Effect of phospholipid methylation on calcium transport and (Ca2+ + Mg2+)-ATPase activity in kidney cortex basolateral membranes

Changes during hibernation in different phospholipid and free and esterified cholesterol serum levels in black bears

During hibernation, fat is known to be the preferred source of energy. A detailed analysis of different phospholipids, as well as free and esterified cholesterol, was conducted to investigate lipid abnormalities during hibernation. The levels of total phospholipids and total cholesterol in the serum of black bears were found to increase significantly in hibernation as compared with the active state. Both free and esterified cholesterol were increased in the hibernating state in comparison with the active state (P < 0.05). The percentage increase during hibernation was more in free cholesterol (57%) than in esterified cholesterol (27%). Analysis of subclasses of serum phospholipids showed that choline containing phospholipids, i.e., sphingomyelin (SPG) (14%) and phosphatidylcholine (PC) (76%), are the major phospholipids in the serum of bear. The minor phospholipids included 8% of phosphatidylserine (PS) + phosphatidylinositol (PI), while phosphatidylethanolamine (PE) was only 2% of the total phospholipids. A comparison of phospholipid subclasses showed that PC, PS + PI and SPG were significantly increased, while PE was significantly decreased (P < 0.05) in the hibernating state as compared with the active state in black bears. These results suggest that the catabolism of phospholipids and cholesterol is decreased during hibernation in black bears, leading to their increased levels in the hibernating state as compared with the active state. In summary, our results indicate that serum cholesterol and phospholipid fractions (except PE) are increased during hibernation in bears. It is proposed that the increase of these lipids may be due to the altered metabolism of lipoproteins that are responsible for the clearance of the lipids.

Sperm phospholipid methyltransferase activity during preparation for exocytosis

The present report describes experiments to evaluate phospholipid methyltransferase activity in golden hamster spermatozoa incubated under different conditions. Washed cauda epididymal sperm were incubated with taurine, in the presence or absence of epinephrine. At various times, the sperm were separated, and phospholipid methyltransferase activity measured. Also, at each time, aliquots of the sperm suspension were assayed for motility, and acrosome reactions. Some sperm incubated in the presence of taurine and epinephrine were capacitated by 3.5 h, because about 40 per cent of them can undergo the acrosome reaction 10 min after addition of the fusogen lysophosphatidylcholine. In epinephrine-free incubations the fusogen failed to stimulate acrosome reactions. On the other hand, epinephrine stimulated by twofold phospholipid methyltransferase activity from '0 time' incubated sperm, in comparison to that observed in taurine-treated cells. Enzyme activities from both taurine or epinephrine plus taurine-treated cells decreased as the incubation time of the sperm suspension increased. Kinetic properties of the sperm phospholipid methyltransferase activity were modified by the presence of taurine and epinephrine when S-adenosylmethionine was used as the substrate. These results suggest that refined molecular events occur in the sperm cell during the acquisition of fertilizing ability.

Human chorionic gonadotropin and free beta subunits stimulate phospholipid methylation in intact rat Leydig cells

The effect of human chorionic gonadotropin (hCG) on intact Leydig cell phospholipid methylation was studied. Hormonal stimulation of rat Leydig cells increased the incorporation of [methyl-3H]methionine into phospholipids threefold. This effect was observed after 10 minutes of incubation time and was time and dose dependent with a maximal stimulation at 67 ng/ml of hCG. In the presence of hCG, 3H-labeled methyl groups were preferentially incorporated into phosphatidyl-N-monomethylethanolamine. This effect of hCG was not reproduced by dibutyryl cyclic adenosine monophosphate (cAMP), cholera toxin, or forskolin. Purified hCG beta subunit but not hCG alpha subunit had stimulatory activity on Leydig cell phospholipid methylation. We conclude that luteinizing hormone (LH)/hCG stimulates specifically Leydig cell phospholipid methylation, because LH-releasing hormone or [Arg8]-vasopressin did not modify these reactions. We postulate that these reactions are occurring at a cellular level that involves hormone-receptor interaction. It is also suggested that this biological response involves hCG beta subunit receptor interaction and does not require cAMP synthesis.

Effect of (1-34) parathyroid hormone-related peptide on the composition and turnover of phospholipids in syncytiotrophoblast brush border and basal plasma membranes of human placenta

The effect of parathyroid hormone-related peptide on the lipid composition and the turnover of phosphoinositides was studied in brush border and basal plasma membranes of human placenta syncytiotrophoblasts. Lipid composition of the two polar membranes differed markedly with respect to the cholesterol/phospholipid ratio (0.57 +/- 0.04 and 0.91 +/- 0.05 in basal plasma membranes and brush border membranes, respectively). Sphingomyelin was the major phospholipid in both membranes. Except for the phosphoinositide-phosphatidylserine complex which was higher in basal plasma membranes, the phospholipid composition was comparable in the brush border membrane and basal plasma membranes. Incubation of the tissue with 10(-8) M parathyroid hormone-related peptide (1-34) resulted in a significant increase in the phosphatidylinositol phosphate content of the two membranes and in the phosphatidylinositol biphosphate concentration in the basal plasma membranes. Finally, when the tissue was preincubated with [3H]myo-inositol in the presence of 10(-8) M parathyroid hormone-related peptide (1-34), the hormone significantly stimulated the inositol phosphate release by the two membranes. These results demonstrate that: (1) in the placental syncytiotrophoblast, as found in other transport epithelia, the lipid composition of the polar membranes is different; (2) parathyroid hormone-related peptide stimulates the phosphoinositide turnover in both membranes.

Effect of sulfasalazine on adaptive and functional changes in intestine of normal and protein-calorie-malnourished rats

The effects of sulfasalazine (500 mg/kg body weight daily for 35 days) and its subsequent recovery for another 35 and 65 days have been investigated on the intestinal uptake of certain end-product nutrients, viz. glucose, leucine, alanine, and calcium, in normal and protein-calorie malnourished (PCM) male albino rats. Sulfasalazine administration caused a reduction in body weight in PCM animals, while intestinal weight and length as well as protein and nucleic-acid contents were reduced in both normal and PCM animals. Serum proteins also showed a decrease in PCM rats. PCM rats showed elevated levels of glucose, amino acids, and calcium uptake by the intestinal segments, but sulfasalazine feeding inhibited the uptake of nutrients both in normal-fed and malnourished animals. All these changes were found to be reversible after the withdrawal of drug treatment. Sulfasalazine caused a decrease in the Na(+)-dependent (active) glucose uptake as well as the Na(+)-independent (passive) process. The kinetic parameters of glucose uptake indicate that the drug might interfere with the transport/carrier protein of these nutrients, because reduction was observed in maximum uptake velocity (Jmax) of these systems without any change in the affinity constant (Kt).

Depression of membrane-bound hydrolases by cimetidine in mouse renal basolateral and brush border

Oral administration of the antiulcerogenic drug, cimetidine, was studied on kidney-bound hydrolytic enzymes at three different dose levels (30 mg, 100 mg, and 2000 mg/kg body weight) and for single administration for 2 and 24 h, and daily administration for 15 days in mice. It significantly inhibited Na+, K(+)-ATPase, Mg(2+)-ATPase, and Ca2+, Mg(2+)-ATPase in the isolated basolateral membrane (BLM). Brush-border-membrane-(BBM)-associated enzymes, sucrase, lactase, maltase, leucine aminopeptidase, and alkaline phosphatase also showed a marked reduction. Substrate saturation kinetics revealed the nature of inhibition was of mixed type in the case of sucrase, lactase, maltase, and alkaline phosphatase (Km was increased, while Vmax decreased), whereas it was of non-competitive type for leucine aminopeptidase (Km was unchanged, while Vmax decreased). In vitro addition of cimetidine (5-20 mM) to the BBM also inhibited the enzyme activity. Dixon plot produced the inhibition constant (Ki) for cimetidine in the case of maltase, alkaline phosphatase, and leucine aminopeptidase in the order of 14.83, 32.83 and 11.5 mM, respectively. Analysis of lipids revealed a significant reduction in BBM-associated phospholipid and phospholipid/cholesterol molar ratio, while the neutral lipid fraction, i.e., cholesterol and triglycerides were not altered. Free fatty acid exhibited an increase after drug treatment, which was significant at higher dose after 24 h of single and 15 days of daily treatment. BLM-associated lipids did not exhibit any significant change. Cimetidine-induced depression in renal BLM- and BBM-associated disaccharidases and ATPases, at least at the higher dose level, may have serious consequences in the absorption of end-product nutrients.

Fluorometric determination of phosphatidylcholine as a measure of phospholipid methylation

The successive methylation of phosphatidylethanolamine to phosphatidylcholine (phospholipid methylation) has been measured by the incorporation of S-[methyl-3H]adenosylmethionine or colorimetric assay of phosphatidylcholine extracted from adipocyte plasma membranes. A fluorometric assay for phosphatidylcholine was developed to measure phospholipid methylation. This assay is 10 times more sensitive than the colorimetric assay and demonstrates no significant interference with other methylated phospholipids. The fluorometric assay was used to determine a biphasic insulin dose response in adipocyte plasma membranes. This fluorometric assay for phosphatidylcholine represents an alternative method for monitoring phospholipid methylation, especially when increased sensitivity is required.

Effect of histamine H2-receptor antagonist, ranitidine on renal brush border and basolateral membranes

The antiulcerogenic drug ranitidine, given orally to mice, brought about reductions of kidney-bound hydrolytic enzymes at three different dose levels, viz. 10 mg, 100 mg, and 1000 mg/kg body weight, and for three different time points (single administration for 2 h and 24 h, and daily administration for 15 days). The activities of Na+, K(+)-ATPase, Ca2(+)-ATPase, and Mg2(+)-ATPase (marker enzymes of basolateral membranes) were reduced, and these reductions were significant at higher doses and after a 24-h single treatment or 15 days' daily treatment. Maltase, alkaline phosphatase, and leucine aminopeptidase (marker enzymes of brush border membrane [BBM]) activities were significantly inhibited after ranitidine treatment. Kinetic analysis of BBM-associated enzymes indicated that ranitidine decreased the maximum of apparent initial enzyme velocity (Vmax) of maltase, alkaline phosphatase, and leucine aminopeptidase. The substrate affinity constant (Km) was decreased in the case of alkaline phosphatase and maltase, while it was not altered in the case of leucine aminopeptidase. In vitro addition of ranitidine to renal BBM also produced significant inhibition of these enzymes, the inhibition constants (Ki) for maltase, alkaline phosphatase, and leucine aminopeptidase being 7.5, 15.5, and 3.5 mM, respectively. Membrane-bound lipid estimation showed a significant increase in phospholipids, triglycerides, and free fatty acids. Cholesterol, however, was decreased in both renal basolateral and brush border membranes.

Reversibility of the effects of gossypol acetic acid, an antispermatogenic/antifertility agent on the intestinal structure and functions of male albino rats

The effects of Gossypol acetic acid (10 mg/kg b. wt. daily for 15 days), an experimental male antifertility agent and its subsequent withdrawal for another 15 days, on the structure and functions of the rat small intestinal tract have been investigated. Gossypol feeding causes a reduction in body weight and intestinal weight, length, protein, and nucleic acid contents. A 27%-50% reduction in the uptake of glucose, alanine, leucine, and calcium is observed after Gossypol feeding which is found to be reversible after 15 days of withdrawal of the drug. Gossypol also causes a significant reduction in the activities of sucrase, lactase, maltase and alkaline phosphatase in the intestinal homogenates as well as in the purified brush border membrane of the microvillus. A decrease in the maximum of apparent enzyme velocity and no change in the substrate affinity constant in these digestive hydrolases are observed on Gossypol treatment. It also causes a shift in the transition temperature in these enzymes and predictably changes the energy of activation both below and above the temperature of transition, although the Arrhenius expression of the temperature dependence still shows proximity, non-linearity, and is parallel to the control group. These changes are reversed on withdrawal of the drug and during the subsequent recovery period. Recovery experiments also show near identical values in kinetic parameters (Kt and Jmax) of 14C-glucose uptake in jejunal segments both in the presence and absence of Na+ ions. Also, no difference is observed between the control and recovery groups with respect to body and intestinal weight, intestinal length, and DNA, RNA, protein, lactate dehydrogenase and glucose-6-phosphate phosphohydrolase values in the intestinal homogenates. Phospholipid, cholesterol and sialic acid levels in both the groups also show nearly identical values. Molecular mechanism of the effects of Gossypol on brush border membrane-bound enzyme/carrier molecules operation is discussed in view of the kinetic and thermodynamic data obtained.

Co-ordinate control of CDP-choline and phosphatidylethanolamine methyltransferase pathways for phosphatidylcholine biosynthesis occurs in response to change in diet fat

Brain microsomal and synaptic plasma membrane phosphatidylcholine composition and biosynthetic activity were examined in relation to the composition of diet fat fed. Phosphocholinetransferase and methyltransferase activities are shown to be modulated by the diet, and by changes in the membrane phospholipid content of long-chain polyunsaturated fatty acids. This physiological modulation is co-ordinated such that the rate of phosphatidylcholine synthesis via one route is inversely regulated with activity of the alternate pathway.

Protein methylation inhibits Na+-Ca2+ exchange activity in cardiac sarcolemmal vesicles

We have examined the effect of membrane methylation on the Na+-Ca2+ exchange activity of canine cardiac sarcolemmal vesicles using S-adenosyl-L-methionine as methyl donor. Methylation leads to approximately 40% inhibition of the initial rate of Nai+-dependent Ca2+ uptake. The inhibition is due to a lowering of the Vmax for the reaction. The inhibition is not due to an effect on membrane permeability and is blocked by S-adenosyl-L-homocysteine, an inhibitor of methylation reactions. The following experiments indicated that inhibition of Na+-Ca2+ exchange was due to methylation of membrane protein and not due to methylated phosphatidylethanolamine (PE) compounds (i.e., phosphatidyl-N-monomethylethanolamine (PMME) or phosphatidyl-N,N'-dimethylethanolamine (PDME]: (1) We solubilized sarcolemma and reconstituted activity into vesicles containing no PE. The inhibition by S-adenosyl-L-methionine was not diminished in this environment. (2) We reconstituted sarcolemma into vesicles containing PMME or PDME. These methylated lipid components had no effect on Na+-Ca2+ exchange activity. (3) We verified that many membrane proteins, probably including the exchanger, become methylated.

Changes in muscle lipid metabolism induced in vitro by 1,25-dihydroxy-vitamin D-3

1,25-Dihydroxy-vitamin D-3 has been shown to increase phosphatidylcholine and decrease phosphatidylethanolamine levels in skeletal muscle in vitro. To elucidate the metabolic pathway and mechanism involved in the effect of the sterol, chick embryo myoblast and vitamin D-deficient chick soleus muscle cultures were treated with 1,25-dihydroxy-vitamin D-3 (5.0 X 10(-10)-3.6 X 10(-11) M, total concentration) for 12-14 h and 1 h, respectively, in the absence and presence of protein and RNA synthesis inhibitors. Lipids were then labelled with [3H]glycerol and [14C]acetate. A significant increase in phosphatidylcholine and triacylglycerol labelling and a decrease in phosphatidylethanolamine labelling were observed in response to the hormone. Cycloheximide suppressed these changes in both types of preparations. Puromycin and actinomycin D were also effective blockers in cultured muscle cells. In addition, double-labelling of control and 1,25-dihydroxy-vitamin D-3-treated myoblasts with [3H]choline and [14C]ethanolamine were performed. The sterol did not affect [3H]choline labelling of total cell lipid extracts and phosphatidylcholine. However, the total lipid fraction of treated cells was labelled to a greater extent with [14C]ethanolamine. In addition, an increased incorporation of this precursor into phosphatidylcholine accompanied by a proportional decrease in phosphatidylethanolamine could be shown in cells pretreated with the metabolite. These changes were abolished by cycloheximide and actinomycin D. The results suggest that 1,25-dihydroxy-vitamin D-3 stimulates methylation of phosphatidylethanolamine into phosphatidylcholine in myoblasts by a nuclear mechanism. The data are consistent with the presence in the cells of a receptor specific for the hormone.

Lipid methylation, sodium transport and the response to PTH in renal tubules

The possible role of progressive methylation of phosphatidylethanolamine to phosphatidylcholine on sodium transport was examined in a suspension of rabbit proximal convoluted tubules. The relation between progressive methylation and the action of parathyroid hormone on sodium transport in this nephron segment was also determined. Incubation of the suspended tubules for 10 minutes at 37 degrees C with 200 microM S-adenosyl-L-[3H]-methyl methionine, a methyl donor, revealed a significant rate of de-novo phosphatidylcholine synthesis. The inactive adenosine analogue, 3-deazaadenosine (DZA), had a significant inhibitory effect on lipid methylation. Despite the inhibition of methylation by DZA, the ouabain sensitive component of oxygen consumption, an index of sodium transport, was not affected. PTH significantly inhibited ouabain sensitive oxygen consumption but had no effect on the methylation of phosphatidylethanolamine. Inhibition of methylation by DZA, did not affect the inhibitory effect of PTH on oxygen consumption. These studies demonstrate that in the proximal convoluted tubule of the rabbit the progressive methylation pathway is present and that inhibition of this pathway does not affect sodium transport. In addition, these studies suggest that the inhibitory effect of PTH on sodium transport is not mediated by the methylation pathway.

Stimulation of Ca2+-pump in rat heart sarcolemma by phosphatidylethanolamine N-methylation

Incubation of purified cardiac sarcolemmal vesicles (SL) in the presence of S-adenosyl-L-methionine, a methyl donor for the enzymatic N-methylation of phosphatidylethanolamine (PE), increased the Ca2+-stimulated ATPase and ATP-dependent Ca2+ accumulation activities. Quantitative analysis of the methylated phospholipids revealed that maximal increase of Ca2+-pump activities was associated with predominant synthesis and intramembranal accumulation of phosphatidyl-N,N-dimethylethanolamine. The stimulation of SL Ca2+-pump activities was prevented by inhibitors of PE N-methylation such as S-adenosyl-L-homocysteine and methyl acetimidate hydrochloride. The results suggest a possible role of PE N-methylation in the regulation of Ca2+-transport across the heart SL membrane.

Composition and physical properties of lipids from plasma membranes of dog kidney

Lipid composition, physical state of major phospholipid classes and transbilayer migration of phosphatidylcholine have been determined in plasma membranes of the dog kidney. The lipid composition of brush-border membranes markedly differs from that of antiluminal membranes with respect to: (a) the total phospholipid content; (b) the cholesterol to phospholipid ratio (C/P); (c) the distribution of the major phospholipid classes. Sphingomyelin present in large amounts in both luminal and antiluminal membranes extracts exhibits a transition of phase between 20 and 44 degrees C approximately. In the range of temperature studied (5-55 degrees C) no phase transitions were detected for the other phospholipid species. Our data suggest that: (1) at physiological temperature the higher C/P ratio of brush-border membranes is in large part responsible for their lower fluidity; (2) both the relatively low cholesterol and high sphingomyelin contents contribute to the thermotropic transitions observed in intact membranes. Finally transbilayer migration of phosphatidylcholine in brush-border membranes is a very slow process with a half time of 6.5 h at 37 degrees C which compares with that of other biological membranes.

Activation of Ca2+-stimulated ATPase by phospholipid N-methylation in cardiac sarcoplasmic reticulum

Incubation of cardiac sarcoplasmic reticulum (SR) in the presence of S-adenosyl-L-methionine, a methyl donor for the enzymatic N-methylation of phosphatidylethanolamine, increased Ca2+-stimulated ATPase activity. The increase in Ca2+-ATPase activity was not due to changes in the affinity for Ca2+ and was prevented by methyl acetimidate, an inhibitor of phospholipid N-methylation. The results suggest a possible regulatory role of phospholipid N-methylation in SR Ca2+-pump mechanism.

Neurochemical aspects of depression: the past and the future?

The role of aberrant neurochemical substrates in the etiology of depression and the neurochemical mechanisms of antidepressant therapies have been the subjects of many hypotheses in the last 30 years. Pharmacological studies of early antidepressant drugs indicated that brain monoamines were significantly affected by these drugs and these led to the formulation of the biogenic amine hypothesis of depression. Although this hypothesis has been of heuristic value in the study of drug mechanisms and has provided a basis for screening drugs for antidepressant potential, deficiencies in it have become apparent. Neuroanatomical and neurochemical considerations favour the view that brain noradrenaline and serotonin systems may serve as bias adjusting systems for each other and numerous other neural systems. As a consequence of such a relationship, a primary defect in some other neural system would appear amplified in measurements of serotonin or noradrenaline. A possible site for this primary defect may be in membrane composition and function. Recent studies have found that typical and other antidepressant therapies have a pronounced effect on membrane lipids. Thus, in view of the important functions of membrane lipids and the fact that they have been linked to the initiation and development of a number of other disease processes, it is now suggested that consideration be given to them as playing primary causal roles in the etiology of depression and as a site of action for antidepressant drugs.

Methylation of phospholipids in microsomes of the rat aorta

The methylation of phospholipids by S-adenosyl-L-methionine was characterized in microsomes prepared from strips of rat aorta. In the presence of 0.5 microM S-adenosyl-L-methionine, endogenous phosphatidylethanolamine was methylated to form three products: phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N-dimethylethanolamine and phosphatidylcholine. In the presence of 150 microM S-adenosyl-L-methionine the methylation activity increased more than 50-fold and the principal radioactive product was phosphatidylcholine. Optimal activity was at pH 9 and no magnesium requirement was detected. Exogenous phosphatidylethanolamine, phosphatidyl-N-monomethylethanolamine and phosphatidyl-N,N-dimethylethanolamine served as substrates for the enzyme. The methylation of exogenous phosphatidyl-N,N-dimethylethanolamine proceeded at a slower rate. Incubation of trypsin with the aorta microsomes reduced the enzymatic activity and reduced the relative yield of phosphatidyl-N-monomethylethanolamine. Phospholipase C degraded the methylated phospholipids, but phosphatidyl-N,N-dimethylethanolamine appeared to be less accessible to the phospholipase. The phospholipid methylation activity was inhibited by the addition of S-adenosyl-L-homocysteine or by L-homocysteinethiolactone. When intact strips of rat aorta were incubated with L-[methyl-3H]methionine, [3H]methyl groups were incorporated into phospholipids. This incorporation was inhibited when L-homocysteinethiolactone was added to the incubation. Polarized fluorescence of diphenylhexatriene in aorta microsomes was measured to determine the apparent membrane fluidity. When intact strips of aorta were incubated with methionine or with L-homocysteinethiolactone, methionine enhanced and L-homocysteinethiolactone decreased apparent fluidity of the microsomal membranes. Phospholipid methylation activity was examined in aorta microsomes prepared from genetically spontaneous hypertensive SHR strain rats. Phospholipid methylation activity was substantially greater in the SHR aorta microsomes than in microsomes prepared from Wistar-Kyoto WKY control strain aorta. Membrane fluidity was greater in the SHR aorta microsomes than in the WKY aorta microsomes. The hypothesis that phospholipid methylation activity influences fluidity of membranes and the possible involvement of methylated phospholipids in aorta membrane functions are discussed.

Induction of increased calcium uptake in liposomes having membrane proteins of chicken erythrocytes by S-adenosylmethionine

Liposomes having membrane proteins of chicken erythrocytes were prepared and the effect of S-adenosylmethionine on 45Ca2+ uptake into the liposomes was investigated. S-Adenosylmethionine, a donor of methyl groups in enzymatic methylation, induced an increase of 45Ca2+ uptake into the proteoliposomes with membrane proteins but not into the liposomes without membrane proteins. Increased release of 45Ca2+ from the inside of the proteoliposomes was also induced by S-adenosylmethionine. These increases of uptake and release of 45Ca2+ were inhibited by S-adenosylhomocystein, an inhibitor of enzymatic methylation. Furthermore, membrane proteins from chicken erythrocytes showed protein and phospholipid methyltransferase activities. The uptake of other materials, 3-0-[methyl-3H]glucose, alpha-[1-14C]aminoisobutyric acid, 42K+ and 54Mn2+, into the proteoliposomes was not increased by S-adenosylmethionine. These results suggest that enzymatic methylation of membrane components may have an important role in the regulation of calcium transport in the chicken erythrocyte membrane and this regulation is rather specific for calcium.