Biosynthesis of retinal phospholipids by base exchange reactions

Levels of ethanolamine intermediates in the human and rat visual system structures: comparison with neural tissues of a lower vertebrate (Mustelus canis) and an invertebrate (Loligo pealei)

Levels of ethanolamine intermediates in the retina and optic nerves of autopsied human donors and in the rat visual system (retina, optic nerve, lateral geniculate body, superior colliculus) were measured. Amounts were also obtained from the retina, optic nerve, and optic tectum of a primitive elasmobranch, the smooth dogfish Mustelus canis, and from the related nervous structures (retina, optic lobe, fin nerve, and stellate ganglia) of a marine invertebrate, the squid Loligo pealei. In all regions of the human and rat nervous system, the pool size of CDP-ethanolamine (values ranging between 10-31 nmol/g wet wt) was much smaller than that of free ethanolamine (values ranging between 197-395 nmol/g wet wt), whereas glycerophosphorylethanolamine was present in relatively high content (values ranging between 125-280 nmol/g wet wt). In nervous system regions of the dogfish and squid, the distribution of values followed the same general trend as observed for humans and rats, even if all regions had less ethanolamine intermediates compared to the mammalian counterpart. In dogfish and squid retina, glycerophosphorylethanolamine showed the highest pool size among the ethanolamine derivatives analyzed (16 and 44 nmol/g wet wt, respectively). The present study confirms the basic similarity of ethanolamine intermediate pool size patterns in the nervous system structures (with the exception of the retina) of animal species which have widely different phylogenetic positions. The data support the proposal that the levels reached by ethanolamine and its derivatives in the nervous tissue is the result of an ancient evolutionary development of metabolic pathways for the maintenance of phosphatidylethanolamine membraneous content.

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

The incorporation of radiolabeled palmitic (16:0), oleic (18:1), and docosahexaenoic (22:6) acids into different molecular species of membrane phospholipids was investigated in isolated bovine rod outer segments (ROS). Phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) were isolated, and their diacylglyceroacetate and diacylglycerobenzoate derivatives were prepared, separated by HPLC, quantified, and assayed for radioactivity. Maximal incorporation of fatty acids occurred within 15-30 min. The rate of incorporation of the fatty acids into PC was three to six times higher than it was into PS or PE. The rate of incorporation of 22:6 into the molecular species, 22:6-22:6, of PC was ten to 15 times higher than into that of PE or PS, and it was three to four times higher than the incorporation rates of 22:6 into the other 22:6-containing molecular species. Similarly, incorporation of 18:1 into 18:1-22:6 was ten to 30 times more rapid in PC than in PE and PS, but in both PE and PS, 18:1 was incorporated into 18:1-22:6 at a rate of 20 to 25 times higher than the incorporation into the other molecular species analysed. For PC, incorporation of 16:0 was most rapid into 16:0-16:0, but for PE and PS it was most rapid into 16:0-20:4; for all cases, incorporation of 16:0 into these molecular species was four to six times more rapid than into the other 16:0-containing molecular species. These results are further evidence for the presence within a membranous organelle, the ROS, of an active acylation-deacylation system that is selective with regard to phospholipid, molecular species of phospholipid, and fatty acid.

Metabolism of lipid molecular species in rat rod outer segments

We have studied the metabolism of selected diacyl molecular species of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) and diacylglycerol (DG) from rat rod outer segments (ROS). Rats were injected intravitreally with [2-3H]glycerol. At 1, 2, 3, 4 and 6 days post-injection, ROS phospholipids and DG were isolated by two-dimensional thin-layer chromatography (TLC), derivatized, and fractionated into molecular species by high-performance liquid chromatography (HPLC). Selected molecular species were quantitated and counted for radioactivity. We found the following. In PC and PE, the specific activities of 22:6-22:6, 18:1-22:6 and 16:0-22:6 were highest at day 1 and then decreased in a nearly exponential manner. In contrast, the specific activities of 18:0-22:6 and 18:0-20:4 were substantially lower than these three molecular species and changed little over the 6-day period. In PS, the specific activities of 22:6-22:6, 18:0-22:6 and 18:1-22:6 were similar and did not reach their maximum until the 3rd or 4th days. In PC, the specific activities of the five molecular species examined were two to three times higher at day 1 than the same species in PE and PS. In PI and DG, the major molecular species were 16:0-20:4 and 18:0-20:4. The specific activities of these two molecular species at day 1 were about ten times higher than those of 20:4-containing species in PE and PC, and showed the most rapid turnover of any of the molecular species examined in this study.(ABSTRACT TRUNCATED AT 250 WORDS)

Unique molecular species composition of glycerolipids of frog rod outer segments

The composition and metabolism of molecular species of glycerolipids, including phosphatidic acid (PA), phosphatidylinositol (PI) and diacylglycerol (DG), were studied in four frog retinal fractions prepared by discontinuous sucrose gradient centrifugation. Six glycerolipid classes were isolated from the lipid extracts of each fraction and converted to their corresponding 1,2-diacylglycerol acetates by acetolysis for quantitation of their molecular species by HPLC. Rod outer segments (ROS) showed a distinctive molecular species composition in all glycerolipid classes except phosphatidylcholine (PC). The relative amounts of dipolyunsaturated species in ROS were higher in phosphatidylethanolamine (PE), phosphatidylserine (PS), and PA, compared to the other retinal fractions. PI and DG of ROS had a similar molecular species composition and contained only small amounts of dipolyunsaturated species. A unique feature of the molecular species of ROS PI and DG was that they had high amounts of species containing docosahexaenoic acid (22: 6 omega 3), while PI and DG from the other retinal membranes consisted mostly of species containing arachidonic acid (20: 4 omega 6). Following in vitro incubation of frog retinas with [2-3H] glycerol, the mass and radioactivity distributions among molecular species were determined following HPLC fractionation. The unique species composition of PS in ROS is determined mainly by selective translocation from the inner segments to ROS, since the dpm %, representative of newly synthesized species composition of the same glycerolipid classes in the other membrane fractions. This suggests that the distinctive species composition of PE and PA in ROS is determined not by selective translocation from the inner segments, but by remodeling processes taking place in the ROS.(ABSTRACT TRUNCATED AT 250 WORDS)

Interrelationship between Ca2+-dependent phospholipid base-exchange reaction and phospholipase D-like activity in bovine retina

Endogenous substrates (phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine) for the Ca2+-dependent base-exchange reaction were investigated using bovine retinal microsomes. The amounts of the three bases, serine, ethanolamine and choline, released from the membranes and the amount of phosphatidic acid generated in the membranes were measured in the presence of Ca2+ with or without exogenous bases. When the membranes were incubated with Ca2+ alone, the three bases were liberated into the water-soluble fractions accompanied by accumulation of phosphatidic acid, suggesting the presence of Ca2+-dependent phospholipase D-like activity. When an exogenous base was added to the reaction mixture, the liberation of the other two bases increased slightly and the formation of phosphatidic acid decreased markedly. The exogenous base also stimulated the liberation of the same base from prelabeled phospholipids. Accompanying these changes, the exogenous base was incorporated into the membrane phospholipid. With respect to pH profile, time course and metal requirements, both the base incorporation and phospholipase D-like activity were quite similar. The amount of base incorporated generally agreed with both the decreased amount of phosphatidic acid formed and the increased amount of base released. These results suggest that, beside the base-exchange reaction, phospholipase D-like activity plays an important role in Ca2+-dependent base incorporation into bovine retinal membranes.

Turnover of palmitate, arachidonate and glycerol in phospholipids of rat rod outer segments

Rat retinas were intravitreally labeled with [3H]palmitic acid, [3H]arachidonic acid or [3H]glycerol to study the turnover of the component parts of the major phospholipids in rod outer segments at times ranging from 2 hr to 12 days post injection. Rod outer-segment and retinal debris fractions were extracted and the major phospholipids separated by two-dimensional thin-layer chromatography. In darkness, [3H]glycerol rapidly labeled phosphatidylinositol in both rod outer-segment and retinal debris fractions. The label in phosphatidylinositol subsequently decreased dramatically, demonstrating a rapid turnover of phosphatidylinositol with a half-life of less than 1 day. Phosphatidylcholine and phosphatidylethanolamine were maximally labeled by glycerol in the retinal debris at the 2-hr time-point and were maximally labeled in rod outer segments between 1 and 5 days post injection, with somewhat longer residence times in the rod outer segments. Phosphatidylserine showed a lag in initial labeling in both rod outer-segment and retinal debris fractions indicating that this phospholipid is not a major precursor of phosphatidylcholine and phosphatidylethanolamine in rat retinas. [3H]Palmitate and [3H]arachidonate labels were rapidly incorporated into outer-segment phospholipids by 1-2 hr post injection. Eighty per cent of the palmitate label was initially associated with phosphatidylcholine at 2 hr. The total amount of palmitate label in rod outer-segment phosphatidylcholine did not change for 12 days post injection. Outer-segment phosphatidylethanolamine steadily increased in palmitate label throughout the 12-day period, suggesting that phosphatidylethanolamine may be utilized for recapture of palmitate released from breakdown of palmitate esters of rhodopsin or vitamin A or from phospholipids. Arachidonate primarily labeled phosphatidylinositol and phosphatidylcholine of both rod outer segments and retinal debris. The arachidonate label did not decrease dramatically during the first day in phosphatidylinositol as did the glycerol label, indicating that arachidonic acid is reutilized by the retina. Turnover of the individual phospholipids, as measured by a decrease in glycerol labeling of the phospholipid backbone, is more rapid than the loss of palmitate label, indicating that there is extensive reutilization of palmitate in both phosphatidylcholine and phosphatidylethanolamine of the rod outer segment. The data indicate that palmitate derived from many sources could be used by the photoreceptor to acylate rhodopsin.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of calmodulin antagonists and calmodulin on phospholipid base-exchange activities in rabbit platelets

Effects of various calmodulin antagonists and calmodulin on the incorporation of serine, ethanolamine and choline into the corresponding phospholipids, such as phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine by Ca2+-stimulated base-exchange reactions in rabbit platelet membranes were studied. Under a Ca2+-EGTA buffer system, the incorporation of three bases were stimulated by Ca2+ in a biphasic manner. Minimum requirement of free Ca2+ for the reactions was found to be around 0.5 microM and maximal incorporation took place at high Ca2+ concentrations (3-5 mM). Various calmodulin antagonists such as chlorpromazine, trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide, not only activated the three reactions but also greatly enhanced their sensitivity to Ca2+ (K0.5, 0.1-0.3 microM). In the absence of Ca2+, however, the drugs did not show any effect on the reactions. The concentrations of the drugs required for half maximal stimulation were approx. 30-40 microM. Although platelet membranes contained endogenous calmodulin (0.3-0.6 microgram/mg of membrane protein), the addition of exogenous calmodulin inhibited choline exchange activity but had no or little effect on serine or ethanolamine exchange activity. The results suggest that in the presence of low Ca2+ concentrations, these drugs markedly stimulate base-exchange activities, and choline exchange activity may be regulated by calmodulin.