Carole Subramanian

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.

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.

Rapid uptke of [1-14C] acetate by the adult rat brain 15 seconds after carotid injection

Abstract [1-14C]Acetate was injected into the carotid artery of adult rats followed by decapitation after 15 sec. Brain, liver and blood lipids were extracted. Analysis of the lipids showed the following: 1. 1. There was considerably more uptake of radioactivity in the brain as compared to the liver or the plasma. 2. 2. High radioactivity in the cholesterol fraction indicating rapid synthesis from radioactive acetate. 3. 3. Incorporation of radioactivity into all major polar lipid fractions of the brain lipids including sphingomyelin and cerebrosides, considered to be myelin lipids. Phosphatidyl serine was the most highly labeled component. 4. 4. Palmitic acid isolated from the brain was synthesized de novo from acetate and stearic acid was formed by chain elongation. All these metabolic reactions occurring so rapidly in the brain are discussed in view of the older concept that adult brain is a tissue characterized by slow metabolism.

Metabolism of 1-14C linolenic acid in developing brain: II. Incorporation of radioactivity from 1-14C linolenate into brain lipids

Metabolism of 1-14C linolenic acid was studied in growing animals by injecting the tracer intraperitoneally into 12–13 day old suckling rats and following up the results by sacrificing groups of animals at 8 hr, 48 hr, 15 day, and 45 day intervals. In the first 15 days, there was a greater decrease in radioactivity of brain total lipids compared to the later period, although the earlier age period is characterized by lipid deposition rather than breakdown. Since the 18∶3 ω3 family of fatty acids occurs largely in the brain total phosphatidyl ethanolamine fraction, we expected that, in the initial period, total phosphatidyl ethanolamine would be the most highly radioactive component. However, results showed that 8 hr after the tracer phosphatidyl choline had the highest specific radioactivity. When the total phosphatidyl ethanolamine fraction was resolved into diacyl and alk-1-enyl species, it was found that radioactivity was not distributed evenly between the two species. There was a progressive increase in radioactivity of the alkenyl and a decrease in the diacyl species. Forty-eight hr after the tracer, however, the radioactivity of phosphatidyl ethanolamine increased and at 45 days remained slightly higher than phosphatidyl choline. Radioactivity of cholesterol, a result of synthesis from acetate undoubtedly derived from the breakdown of tracer linolenate, was also high 48 hr after tracer and remained high until 45 days.

Metabolism of [1-14c]palmitic acid in the developing brain:Persistence of radioactivity in the carboxyl carbon

Abstract [1- 14 C]Palmitic acid was injected intraperitoneally into young rats (12–13 daysold) to study the label distribution over an extended period of 2 months during whichtime rapid growth and active myelination occurs. It was found that, 24 h after injection,brain palmitate still had 78% relative carboxyl activity. It was also noted thatlabel persisted in the carboxyl carbon of brain palmitate even after an extended periodof 2 months. Radioactivity distribution and specific activities of 18:0, 18:1, 20:4 and 22:6,conclusively showed that these fatty acids were synthesized by chain elongation.Similarly, radioactivity in the cholesterol fraction indicated that injected palmitatewas degraded to form radioactive acetate; thus it was surprising to find that radioactivityin brain palmitate was not redistributed by de novo synthesis from radioactiveacetate. Hypothetical possibilities for this finding are discussed.