Seasonal dynamics of fatty acid composition in female northern pike (Esox lucius L.)

Seasonal changes in the fatty acid composition of neutral and polar lipids were measured in the ovary, liver, white muscle, and adipopancreatic tissue of northern pike. The role of environmental and physiological factors underlying these changes was evaluated. From late summer (August–September) to winter (January–March), the weight percentage of n-3 polyunsaturated fatty acids (especially 22:6n3) declined significantly in the neutral lipids of all somatic tissues examined. However, large quantities of n-3 polyunsaturated fatty acids accumulated in the recrude cing ovaries during fall and the weight percentage of n-3 polyunsaturated fatty acids in ovary polar lipids also increased significantly. Additionally, the n-3 polyunsaturated fatty acid content of somatic polar lipids increased significantly during fall due to increases in the total polar lipid content of the somatic tissues. This suggests that during fall n-3 polyunsaturated fatty acid are diverted away from somatic neutral lipids and thereby conserved for use in ovary construction and for incorporation into tissue polar lipids. The percentage of n-3 polyunsaturated fatty acid in ovary neutral lipids also declined during fall and early winter, perhaps as an adaptation to conserve these fatty acids for storage in oocyte polar lipids and later incorporation into cellular membranes of the developing embryo. Reductions in the n-3 polyunsaturated fatty acids content of somatic and ovarian neutral lipids during fall were compensated for specifically by increases in the percentage of monounsaturated fatty acids rather than saturated fatty acids. This suggests that the ratio of saturated to unsaturated fatty acids in pike neutral lipid, is regulated physiologically, and hence may influence the physiological functioning of these lipids. During fall and early winter the percentage of saturated fatty acids declined significantly in the polar lipids of all tissues examined. This change was consistent with the known effects of cold acclimation on the fatty acid composition of cellular membranes. As the ovaries were recrudescing from September to January, liver polar lipids exhibited significant decreases in the percentage of total polyunsaturated fatty acids and n-3 polyunsaturated fatty acids and increases in monounsaturated fatty acids, and acquired a fatty acid composition very similar to that of ovary polar lipids. Therefore, seasonal changes in the percentage of polyunsaturated and monounsaturated fatty acids in liver polar lipids probably reflect the liver's role in vitellogenesis rather than the effects of temperature on membrane fatty acid composition. At all times of year, the fatty acid compositions of white muscle and adipopancreatic tissue neutral lipids were very similar, which may indicate a close metabolic relationship between these lipid compartments.Abbreviations AP adipopancreatic - BHT butylated hydroxytoluene - CI confidence interval - EFA essential fatty acids - MUFA monounsaturated fatty acids - NL neutral lipids - PL polar lipids - PUFA polyunsaturated fatty acids - SFA saturated fatty acids     

Interplay of unsaturated phospholipids and cholesterol in membranes: effect of the double-bond position

The structural and dynamical properties of lipid membranes rich in phospholipids and cholesterol are known to be strongly affected by the unsaturation of lipid acyl chains. We show that not only unsaturation but also the position of a double bond has a pronounced effect on membrane properties. We consider how cholesterol interacts with phosphatidylcholines comprising two 18-carbon long monounsaturated acyl chains, where the position of the double bond is varied systematically along the acyl chains. Atomistic molecular dynamics simulations indicate that when the double bond is not in contact with the cholesterol ring, and especially with the C18 group on its rough β-side, the membrane properties are closest to those of the saturated bilayer. However, any interaction between the double bond and the ring promotes membrane disorder and fluidity. Maximal disorder is found when the double bond is located in the middle of a lipid acyl chain, the case most commonly found in monounsaturated acyl chains of phospholipids. The results suggest a cholesterol-mediated lipid selection mechanism in eukaryotic cell membranes. With saturated lipids, cholesterol promotes the formation of highly ordered raft-like membrane domains, whereas domains rich in unsaturated lipids with a double bond in the middle remain highly fluid despite the presence of cholesterol.     

Dynamics of lipid and fatty acid composition of shallow-water corals under thermal stress: an experimental approach

Coral bleaching induces changes in lipid and fatty acid composition that result in low lipid content, reducing the likelihood of coral survival. Species-specific differences in the metabolism of lipid reserves may contribute to the differential resistance of corals under acute heat exposures. Here, we examined the dynamics of lipids and fatty acid abundance in corals subjected to short-term heat stress. The stony corals Acropora intermedia, Montipora digitata, and the soft coral Sinularia capitalis all showed a 60–75% decline in both storage and structural lipids. However, S. capitalis and M. digitata exhibited no significant change in the percentages of structural lipids (i.e., polar lipids and sterols) until they had lost 90–95% of their endosymbionts, whereas A. intermedia showed a rapid decline in structural lipids after a 50% loss of symbionts. After a 90–95% loss of symbionts under heat stress, all three corals showed a relative depletion of polyunsaturated fatty acids that had symbiont biomarkers, suggesting that polyunsaturated fatty acids were translocated from the symbiont to the coral host tissue.     

Seasonal dynamics of fatty acid biomarkers in the soft coral Sinularia flexibilis,a common species of Indo-Pacific coral reefs

Year-round fluctuation in environmental conditions have a considerable influence on the level of biomarkers, that are usually applied for monitoring marine invertebrates. To date, no data on annual cycles of fatty acid (FA) biomarkers in tropical soft corals are available. The FA composition of the alcyonarian Sinularia flexibilis containing symbiotic microalgae was studied during a year. Two major FAs, 20:4n-6 and 16:0, determined the level of polyunsaturated and saturated FAs (PUFAs and SFAs). The insignificant variations in PUFA content indicated a relatively stable level of phospholipids that formed the structural base of coral cell biomembranes. No clear relationship between the PUFA markers of photosynthetic symbionts and the solar radiation was found. The three-fold increase in the SFA level in the summer correlated with the water temperature and indicated a rise in the level of neutral lipids that made up the energy reserve of corals. The seasonal changes in FA biosynthesis, membrane fluidity, and coral nutrition are discussed.     

Characterization of fatty acid composition in healthy and bleached corals from Okinawa, Japan

Under bleaching conditions, corals lose their symbiotic zooxanthellae, and thus, the ability to synthesize fatty acids (FAs) from photosynthetically derived carbon. This study investigated the lipid content and FA composition in healthy and bleached corals from the Odo reef flat in Okinawa, southern Japan, following a bleaching event. It was hypothesized that the FA composition and abundance would change as algae are lost or die, and possibly microbial abundance would increase in corals as a consequence of bleaching. The lipid content and FA composition of three healthy coral species (Pavona frondifera, Acropora pulchra, and Goniastrea aspera) and of partially bleached and completely bleached colonies of P. frondifera were examined. The FA composition did not differ among healthy corals, but differed significantly among healthy, partially bleached, and completely bleached specimens of P. frondifera. Completely bleached corals contained significantly lower lipid and total FA content, as well as lower relative amounts of polyunsaturated FAs and higher relative amounts of saturated FAs, than healthy and partially bleached corals. Furthermore, there was a significantly higher relative concentration of monounsaturated FAs and odd-numbered branched FAs in completely bleached corals, indicating an increase in bacterial colonization in the bleached corals.     

Lipids in plant tissue cultures. II. Unusual fatty acids in lipids of Hydnocarpus anthelminthica cultures

The lipids in callus cultures of Hydnocarpus anthelminthica were studied after 60, 160 and 460 days of growth. In each of the cultures the lipid classes usually found in plant tissue cultures were detected. With increasing age of the cultures the total lipid content as well as the proportions of triglycerides decreased. The major constituent fatty acids of the total lipids were palmitic and linoleic acids. Small amounts of cyclopentenyl fatty acids were also present. The proportions of saturated straight-chain fatty acids increased with the age of the cultures whereas the proportions of monounsaturated straight-chain fatty acids decreased. Only small changes were observed with polyunsaturated fatty acids. The content of cyclopentenyl fatty acids rose with the age of the cultures. The monounsaturated straight-chain fatty acids consisted of mixtures of isomers whose composition changed with the age of the cultures. In contrast, the polyunsaturated straight-chain fatty acids belonged exclusively to the Δ9 series, regardless of the age of the cultures.     

Different strategies of energy storage in cultured and freshly isolated Symbiodinium sp.

The endosymbiotic relationship between cnidarians and Symbiodinium is critical for the survival of coral reefs. In this study, we developed a protocol to rapidly and freshly separate Symbiodinium from corals and sea anemones. Furthermore, we compared these freshly‐isolated Symbiodinium with cultured Symbiodinium to investigate host and Symbiodinium interaction. Clade B Symbiodinium had higher starch content and lower lipid content than those of clades C and D in both freshly isolated and cultured forms. Clade C had the highest lipid content, particularly when associated with corals. Moreover, the coral‐associated Symbiodinium had higher protein content than did cultured and sea anemone‐associated Symbiodinium. Regarding fatty acid composition, cultured Symbiodinium and clades B, C, and D shared similar patterns, whereas sea anemone‐associated Symbiodinium had a distinct pattern compared coral‐associated Symbiodinium. Specifically, the levels of monounsaturated fatty acids were lower than those of the saturated fatty acids, and the level of polyunsaturated fatty acids (PUFAs) were the highest in all examined Symbiodinium. Furthermore, PUFAs levels were higher in coral‐associated Symbiodinium than in cultured Symbiodinium. These results altogether indicated that different Symbiodinium clades used different energy storage strategies, which might be modified by hosts.     

Fatty acids and other lipids of corals: Composition, distribution, and biosynthesis

This paper reviews the literature data on the composition, structure, and distribution of the main lipid classes and their fatty acids in reef-building and soft corals and in hydrocorals of the genus Millepora. The review presents information about more than 150 coral species from tropical and cold waters of the World Ocean, as well as data on the influence of environmental factors on the lipid compositions of corals. The possible pathways of polyunsaturated fatty acid biosynthesis in corals are discussed.     

Effects of fatty acids on the growth of Caco-2 cells

Epidemiological studies suggest that polyunsaturated fatty acids may protect against colorectal neoplasia. In order to explore this observation, cell proliferation and viability, lipid composition, membrane fluidity, and lipid peroxidation were measured in Caco-2 cells after 48h incubation with various fatty acids. Saturated and monounsaturated fatty acids incorporated less well in the membranes than polyunsaturated fatty acids (PUFAs). All of the PUFAs tested had an inhibitory effect on cell proliferation/viability whereas the saturated and monounsaturated fatty acids did not. Addition of palmitic acid had no significant effect on membrane fluidity whereas unsaturated fatty acids increased membrane fluidity in a dose-dependent manner. PUFAs strongly increased tumor cell lipid peroxidation in a dose-dependent manner. Saturated and monounsaturated fatty acids increased lipid peroxidation in this cell line only at high concentration. Preincubation of Caco-2 cells with vitamin E prevented the inhibition of proliferation/viability, the elevation of the MDA concentration and the increased membrane fluidity induced by PUFAs. Our data indicate that PUFAs are potent inhibitors of the growth of colon cancer cells in vitro.     

Metabolism of palmitic and docosahexaenoic acids in Reuber H35 hepatoma cells

In this work, we have modified the fatty acid composition of Reuber H35 hepatoma cells by supplementation of the culture medium with a saturated (palmitic) or a polyunsaturated (docosahexaenoic) acid. These fatty acids were incorporated into total lipids and phospholipids of hepatoma cells. Palmitic acid readily increased the percentage of its monounsaturated derivative (16:1 n-7). When both fatty acids were supplemented at the same concentration, the percentage of docosahexaenoic acid in the total lipids and phospholipids of Reuber H35 cells increased more than that of palmitic acid. Although the levels of 16:0 increased, the addition of docosahexaenoic acid to the culture medium decreased the percentages of monoenoic acids. From our results, it can be concluded that palmitic and docosahexaenoic acids modify the fatty acid composition of Reuber H35 hepatoma cells. The profound changes induced by docosahexaenoic acid, especially those in the phospholipid fraction, may be of great interest given the main role of these components in the regulation of chemical and physical properties of biological membranes and/or membrane systems.     

A Comparison of the Composition of Wax Ester Molecular Species of Different Coral Groups (Subclasses Hexacorallia and Octocorallia)

Wax esters, which are esters of fatty alcohols and fatty acids (FAs), are one of the main classes of reserve lipids in all coral species. The chemical structures and the content of wax ester molecular species were determined for the first time in nine coral species from three taxonomic groups: symbiotic reef-building corals, (Hexacorallia subclasses), symbiotic soft coral alcyonarians, and asymbiotic soft coral gorgonians (Octocorallia subclasses) collected in the South China Sea (Vietnam). Our comparison of these groups showed that the absence of symbiotic microalgae (zooxanthellae) and the exoskeleton affects the profile of molecular species of wax esters considerably. The main components of wax esters of all corals were cetyl palmitate (16:0-16:0) and other saturated wax esters containing 30, 34, and 36 carbon atoms. The content of unsaturated molecular species 6:0–16:1, 16:0–18:1, and 16:0–20:1 in wax esters of symbiotic soft corals (alcyonarians) was greater than that in wax esters of reef-building corals. In contrast to symbiotic coral species, wax esters of asymbiotic soft corals, namely azooxanthellate gorgonians, contained a considerable amount of long-chain molecular species (C₃₇-C₄₁) with an odd number of carbon atoms. The presence of such molecular species indicates that asymbiotic gorgonians may use bacterial FAs in biosynthesis of their own wax esters. This observation confirms our hypothesis that bacterial community is important for maintaining the energy balance of azooxanthellate corals.     

Lipid biosynthesis and composition in oil bodies and microsomes of olive fruit

Both lipid synthesis and composition in oil bodies and microsomes of olive fruit at the first stage of development have been studied. The rate of fatty-acid synthesis in isolated oil bodies was saturated by 4.0 microM [2-14C]-malonyl-CoA. The fatty-acids synthesized of phospholipids and neutral lipids were saturated and monounsaturated. Neutral lipids, galactolipids and, above all, phospholipids were the major acyl-lipid components of microsomal fraction, oleic and palmitic being their principal fatty-acids. When the lipids of microsomes were labelled in vivo with [1-14C]-acetate, phospholipids and neutral lipids exhibited a higher biosynthesis rate relative to the galactolipids. The increase in saturated and monounsaturated fatty-acid synthesis in microsomes, was also accompanied by an important [1-14C]-acetate incorporation into polyunsaturated acids. The data presented here, in conjunction with our previous morphological results, suggest the possibility that olive fruit oil bodies could contain the necessary enzymes for the reserve lipid biosynthesis.     

Polyunsaturated fats,membrane lipids and animal longevity

Fatty acids are essential for life because they are essential components of cellular membranes. Lower animals can synthesize all four classes of fatty acids from non-lipid sources, but both omega-6 and omega-3 cannot be synthesized de novo by ‘higher’ animals and are therefore essential components of their diet. The relationship between normal variation in diet fatty acid composition and membrane fatty acid composition is little investigated. Studies in the rat show that, with respect to the general classes of fatty acids (saturated, monounsaturated and polyunsaturated) membrane fatty acid composition is homeostatically regulated despite diet variation. This is not the case for fatty acid composition of storage lipids, which responds to diet variation. Polyunsaturated fatty acids are important determinants of physical and chemical properties of membranes. They are the substrates for lipid peroxidation and it is possible to calculate a peroxidation index (PI) for a particular membrane composition. Membrane PI appears to be homeostatically regulated with respect to diet PI. Membrane fatty acid composition varies among species and membrane PI is inversely correlated to longevity in mammals, birds, bivalve molluscs, honeybees and the nematode Caenorhabditis elegans.     

Lipid class and fatty acid compositions of the zoanthid Palythoa caesia (Anthozoa: Hexacorallia: Zoanthidea) and its chemotaxonomic relations with corals

Lipid class and fatty acid (FA) compositions of the zoanthid Palythoa caesia from the South China Sea (Vietnam) were determined and compared with those of zooxanthellate reef-building and soft corals from the same region to clarify chemotaxonomic relations between zoanthids and corals. Total lipids of P. caesia and corals were formed by the same lipid classes. The lipids of P. caesia contained specific for cnidarians monoalkyldiacylglycerols and phosphonolipids. The proportion between reserve and structural lipid classes of P. caesia was similar to that of alcyonarians (soft corals), but significantly differed from reef-building corals. This similarity may be caused by the similar energy budget of cnidarian colonies without hard exoskeleton. The distribution of polyunsaturated FAs (PUFAs), such as 22:4n-6 and 22:5n-3, indicated pathways of biosynthesis of PUFAs in Palythoa to be closer to those in reef-building corals than in soft corals. The differences in the profiles of common FAs between P. caesia and reef-building corals were showed. Zoanthid lipids contained rare Δ5,9 non-methylene-interrupted (NMI) PUFAs (with two and three double bounds), which probably originated from microorganisms associated with P. caesia. Trienoic acids Δ5,9,15–24:3 and Δ5,9,17–24:3 were found in Palythoa for the first time. NMI PUFAs, as well as total FA profile, can be used for chemotaxonomy of Palythoa.     

Rhythmical changes in the division and degradation of symbiotic algae in hermatypic corals

Three types of rhythmical changes in division and degradation (digestion) have been found and described in the symbiotic zooxanthellae of the scleractinian coral Seriatopora caliendrum and in the colonial hydroid Myllepora intricata collected at the reef of Sesoko Island (Okinawa, Japan). The first type of changes reported for S. caliendrum is daily variation of the intensity of these two processes peaking at night. The second type of rhythm changes described in S. caliendrum and M. intricata is opposing variations of cell division and degradation levels with a period of three days. The rhythm is disturbed by sudden weather changes or by variations in light. Rhythmical alterations of zooxanthella proliferation and degradation of the third type found in S. caliendrum have a period of 5–6 days and are connected with oppositely directed changes as well as with alterations of the second type. The rhythm of such alterations is not disturbed by abrupt weather changes. All three types of rhythmical changes in zooxanthellae division and digestion observed in hermatypic coral are supposed to be directed to the regulation of the population density of zooxanthellae in organism tissues.     

Effects of ethanol on the remodeling of neutral lipids and phospholipids in brain mitochondria and microsomes

We have analyzed the effects of ethanol in vitro on the remodeling of neutral lipids and phospholipids in mitochondria and microsomes isolated from chick brain. We used three different fatty acyl-CoAs of similar chain lengths but different degrees of unsaturation. Our results demonstrate the existence of active mechanisms for acyl-CoA transfer into neutral lipids and phospholipids in both mitochondria and microsomes. The profile of fatty acid incorporation was clearly different according to the membrane and lipid fraction in question. Thus, in mitochondrial lipids, the remodeling processes showed a clear preference for the saturated fatty acid whilst the polyunsaturated one was the preferred substrate for microsomal lipid acylation. With regard to the effects of ethanol in vitro, we were able to demonstrate that exposure of the membrane to ethanol led to an increase in the incorporation of polyunsaturated fatty acid into triacylglycerol (TG) in both mitochondria and microsomes, indicating that it directly stimulates the acylation of diacylglycerol (DG) to give TG. This effect may then contribute to the widely reported stimulation of TG biosynthesis in cases of both acute and chronic ethanol ingestion. It is noteworthy that the exposure of microsomes to ethanol in vitro also stimulated the incorporation of oleoyl-CoA into the aminophospholipids phosphatidylethanolamine (PE) and phosphatidylserine (PS). We also demonstrate that both mitochondria and microsomes synthesize fatty acid ethyl esters (FAEEs) from fatty acyl-CoA, although there is a clear difference in preference for the fatty acid used as substrate in the esterification of the alcohol. Thus, mitochondria were capable of forming FAEEs from the polyunsaturated fatty acid whilst in microsomes the saturated fatty acid was the preferred substrate. In both types of membrane, FAEE production was lowest with the monounsaturated fatty acyl-CoA.     

Lipid biomarkers in Symbiodinium dinoflagellates: new indicators of thermal stress

Lipid content and fatty acid profiles of corals and their dinoflagellate endosymbionts are known to vary in response to high-temperature stress. To better understand the heat-stress response in these symbionts, we investigated cultures of Symbiodinium goreauii type C1 and Symbiodinium sp. clade subtype D1 grown under a range of temperatures and durations. The predominant lipids produced by Symbiodinium are palmitic (C16) and stearic (C18) saturated fatty acids and their unsaturated analogs, the polyunsaturated fatty acid docosahexaenoic acid (C22:6, n-3; DHA), and a variety of sterols. Prolonged exposure to high temperature causes the relative amount of unsaturated acids within the C18 fatty acids in Symbiodinium tissue to decrease. Thermal stress also causes a decrease in abundance of fatty acids relative to sterols, as well as the more specific ratio of DHA to an algal 4-methyl sterol. These shifts in fatty acid unsaturation and fatty acid-to-sterol ratios are common to both types C1 and D1, but the apparent thermal threshold of lipid changes is lower for type C1. This work indicates that ratios among free fatty acids and sterols in Symbiodinium can be used as sensitive indicators of thermal stress. If the Symbiodinium lipid stress response is unchanged in hospite, the algal heat-stress biomarkers we have identified could be measured to detect thermal stress within the coral holobiont. These results provide new insights into the potential role of lipids in the overall Symbiodinium thermal stress response.     

Experimental responses to elevated water temperature in genotypes of the reef coral<Emphasis Type="Italic"> Pocillopora damicornis</Emphasis> from upwelling and non-upwelling environments in Panama

The authors investigated the response to experimentally elevated water temperature in genotypes of Pocillopora damicornis from three coral reefs in the upwelling Gulf of Panama and four coral reefs in the non-upwelling Gulf of Chiriquí, Panamanian Pacific. Sea-surface temperature in the Gulf of Panama declines below 20 °C during seasonal upwelling, while in the thermally stable Gulf of Chiriquí, the temperature ranges from 27 to 29 °C. Genotypes of P. damicornis from the seven locations were determined by allozyme electrophoresis. The most abundant genotype at each location was selected for a thermal tolerance experiment where corals were exposed to water temperature of 30 °C (1 °C above ambient) for 43 days. Four site coral genotypes can be uniquely differentiated by the GPI locus, two by the LGG-2 locus, and two by a combination of the MDH-1, LGG-2, and LTY-3 loci. A visual assessment of the coral condition after exposure to an elevated temperature showed that corals from localities in the non-upwelling environment retained a normal to slightly pale appearance, while corals from the upwelling environment bleached and their polyps were mostly retracted. A two-way ANOVA confirmed that corals were significantly affected by water temperature and locality. The zooxanthellae were also significantly affected by the interaction of elevated temperature and locality of the corals. Mean zooxanthellae density decreased by 25 and 55%, respectively, in experimentally heated corals from the non-upwelling and upwelling environments. Low concentrations of photosynthetic pigments per live area of the corals were the norm in corals under elevated temperature. The mean concentration of chlorophyll a per live area of the corals was reduced by 17 and 49%, respectively, in heated corals from the non-upwelling and upwelling sites. Coral genotypes from the upwelling Gulf of Panama demonstrated higher vulnerability to thermal stress than coral genotypes from the non-upwelling Gulf of Chiriquí. However, the latter showed greater differences in their responses. Thus, even at small geographic scales, corals can display different levels of tolerance to thermal stress. The difference in thermal tolerance between corals from upwelling and non-upwelling environments is concomitant with greater genetic differences in experimental corals from the thermally stable Gulf of Chiriquí compared with corals from the upwelling Gulf of Panama.Communicated by K.S. Sealey     

Membrane lipids and sodium pumps of cattle and crocodiles: an experimental test of the membrane pacemaker theory of metabolism

The influence of membrane lipid composition on the molecular activity of a major membrane protein (the sodium pump) was examined as a test of the membrane pacemaker theory of metabolism. Microsomal membranes from the kidneys of cattle (Bos taurus) and crocodiles (Crocodylus porosus) were found to possess similar sodium pump concentrations, but cattle membranes showed a four- to fivefold higher enzyme (Na(+)-K(+)-ATPase) activity when measured at 37 degrees C. The molecular activity of the sodium pumps (ATP/min) from both species was fully recoverable when delipidated pumps were reconstituted with membrane from the original source (same species). The results of experiments involving species membrane crossovers showed cattle sodium pump molecular activity to progressively decrease from 3,245 to 1,953 (P < 0.005) to 1,031 (P < 0.003) ATP/min when subjected to two cycles of delipidation and reconstitution with crocodile membrane as a lipid source. In contrast, the molecular activity of crocodile sodium pumps progressively increased from 729 to 908 (P < 0.01) to 1,476 (P = 0.01) ATP/min when subjected to two cycles of delipidation and reconstitution with cattle membrane as a lipid source. The lipid composition of the two membrane preparations showed similar levels of saturated ( approximately 31-34%) and monounsaturated ( approximately 23-25%) fatty acids. Cattle membrane had fourfold more n-3 polyunsaturated fatty acids (11.2 vs. 2.9%) but had a reduced n-6 polyunsaturate content (29 vs. 43%). The results support the membrane pacemaker theory of metabolism and suggest membrane lipids and their polyunsaturates play a significant role in determining the molecular activity of the sodium pump.