Phospholipid molecular species from isolated bovine rod outer segments incorporate exogenous fatty acids at different rates

Dietary omega 3 fatty acids and the developing brain

The omega-3 fatty acids are essential dietary nutrients and one of their important roles is providing the fatty acid with 22 carbons and 6 double bonds known as docosahexaenoic acid (DHA) for nervous tissue growth and function. Inadequate intakes of omega-3 fatty acids decrease DHA and increase omega-6 fatty acids in the brain. Decreased DHA in the developing brain leads to deficits in neurogenesis, neurotransmitter metabolism, and altered learning and visual function in animals. Western diets are low in omega-3 fatty acids, including the 18 carbon omega-3 fatty acid alpha linolenic acid found mainly in plant oils, and DHA, which is found mainly in fish. The DHA status of the newborn and breast-fed infant depends on the maternal intake of DHA and varies widely. Epidemiological studies have linked low maternal DHA to increased risk of poor child neural development. Intervention studies have shown improving maternal DHA nutrition decreases the risk of poor infant and child visual and neural development. Thus, sufficient evidence is available to conclude that maternal fatty acid nutrition is important to DHA transfer to the infant before and after birth, with short and long-term implications for neural function. However, genetic variation in genes encoding fatty acid desaturases also influence essential fatty acid metabolism, and may increase requirements in some individuals. Consideration of omega-3 fatty acid to include brain development, optimizing omega-3 and omega-6 fatty acids in gestation and lactation, and in fatty acid nutrition support for intravenous and formula-fed neonates is important.

Effect of light and protein phosphorylation on photoreceptor rod outer segment acyltransferase activity

Rod outer segments (ROS) exhibit high acyltransferase (AT) activity, the preferred substrate of which being lysophosphatidylcholine. To study factors possibly regulating ROS AT activity purified ROS membranes were assayed under conditions under which protein kinase C (PKC), cAMP-dependent protein kinase (PKA), and phosphatases were stimulated or inhibited. PKC activation produced a significant increase in the acylation of phosphatidylethanolamine (PE) and phosphatidylinositol (PI) with oleate, it inhibited phosphatidylcholine (PC) acylation, and phosphatidylserine (PS) and phosphatidic acid (PA) acylation remained unchanged. ROS PKA activation resulted in increased oleate incorporation into PS and PI while the acylation of PC, PE, and PA remained unchanged. Inhibition of ROS PKC or PKA produced, as a general trait, inverse effects with respect to those observed under kinase-stimulatory conditions. ROS phosphatase 2A was inhibited by using okadaic acid, and the changes observed in AT activity are described. These findings suggest that changes in ROS protein phosphorylation produce specific changes in AT activity depending on the phospholipid substrate. The effect of light on AT activity in ROS membranes was also studied and it is reported that acylation in these membranes remains unchanged independent of the illumination condition used.

Effect of diet on the fatty acid and molecular species composition of dog retina phospholipids

Dogs were born to mothers fed commercial diets low or enriched in n-3 fatty acids and raised on those diets until they were about 50 d old. Retinas were removed, lipids were extracted, and total phospholipids were analyzed for fatty acid and molecular species composition. Animals from the low n-3 group had significantly lower retinal levels of 22:6n-3 and higher levels of n-6 fatty acids, especially 20:4n-6 and 22:5n-6. There was no difference in the retinal levels of 18:2n-6, and only small differences were found in saturated and monounsaturated fatty acids. The most dramatic differences in molecular species occurred in 22:6n-3-22:6n-3 (4.7 vs. 0.8%) and 18:0-22:6n-3 (27.6 vs. 14.4%); total molecular species containing 22:6n-3 were significantly lower in the low n-3 group (45.5 vs. 24.0%). Molecular species containing 20:4n-6 and 22:5n-6 were greater in the low n-3 animals (13.0 vs. 25.7%), as were molecular species containing only saturated and monounsaturated fatty acids (40.8 vs. 35.4%). These results show that modest differences in the amount of n-3 fatty acids in the diets of dogs can have profound effects on the fatty acid and molecular species composition of their retinas.

Post-Golgi vesicles cotransport docosahexaenoyl-phospholipids and rhodopsin during frog photoreceptor membrane biogenesis

Post-Golgi vesicles budding from the trans-Golgi network (TGN) are involved in the vectorial transport and delivery of rhodopsin to photoreceptor rod outer segments (ROS). We report here that newly synthesized docosahexaenoyl (DHA) phospholipids are sequestered and cotransported by rhodopsin-bearing post-Golgi vesicles to ROS. Frog retinas were pulse-labeled with [35S]methionine/cysteine and [3H]DHA prior to ROS isolation and subcellular fractionation. After a 1-h pulse, relatively uniform [3H]DHA-lipid labeling (DPM/microg protein) was observed in all fractions enriched in post-Golgi vesicles, TGN, Golgi, and endoplasmic reticulum (ER) membranes. During the subsequent 2-h chase translocation of free [3H]DHA from ROS to the photoreceptor inner segment contributed to an additional overall increase in labeling of lipids. The specific activity (dpm/nmol DHA) in ER-enriched fraction was similar or higher than in other subcellular fractions after both the pulse and the chase, indicating that the bulk of [3H]DHA-lipids was synthesized in the ER. After the chase a 2-fold increase in labeling of lipids in the ER and Golgi and a 2.6-fold in lighter TGN-enriched fractions was observed. The highest labeling was in the post-Golgi vesicle fraction (4-fold increase), with [3H]DHA-phosphatidylcholine and [3H]DHA-phosphatidylethanolamine showing the greatest increase. At the same time, newly synthesized [35S]rhodopsin shifted from the ER and Golgi toward TGN and post-Golgi fractions. Therefore, sequestration and association of [35S]rhodopsin and [3H]DHA-lipids in a TGN membrane domain occurs prior to their exit and subsequent vectorial cotransport on post-Golgi vesicles to ROS. Labeling of ROS lipids was very low, with phosphatidylinositol and diacylglycerols displaying the highest labeling. This indicates that other mechanisms by-passing Golgi, i.e. facilitated by lipid carrier proteins, may also contribute to molecular replacement of disc membrane DHA-phospholipids, particularly phosphatidylinositol.

Acyl-CoA:lysophosphatidylcholine acyltransferase activity in bovine retina rod outer segments

In the present paper the properties of acyl-CoA:lysophosphatidylcholine acyltransferase activity associated with rod outer segments (ROS) have been studied. Under adequate experimental conditions, ROS acyl-CoA:lysophosphatidylcholine acyltransferase activity presented a maximum at pH 7.0. The enzyme was able to incorporate as much as 60% of the label offered as [1-14C]oleoyl-CoA into phosphatidylcholine after 5 min of incubation. The use of varying concentrations of oleoyl-CoA and 46 microM lysophosphatidylcholine gave an apparent K(m) value for oleoyl-CoA of 100 microM and a Vmax value of 153 nmol x h-1 x (mg protein)-1. The use of varying concentrations of lysophosphatidylcholine and 100 microM oleoyl-CoA gave an apparent K(m) value for lysophosphatidylcholine of 27 microM and a Vmax value of 155 nmol x h-1 x (mg protein)-1. The enzyme was inhibited by 25% when ROS membranes were incubated in the presence of 10 mM MgCl2. The acyltransferase was able to incorporate other acyl-CoAs (palmitoyl-CoA and arachidonoyl-CoA) into ROS phospholipids and to acylate other lysophospholipids but less efficiently than lysophosphatidylcholine. Lysophoshatidylcholine was preferentially acylated with arachidonic acid followed by oleic acid and, less efficiently, with palmitic acid. The high specific activity of acyl-CoA lysophosphatidylcholine acyltransferase found in purified ROS compared to the activity found in other subcellular fractions of the bovine retina suggests that this enzymatic activity is native to the ROS.

Phosphatidylethanolamine N-methyltransferase activity in isolated rod outer segments from bovine retina

Phosphatidylcholine (PC) can be synthesized in isolated rod outer segments from bovine retina by successive transfer of methyl groups from S-adenosyl-L-methionine (SAM) to phosphatidylethanolamine (PE), with the intermediate formation of phosphatidyl-N-monomethylethanolamine (PMME) and phosphatidyl-N,N-dimethylethanolamine (PDME). This reaction is time-protein-and SAM concentration-dependent. Phosphatidylethanolamine N-methyltransferase (PE N-MTase) has two pH optima, 8.5 and 10, at low (10 microM) and high (200 microM) SAM concentrations and requires magnesium ions for full activity. When ROS membranes were incubated at 5 to 200 microM SAM concentrations at pH 8.5 or pH 10, the major methylated product was PMME, followed by PC and PDME. The apparent Kms for SAM at pH 8.5 and at pH 10 were similar (37 and 38 microM, respectively). The Vmax was 13 pmol h-1 (mg protein)-1 at pH 8.5 and 12.50 pmol h-1 (mg protein)-1 at pH 10. Pulse-chase experiments demonstrated a precursor-product relationship with [3H]PC as the end product. The level of PE N-Mtase activity in the purified ROS preparation obtained from crude ROS fractions by discontinuous sucrose gradient centrifugation, was as high as 65% of the level found in the microsomal fraction obtained from the remainder of the retinas. The presence of microsomal and mitochondrial marker enzymes, however, was minimal in the ROS preparation. The radioactivity incorporated into ROS PC was measured in an upper and lower band of PC obtained by two-dimensional TLC. We found that the amount of [methyl-3H] groups incorporated into the upper PC band was 2.5-fold greater than that incorporated into the lower one. The fatty acid composition of the upper band was very different from that of the lower band, the former being enriched in very long-chain polyunsaturated fatty acids and the latter in saturated fatty acids. Phosphatidyl-ethanolamine N-methyltransferase activity increased in the presence of exogenous phospholipid substrates. PDME being augmented ten-fold and PC eight-fold when the incubations were carried out in the presence of PMME and PDME, respectively. At a 2 mM concentration, S-adenosyl-L-homocysteine (SAH) inhibited the methyl groups' incorporation into the endogenous phospholipids by 40%. When ROS membranes were selectively depleted of soluble or peripheral and soluble proteins, the PE N-MTase activity remained mainly associated to the membrane, suggesting that this enzyme (s) is an intrinsic membrane protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Conservation of docosahexaenoic acid in the retina

Over the last several years, evidence has accumulated that n-3 fatty acids, particularly 22:6n-3, are essential for the development of the structure and function of the visual system. The importance of 22:6n-3 is reflected in the tenacious manner in which the retina conserves this fatty acid during n-3 deficiency. We have shown that conservation is achieved by recycling 22:6n-3 within the retina or between the retina and the pigment epithelium. Within the retina, recycling could be accomplished by deacylation-reacylation reactions (Louie et al., 1991; Zimmerman and Keys, 1988). Recycling between the retina and the RPE may be achieved through specific transport proteins, possibly interphotoreceptor retinoid-binding protein (Bazan et al., 1985) and/or apolipoprotein E (Bazan et al., 1991).