The (non)specificity in the lipid-requirement of calcium- and (sodium plus potassium)-transporting adenosine triphosphatases

Phospholipid and detergent effects on (Ca2+ + Mg2+)ATPase purified from human erythrocytes

(Ca2+ + Mg2+)ATPase (EC 3.6.1.3) was solubilized from human erythrocyte membranes by detergent extraction with Triton N-101 (0.5 mg/mg membrane protein) and purified by calmodulin affinity chromatography. ATPase activity was assayed in mixtures of Triton N-101 and phospholipid, without reconstitution into bilayer vesicles. At low levels of phospholipid (5 micrograms/ml), the ATPase activity was highly sensitive to the detergent concentration, with maximal activity occurring at or near the critical micelle concentration of the detergent. With increased amounts of phospholipid (50 micrograms/ml), detergent concentrations greater than the critical micelle concentration were required for maximal activity. Detergent alone did not support ATPase activity. Sonicated phospholipid in the form of vesicles was equally ineffective. Activity seemed to be dependent on the presence of detergent/phospholipid mixed micelles. The acidic phospholipids, phosphatidylserine and phosphatidylinositol, as well as the commercial phospholipid preparation, Asolectin, gave activities five to eight times greater than the same amount of phosphatidylcholine. Mixtures of phosphatidylserine and phosphatidylcholine produced intermediate ATPase activities, with the maximal value dependent on the phosphatidylserine concentration. Addition of phosphatidylcholine to fixed concentrations of phosphatidylserine caused a rise in activity that was independent of the ratio of the two phospholipids or the total phospholipid concentration. Phosphatidylcholine may therefore be irreplaceable for some aspect of ATPase function. The number of phospholipid molecules present in mixed micelles at maximal ATPase activity was calculated to be near 50. This value implied that the hydrophobic surface of the ATPase molecule must be completely coated by a single layer of phospholipid molecules for maximum activity to occur.

Stimulation of (Ca2+ + Mg2+)-ATPase activity in human erythrocyte membranes by synthetic lysophosphatidic acids and lysophosphatidylcholines. Effects of chain length and degree of unsaturation of the fatty acid groups

Synthetic lysophosphatidic acids and lysophosphatidylcholines were examined for their effects on the (Ca2+ + Mg2+)-ATPase of human erythrocyte membranes. Addition of these compounds to erythrocyte ghosts caused significant changes in ATPase activity. The degree of unsaturation and the length of the sn-1 long chain hydrocarbon moiety were both contributing factors. All lysophosphatidic acids tested stimulated (Ca2+ + Mg2+)-ATPase activity. Of the species having a saturated acyl group, the most active was the myristoyl derivative. Linoleoyllysophosphatidic acid was the most potent of the unsaturated species. Saturated lysophosphatidylcholines with a short chain fatty acyl group (C10 to C14) exhibited only a moderate stimulatory activity, whereas the longer chain homologues, i.e., C16 and C18 were inhibitory at high concentrations. On the other hand, unsaturated lysophosphatidylcholines had stimulatory activities comparable to the unsaturated lysophosphatidic acids. These results suggest that the acidic moiety of lysophosphatidic acid is not an important structural determinant for expressing ATPase stimulatory activity in ghosts. Rather the nature of the hydrocarbon chain as well as the lyso structure of these compounds appear most critical under these conditions. The stimulatory effects of lysophosphatidic acids or lysophosphatidylcholines were additive to that induced with calmodulin, suggesting that these lysophospholipids affect the (Ca2+ + Mg2+)-ATPase by a mechanism which is different from that seen with calmodulin.

Species variation in the ouabain sensitivity of cardiac Na+/K+-ATPase. A possible role for membrane lipids

The role of membrane lipid composition on the modulation of ouabain sensitivity of cardiac Na+/K+-ATPase has been studied in vitro using several animal species. The animals can be grouped as ouabain-sensitive and ouabain-insensitive species. Ouabain-sensitive species (I50; 0.5-2.2 microM) include sheep, marmoset, pig and the guinea pig, whilst rat and mouse form the ouabain-insensitive group (I50; 100-105 microM). Although no species variation in the distribution of major phospholipid classes was observed, significant differences were apparent in the proportions of certain saturated and unsaturated phospholipid fatty acids. Thus, there was a marked increase in the relative proportion of docosahexaenoic (22:6, omega-3) acid in the Na+/K+-ATPase preparations from the rat and mouse compared to ouabain-sensitive species. Despite these differences, all animals had similar proportions of total saturated (sigma SAT) and total unsaturated (sigma Unsat) fatty acids. On the other hand, a good correlation between the unsaturation index of membrane lipids and I50 value for ouabain was observed. It is proposed that acyl chain characteristics (unsaturation and/or chain length) rather than the head group of the phospholipid molecule play a major role in the modulation of Na+/K+-ATPase to inhibition by ouabain.

Rabbit platelet calcium ATPase differs from the human erythrocyte (Ca2+ + Mg2+)-ATPase in its response to three purified phospholipases A2, exogenous phospholipids and calmodulin

Human erythrocyte (Ca2+ + Mg2+)-ATPase and calcium ATPase of rabbit platelets were compared by their responses to a variety of treatments. These included three purified phospholipases A2 (acidic, neutral and basic) from Agkistrodon halys blomhoffii, as well as several phospholipids and lysophospholipids. The erythrocyte enzyme was stimulated 2-3-fold by all three phospholipases with maximal stimulation occurring at different concentrations of the three enzymes. The basic phospholipase was the most potent, followed by the neutral and acidic enzymes in that order. The calcium ATPase activity of the platelet was also stimulated by phospholipase treatment, but only by 10-20%. The stimulatory activity was attributable to hydrolysis of a very small portion of the total membrane phospholipid. Inactivation of the phospholipases by heating or chemical modification with p-bromophenacyl bromide abolished their ability to stimulate. Addition of polyphosphoinositides stimulated both ATPases. However, another acidic phospholipid, lysophosphatidic acid, stimulated only the erythrocyte enzyme and failed to affect the platelet calcium ATPase. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) had no effect on either enzyme, while the platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), its lyso compound and lysoPC inhibited both ATPases. Calmodulin stimulated the erythrocyte enzyme, but did not affect the platelet calcium ATPase. These results demonstrate that the protein-lipid interactions operative in the erythrocyte and platelet calcium ATPases are quite different.

Response of rat heart membranes and associated ion-transporting ATPases to dietary lipid

The effects of different dietary fat intake on the lipid composition and enzyme behaviour of sarcolemmal (Na+ + K+)ATPase and sarcoplasmic reticulum Ca2+-ATPase from rat heart were investigated. Rat diets were supplemented with either sunflower seed oil (unsatd./satd. 5.6) or sheep kidney fat (unsatd./satd. 0.8). Significant changes in the phospholipid fatty acid composition were observed in both membranes after 9 weeks dietary lipid treatment. For both membranes, the total saturated/unsaturated fatty acid levels were unaffected by the dietary lipid treatment, however the proportions of the major unsaturated fatty acids were altered. Animals fed the sunflower seed oil diet exhibited an increase in n-6 fatty acids, including linoleic (18:2(n-6] and arachidonic (20:4(n-6] while the sheep kidney fat dietary rats were higher in n-3 fatty acids, principally docosahexaenoic (22:6), with the net result being a higher n-6/n-3 ratio in the sunflower seed oil group compared to sheep kidney fat dietary animals. Fluorescence polarization indicated that the fluidity of sarcoplasmic reticular membrane was greater than that of sarcolemmal membrane, with a dietary lipid-induced decrease in fluidity being observed in the sarcoplasmic reticular membrane from sheep kidney fat dietary animals. Despite these significant changes in membrane composition and physical properties, neither the specific activity nor the temperature-activity relationship (Arrhenius profile) of the associated ATPases were altered. These results suggest that with regard to the parameters measured in this study, the two ion-transporting ATPases are not modulated by changes which occur in the membrane lipid composition as a result of the diet.

Cerebral phospholipid content and Na+,K+-ATPase activity during ischemia and postischemic reperfusion in the mongolian gerbil

Using bilateral carotid artery occlusion in adult gerbils we examined the effects of ischemia and ischemia/reperfusion on cerebral phospholipid content and Na+,K+-ATPase (EC 3.6.1.3) activity. In contrast to the large changes in phospholipid content and membrane-bound enzyme activity that have been observed in liver and heart tissues, we observed relatively small changes in the cerebral content of total phospholipid, phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylethanolamine (PE) following ischemic intervals of up to 240 min. Following 15 min of ischemia the cerebral content of sphingomyelin (SM) was decreased to less than 50% of control values but returned to near-normal levels with longer ischemic periods. Significant decreases in the cerebral content of phosphatidylinositol (PI) and phosphatidic acid (PA) were observed following shorter intervals of ischemia (15-45 min). Na+,K+-ATPase activity of cerebral homogenates prepared from the brains of gerbils subjected to 30-240 min of ischemia was decreased but significantly different from control activity only after 30 min of ischemia (-29%, p less than or equal to 0.05). With the exception of PS, reperfusion for 60 min following 60 min of ischemia resulted in marked increases in cerebral phospholipid content with PC, SM, PI, and PA levels exceeding and PE levels equal to preischemic values. Longer periods of reperfusion (180 min) resulted in decreases in cerebral phospholipid content toward (PC, SM, PI, and PA) or below (PE) preischemic levels. In contrast, the cerebral content of PS significantly decreased during reperfusion (-51% at 60 min, p less than or equal to 0.05) and remained below preischemic values even after 180 min of reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischemia-induced changes in sarcolemmal (Na+, K+)-ATPase, K+-pNPPase, sialic acid, and phospholipid in the dog and effects of the nisoldipine and chlorpromazine treatment

This work was undertaken to study functional and structural changes of the cardiac sarcolemmal membrane which was isolated from the ischemic lesion in the dog. The sarcolemmal fraction was prepared, by adopting the method devised by Reeves and Sutko , from the right ventricle and the subendocardial and subepicardial layers of the left ventricle. Ischemic lesion was produced by occlusion of a branch of the left anterior descending coronary artery for a period of 1.5 hr in the thoracotomized dog, followed by release of the occlusion for 3 hr. Nisoldipine, 5 micrograms/kg, was given twice intravenously, and chlorpromazine was infused at a rate of 10 micrograms/kg X min, in addition to the administration of twice bolus doses of 400 micrograms/kg each. Nisoldipine significantly decreased the incidence of premature ventricular contractions and microvascular hemorrhage. Sarcolemmal purity was monitored by using enzyme and chemical markers; the results indicated that the membrane preparation was tenfold purified over the homogenate. Although the activities of ouabain-sensitive (Na+, K+)-ATPase and ouabain-sensitive K+-p-nitrophenylphosphatase ( pNPPase ) of the sarcolemmal preparation isolated from the subendocardial layer were similar to those from the subepicardial layer in the nonischemic left ventricle, a significant decrease in these activities was observed only when the sarcolemmal fraction isolated from the subendocardial layer of ischemic area was compared with that from the subendocardial layer of nonischemic area. In contrast, the sialic acid content of the sarcolemma from the ischemic subendocardial layer was significantly increased compared to that of the nonischemic subendocardial layer. No such changes occurred in sarcolemma prepared from the ischemic subepicardial layer. The total phospholipid content as well as phosphatidylcholine and -ethanolamine contents of the sarcolemmal membrane prepared from the subendocardial layer of ischemic area were significantly decreased compared to nonischemic area. Nisoldipine prevented the ischemia-induced alterations in sarcolemmal (Na+, K+)-ATPase, pNPPase , sialic acid and phospholipids of the subendocardial layer. Chlorpromazine showed a less consistent effect than did Nisoldipine under our experimental conditions. Our study thus demonstrates that the lipid component and function of cardiac sarcolemmal membrane are altered in the early ischemic lesion and that these alterations are nonuniform in distribution and are alleviated by some pharmacological intervention.

Competition between cholesterol and phosphatidylcholine for the hydrophobic surface of sarcoplasmic reticulum Ca2+-ATPase

A multiple equilibrium binding model is used to examine phospholipid and cholesterol binding with the transmembranous protein Ca2+-ATPase (calcium pump). The protein was reconstituted in egg phosphatidylcholine bilayers by lipid substitution of rabbit muscle sarcoplasmic reticulum. Electron spin resonance spectra of a phosphatidylcholine spin-label and a recently developed cholesterol spin-label show two major spectral contributions, a motionally restricted component consistent with interactions between the label and the protein surface and another component characteristic of motion of the label in a fluid lipid bilayer. The number of lipid binding (or contact) sites at the hydrophobic surface of the protein is calculated to be N = 22 +/- 2. Experiments with intact sarcoplasmic reticulum membranes give approximately the same value for N. The relative binding constants are Kav approximately 1 for the phosphatidylcholine label and Kav approximately 0.65 for the cholesterol spin-label. Thus, cholesterol does contact the surface of the protein, but with a somewhat lower probability than phosphatidylcholine. This is confirmed by competition experiments where unlabeled cholesterol and the phospholipid spin-label are both present in the bilayer. Evidently the flexible acyl chains of the phospholipid molecules accommodate more readily to the irregular surface of the protein than does the rigid steroid structure of cholesterol.

Inhibition of rat brain microsomal Na+,K+-ATPase by S-adenosylmethionine

Rat brain microsomes were preincubated with S-adenosylmethionine (SAM), MgCl2, and CaCl2, then reisolated, and the activity of Na+,K+-ATPase determined. SAM inhibited the Na+,K+-ATPase activity compared with microsomes subjected to similar treatment in the absence of SAM. A biphasic inhibitory effect was observed with a 50% decrease at a SAM concentration range of 0.4 microM-3.2 microM and a 70% reduction at a concentration range above 100 microM. Inclusion of either S-adenosylhomocysteine or 3-deazaadenosine in the preincubations prevented the SAM inhibition of Na+,K+-ATPase activity. The inhibition by SAM appeared to be Mg2+- or Ca2+-dependent.

Diabetic neuropathy

Peripheral nerve disorders are important late complications of diabetes mellitus. Polyneuropathy, which may involve varying proportions of sensory, motor, and autonomic fibers, is considered the consequence of metabolic derangements that result from chronic hyperglycemia. Symmetrical proximal motor neuropathy ("diabetic amyotrophy") also may have a metabolic basis. Mononeuropathies in diabetes may have an ischemic or compressive cause. Advances have been made in understanding the biochemical basis for diabetic polyneuropathy. The treatment of symptomatic diabetic neuropathy should be directed toward long-term normalization of blood glucose until more specific therapies become available.

Impaired rat sciatic nerve sodium-potassium adenosine triphosphatase in acute streptozocin diabetes and its correction by dietary myo-inositol supplementation

Nerve conduction impairment in experimental diabetes has been empirically but not mechanistically linked to altered nerve myo-inositol metabolism. The phospholipid-dependent membrane-bound sodium-potassium ATPase provides a potential mechanism to relate defects in diabetic peripheral nerve myo-inositol-phospholipid metabolism, impulse conduction, and energy utilization. Therefore, the effect of streptozocin-induced diabetes mellitus and dietary myo-inositol supplementation on rat sciatic nerve sodium-potassium ATPase was studied. ATPase activity was measured enzymatically in sciatic nerve homogenates from 4-wk streptozocin diabetic rats and age-matched controls either fed a standard or 1% myo-inositol supplemented diet. The sodium-potassium ATPase components were assessed by ouabain inhibition or the omission of sodium and potassium ions. Diabetes reduced the composite ATPase activity recovered in crude homogenates of sciatic nerve. The 40% reduction in the sodium-potassium ATPase was selectively prevented by 1% myo-inositol supplementation (which preserved normal nerve conduction). Thus, in diabetic peripheral nerve, abnormal myo-inositol metabolism is associated with abnormal sodium-potassium ATPase activity. The mechanism of the effect of dietary myo-inositol to correct diabetic nerve conduction may be through changes in a sodium-potassium ATPase, possibly via changes in myo-inositol-containing phospholipids.

Effects of base exchange reaction on the Na+, K+ ATPase in rat brain microsomes

Incorporation of ethanolamine and monomethylethanolamine into their corresponding phospholipid by the base exchange enzymes activated an Na+, K+-ATPase associated with a rat brain microsomes enriched preparation. The serine and dimethylethanolamine base exchange catalyzed incorporation reactions inhibited this particular Na+, K+-ATPase. These effects require Ca2+ and several other structural analogues which are not incorporated into phospholipid were without affect on this ATPase.

Defective sarcoplasmic reticular calcium transport in diabetic cardiomyopathy

The effects of insulin and thyroid hormone treatments on cardiac sarcoplasmic reticular function were investigated in chronic streptozotocin-induced diabetes in rats. ATP-dependent Ca2+ transport and Ca2+-stimulated ATPase activities were depressed significantly in microsomal samples from diabetic rats in comparison with control (P less than 0.05). This defect was seen at various times of incubation (1-20 min) and different concentrations of free Ca2+ (10(-7) to 10(-5) M Ca2+) and was accompanied by changes in the protein composition and phospholipid contents of the microsomal fraction. The defect in calcium transport in microsomal vesicles was not evident until 28 days after streptozotocin (65 mg/kg iv) injection, whereas increases in plasma glucose levels due to insulin-deficiency occurred within 3 days. All changes in function and composition of the sarcoplasmic reticulum were reversed by insulin administration to the diabetic rats. Although the plasma level of thyroid hormone was decreased in the diabetic rat, thyroid hormone treatment did not restore microsomal calcium transport in the diabetic animals. The results of this study provide some evidence that the depression in cardiac sarcoplasmic reticular calcium accumulation during diabetes is a consequence of insulin deficiency and associated chronic metabolic changes but the hypothyroid condition that accompanies experimental diabetes does not appear to play any role in this defect.

The effect of a calcium channel antagonist, Nisoldipine, on the ischemia-induced change of canine sarcolemmal membrane

Ischemic injury was produced in the dog heart by occluding the left anterior descending coronary artery just below the second diagonal branch for a duration of 1.5 h followed by the release of occlusion. Nisoldipine, 3.5 micrograms/kg was injected intravenously 10 min before the occlusion and again 10 min before the commencement of reperfusion. The activity of serum creatine phosphokinase greatly increased after the reperfusion, and this increase was significantly suppressed by Nisoldipine. This drug, in addition, prevented ischemia-induced myocardial hemorrhage and premature ventricular contraction. Sarcolemmal membrane vesicles were prepared from an ischemic and non-ischemic portions of the myocardium 3 h after the commencement of reflow. The fraction was purified approximately 12-fold with respect to ouabain-sensitive (Na+K+)-ATPase as an indicator; contamination of mitochondria was minimum with cytochrome c oxidase as an indicator. Without treatment of Nisoldipine, the total amount of sarcolemmal phospholipid obtained from the ischemic area, as well as the amounts of phosphatidyl-choline and phosphatidyl-ethanolamine, were significantly decreased as compared with those obtained from the non-ischemic area. Nisoldipine treatment abolished the decrease in the sarcolemmal phospholipids, total as well as phosphatidyl-choline and -ethanolamine, induced by ischemia plus reperfusion. Therefore, our work indicates that the Ca++ channel antagonist, Nisoldipine, suppresses the ischemia-induced increase in phospholipid breakdown of cardiac sarcolemma probably through its inhibitory effect on the Ca++-mediated activation of membrane phospholipase, through its vasodilatory action, or both.

Phospholipid synthesis in the squid giant axon: enzymes of phosphatidylinositol metabolism

We examined the properties of several enzymes of phospholipid metabolism in axoplasm extruded from squid giant axons. The following synthetic enzymes, CDP-diglyceride: inositol transferase (EC 2.7.8.11), ATP:diglyceride phosphotransferase, diglyceride kinase (EC 2.7.2.-), and phosphatidylinositol kinase (EC 2.7.1.67), were all present in axoplasm. Phospholipid exchange proteins, which catalyzed the transfer of phosphatidylinositol and phosphatidylcholine between membrane preparations and unilamellar lipid vesicles, were also found. However, we did not find conditions under which the synthesis of CDP-diglyceride, phosphatidylserine, and phosphatidylinositol-4,5-diphosphate could be measured. Subcellular fractionation by differential centrifugation showed that the axoplasmic inositol transferase and phosphatidylinositol kinase activities were largely "microsomal," while the diglyceride kinase and exchange protein activities were primarily "cytosolic."

Membrane regulation of the Na+,K+-ATPase during the neuroblastoma cell cycle: correlation with protein lateral mobility

The pumping activity of the plasma membrane-bound Na+,K+-ATPase shows considerable variation during the cell cycle of mouse neuroblastoma Neuro-2A cells. Addition of external ATP at millimolar concentrations, which selectively enhances the plasma membrane permeability of Neuro-2A cells for sodium ions, stimulates the Na+,K+-ATPase pumping activity at all phases of the cell cycle from a factor of 1.05 in mitosis up to 2.2 in G1 phase. Determination of the number of Na+,K+-ATPase copies per cell by direct 3H-ouabain binding studies in the presence of external ATP shows a gradual increase in the number of pump sites on passing from mitosis to the late S/G2-phase by approximately a factor of 2. From these data the pumping activity per copy of Na+,K+-ATPase, optimally stimulated with respect to its various substrate ions, has been determined during the various phases of the cell cycle. This optimally stimulated pumping activity per enzyme copy, which is a reflection of the physicochemical state of the plasma membrane, is high in mitosis, almost twofold lower in early G1 phase, and increases gradually again during the other phases of the cell cycle. This shows that the observed regulation of Na+,K+-ATPase activity during the cell cycle is caused by a combination of three independent factors--namely variation in intracellular substrate availability (Na+), changes in number of enzyme copies per cell, and modulation of the plasma membrane environment of the protein molecules. The modulation of the optimal pumping activity per enzyme copy shows a good correlation (rho = 0.96) with the known modulation of protein lateral mobility during the cell cycle, such that a high protein lateral mobility correlates with a low enzyme activity. It is concluded that changes in plasma membrane properties take place during the Neuro-2A cell cycle that result in changes in the rate of protein lateral diffusion and Na+,K+-ATPase activity in directly correlated way.