.

Biosynthesis of polyunsaturated fatty acids in the developing brain: I. Metabolic transformations of intracranially administered 1-14C linolenic acid

Thirteen-day old rats were given intracranial injections of 1-14C linolenic acid (allcis 9,12,15 octa decatrienoic acid) and were sacrificed after 8 hr. Analysis of brain fatty acids showed that 16∶0, 18∶0, 18∶1, 18∶3, 20∶3, 20∶4, 20∶5, 22∶5, and 22∶6 were labeled. The total fatty acid methyl esters were separated into classes according to degree of unsaturation on a AgNO3∶SiO2 impregnated plate. The bands were scraped off and the eluted fatty acids were first analyzed by radiogas liquid chromatography and then subjected to reductive ozonolysis to determine double bond position. The saturated acids, 16∶0, and 18∶0, as well as the mono-unsaturated 18∶1, must have been formed from radioactive acetate produced by β oxidation of the injected linolenate. Among the polyunsaturated fatty acids, the triene fraction was characterized and identified as 18∶3 ε3 (Δ9,12,15), the starting material, and 20∶3 ω3 (Δ11,14,17); the tetraene fraction was identified as 20∶4 ω3 (Δ8,11,14,17); the pentaene fraction was identified as 20∶5 ω3 (Δ5,8,11,14,17) and 22∶5 ω3 (Δ7,10,13,16,19); and, finally, the hexaene fraction was shown to be 22∶6 ω3 (Δ4,7,10,13,16,19). The biosynthesis of these ω3 family fatty acids in the brain in situ is discussed.

Thyroid control over biomembranes: VI. Lipids in liver mitochondria and microsomes of hypothyroid rats

The lipids of liver mitochondria prepared from normal rats and from rats made hypothyroid by thyroidectomy and injection with131INa contained similar amounts, per mg protein, of total lipids, phospholipids, neutral lipids and lipid phosphorus. Hypothyroidism caused a doubling of the relative amounts of mitochondrial cardiolipins (CL; to 20.5% of the phospholipid P) and an accompanying trend (although statistically not significant) toward decreased amounts of both phosphatidylcholines (PC) and phosphatidylserines (PS), with phosphatidylethanolamines (PE) remaining unchanged. The pattern of elevated 18∶2 fatty acyl content and depleted 20∶4 acyl groups of the mitochondrial phospholipids of hypothyroid preparations was reflected to varying degrees in the resolved phospholipids, with PC showing greater degrees of abnormality than PE, and CL showing none. Hypothyroidism produced the same abnormal pattern of fatty acyl distributions in liver microsomal total lipids as was found in the mitochondria. Hypothyroid rats, when killed 6 hr after injection of [1-14C] labeled linoleate, showed the following abnormalities: the liver incorporated less label into lipids, and converted 18∶2 not exclusively to 20∶4 (as normals do) but instead incorporated the label mainly into saturated fatty acids. These data, together with the known decrease in β-oxidation, suggest that hypothyroidism involves possible defective step(s) in the conversion of 18∶2 to 20∶4.

Fatty acid uptake by the brain V. incorporation of [1-14c]linolenic acid into adult rat brain

Abstract Linolenic acid seems to have special importance in the brain lipids since the ω-3 polyunsaturated acids are predominant brain fatty acids. Thus [I-14C]linolenic acid was fed to adult rats to investigate its passage from blood to the brain. Unlike palmitic and oleic acids that are present in large amounts linolenic acid is present in only trace amounts but its product docosahexaenoic acid is present in large amounts. Label distribution in brain docosahexaenoic acid 4 and 24 h after feeding linolenic acid showed the percentage of the total radioactivity in the carboxyl carbon was 23% and 12.4%, respectively, compared to 70.2% and 81.9% when [I-14C]acetate was injected. This pattern of distribution clearly shows that the fed material, [I-14C]linolenic acid, was incorporated into the brain lipids and directly converted into docosahexaenoic acid without appreciable oxidation to acetate. It was surprising to note that cerebrosides had a rather high specific activity when linoleic or linolenic acid were fed but not when acetate was injected.

Incorporation of [1-14C]acetate into the fatty acids of the developing rat brain

Abstract In order to gain information about passage of fatty acids from blood to the brain in weanling and adult rats, it was necessary to determine the pattern of fatty acid synthesis from [1-14C]acetate. The results of such a study were as follows: 1. 1. The uptake of intraperitoneally administered [1-14C]acetate was high in the brain but low in the liver of weanling rats. The reverse was noted in the adult animals. 2. 2. A decrease in the palmitate to stearate ratio and an increase in the oleate to stearate ratio was found in the older brains. Weanling rat liver, unlike the adult liver, was very rich in docosahexaenoic acid. 3. 3. Palmitic acid, both from liver and brain, irrespective of age, had the highest specific activity. A drop in the specific activity of fatty acids with increasing chain length from palmitic to docosahexaenoic was more pronounced in the brains of the young animals. 4. 4. The distribution of 14C among the carbons of palmitic acid confirmed its synthesis by the de novo synthetic processes except in the case of the weanling livers. With increasing chain length, the chain elongation process contributed most of the 14C incorporation. 5. 5. The chain elongation process, with respect to stearate and oleate, seemed to be more pronounced in the adult brain than in the weanling rats. However, the opposite seems to be true in the liver.

INTRACRANIAL CONVERSION OF LINOLEIC ACID TO ARACHIDONIC ACID: EVIDENCE FOR LACK OF Δ8 DESATURASE IN THE BRAIN

Eleven‐day old rats were given intracranial injection of [1‐14C]linoleic acid (all cis 9,12 octadecadienoic acid) and sacrificed after 8 h. Analysis of brain fatty acids showed that 16:0, 18:2, 20:2,20:3 and 20:4 were labeled. Separation by AgN03:Si02 TLC plates followed by reductive ozonolysis characterized thc polyunsaturated fatty acids as 18:2 (Δ9,12), 20:2 (Δ11,14), 20:3 (Δ8,11,14) and 20:4 (Δ5,8,11,14). A smaller amount of 18:3 (Δ6,9,12) was also identified. This initially suggested 20:2 (A1 1,14) as an intermediate in the optional pathway of biosynthesis of arachidonate. However, when [l‐14C]eicosadienoic acid (Δ1 1,141 itself was injected in the brain it was converted to 20:3 (Δ5,11,14) (a non‐methylene interrupted double bond system) rather than the expected 20:3 (Δ8,11,14). Only a small amount of arachidonate was formed from 20:2 (Δ11,14). Thus it was concluded that 20:2 (Δ11,14) was not an intermediate in the pathways of arachidonate biosynthesis due to lack of Δ5 desaturase in thc brain which agrees with the findings of SPKECRER & LEE (1975) in rat liver.

Metabolism of linolenic acid in developing brain: I. Incorporation of radioactivity from 1-14C linolenic acid into brain fatty acids

Twelve-thirteen day old rats were given 1-14C linolenic acid by intraperitoneal injection. Fatty acids were isolated from the brains of animals sacrificed at the end of 8 and 48 hr and 15 and 45 days. Eight hr after the tracer, radioactivity was found neither in 18∶3 nor its endproduct, 22∶6, and palmitate was the most highly radioactive component. At longer intervals, 22∶6 seemed to retain much of the radioactivity, whereas palmitate showed a precipitous decline in radioactivity. Initial oxidation of linolenate and sparing of the linolenate complexed with polar lipids are discussed.

Fatty acid uptake by the brain IV. Incorporation of [1-14C]linoleic acid into the adult rat brain

Abstract In order to investigate the passage of the essential fatty acid, linoleic acid, into the brain from the blood and its incorporation into the brain lipids, [1-14C]linoleic acid was fed to adult rats and the distribution of the label in their brain linoleic and arachidonic acids was investigated. I. 92% of the total radioactivity of brain linoleic acid was in the carboxyl carbon 4 h after feeding [1-14C]linoleic acid and 88% of the total radioactivity was retained in the carboxyl carbon even after 24 h. Arachidonic acid, formed from chain elongation and desaturation of linoleic acid, showed low activities (28% after 4 h and 3.5% after 24 h) in the carboxyl carbon. This distribution can be obtained only if the fed material, [1-14C]linoleic acid, is taken up directly by the brain and transformed into arachidonic acid by addition of acetate (derived in part by oxidative degradation of the fed material) and desaturation. In contrast, it was shown, using [1-14C] acetate, that in the absence of a highly radioactive precursor, linoleic acid, the arachidonic acid had most of the label in the carboxyl carbon.

Fatty acid transport into the brain

Abstract 15 μC of [I- 14 C]palmitate was injected into three groups of animals. Group I served as control animals, Group II animals were subjected to functional hepatectomy, and animals of both groups received the tracer intravenously and were killed 1 h after injection. Group III animals received the tracer by an intracarotid injection followed by decapitation 15 sec later. The circulating blood lipids in the control group had most of the radioactivity in the triglyceride fraction whereas in both hepatectomized and carotid injected animals the blood free fatty acid fraction had most of the radioactivity. Under these conditions the uptake of radioactivity by the brain was about 6 times higher in hepatectomized animals and 14 times higher in the intracarotid injected animals than in the control group. This indicates that free fatty acid is a preferred form of fatty acid transport to the brain.