Base-exchange reactions of the phospholipids in cardiac membranes

Pathways and mechanisms of N-acylethanolamine biosynthesis: can anandamide be generated selectively?

Long-chain N-acylethanolamines (NAEs) and their precursors, N-acylethanolamine phospholipids, are ubiquitous trace constituents of animal and human cells, tissues and body fluids. Their cellular levels appear to be tightly regulated and they accumulate as the result of injury. Saturated and monounsaturated congeners which represent the vast majority of cellular NAEs can have cytoprotective effects while polyunsaturated NAEs, especially 20:4n-6 NAE (anandamide), elicit physiological effects by binding to and activating cannabinoid receptors. It is the purpose of this article to review published data on the pathways and mechanisms of NAE biosynthesis in mammals and to evaluate this information for its physiological significance. The generation and turnover of NAE via N-acyl PE through the transacylation-phosphodiesterase pathway may represent a novel cannabinoid receptor-independent signalling system, analogous to and possibly related to ceramide-mediated cell signalling.

The effect of calcium ions and the calcium ionophore A23187 on choline uptake and phosphatidylcholine biosynthesis in chick embryo hearts

The effect of increases in extracellular calcium [Ca]o and the calcium ionophore A23187 on choline uptake and phosphatidylcholine biosynthesis was assessed in isolated cardiac myocytes. The cells were obtained from 7-day old chick embryos and were maintained in culture. Choline uptake was examined using [methyl 3H] choline. A23187 was found to increase choline uptake through the saturable choline uptake process. Pulse chase experiments using [methyl 3H] choline showed that after a 2 h incubation with choline, about 85% of the label was recovered in phosphocholine with most of the rest in phospholipid and a small amount in CDP-choline and glycerol phosphocholine. Increases in [Ca]o up to 10 mM did not affect the amount of label in phosphocholine or phospholipid, the rate of disappearance of label from phosphocholine, or the rate of appearance of labelled choline in phospholipid. In contrast, A23187, at concentrations up to 10(-4) M, was associated with a significant (p less than 0.05) increase in choline in the phosphocholine and phospholipid pool compared to control cells. The time course of the disappearance of choline from the phosphocholine pool and appearance in phospholipid pool was not significantly different between control cells and those treated with A23187. A23187 increased choline uptake via the specific uptake process. The effect on choline uptake may be attributed to the action of A23187 to facilitate the release of calcium from specific intracellular calcium storage sites rather than a nonspecific increase in [Ca]i that may have resulted from the increase in [Ca]o.

Presence of base-exchange activity in rat brain nerve endings: dependence on soluble substrate concentrations and effect of cations

The calcium-dependent, energy-independent incorporations of 14C-labeled bases, choline, ethanolamine, and serine, into their corresponding membrane phospholipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine, were compared in microsomes and in subcellular fractions prepared from a lysed crude mitochondrial (P2) pellet of whole rat brain. When activities were measured in the presence of an extracellular (1.25 mM) concentration of Ca2+, recovered activities were highest in the microsomal fraction, although substantial activity remained associated with the P2 homogenate even after repeated washing of the pellet. When this washed P2 homogenate was subfractionated, enrichment of all three exchange activities was obtained only in a fraction that was fivefold enriched over the homogenate and sevenfold enriched over the microsomal fraction in Na+, K+-ATPase, a plasma membrane marker. This strongly suggests that the base-exchange enzymes are normal constituents of synaptosomal plasma membranes. The three exchange activities were measured in synaptosomes prepared from whole rat brain in the presence of various substrate (base) concentrations, and kinetic constants were calculated. The Vmax values for choline, ethanolamine, and serine exchange were, respectively, 1.27 +/- 0.09, 1.60 +/- 0.17, and 0.56 +/- 0.06 nmol/mg of protein/h; the respective Km (apparent) values were 241 +/- 29, 65 +/- 18, and 77 +/- 22 microM. Endogenous levels of the three bases, choline, ethanolamine, and serine, in whole (microwaved) rat brains were 20 +/- 8, 78 +/- 28, and 639 +/- 106 nmol, respectively. That ethanolamine and serine incorporations had lower Km values than choline incorporation suggests that these bases are preferentially incorporated into their respective phospholipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulatory effect of calmodulin antagonists on phospholipid base-exchange reactions in rabbit platelet membranes

The properties of Ca2+-dependent incorporation of [3H]serine, [3H]ethanolamine and [3H]choline into the corresponding phospholipids mediated by base-exchange enzymes in rabbit platelet membranes were studied in the presence or absence of the calmodulin antagonists chlorpromazine, trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), all of which markedly activate three base-exchange reactions. The base-exchange activities were dependent on Ca2+ both in the presence and absence of the drugs. Other metal ions tested did not stimulate the base-exchange reactions, even in the presence of the drugs. Apparent Km values for serine, ethanolamine and choline were not affected significantly by the concentration of Ca2+, with or without the drugs. [3H]Serine incorporation into phospholipid was competitively inhibited by ethanolamine and choline, [3H]choline incorporation was competitively inhibited by serine and ethanolamine, whereas [3H]ethanolamine incorporation was competitively inhibited by serine and noncompetitively by choline. These competitive and noncompetitive relations between each base were also not affected by the drugs. The amount of 45Ca2+ binding to platelet membranes was decreased by the drugs dose dependently. A weaker calmodulin antagonist, N-(6-aminohexyl)-1-naphthalenesulfonamide (W-5), only slightly stimulated the base-exchange reactions, but did clearly inhibit 45Ca2+ binding to the membranes, in the same manner as that of the other calmodulin antagonists used. The concentration of chlorpromazine, trifluoperazine, W-7 and W-5, required to produce half-maximal inhibition of Ca2+ binding, was approximately 30 microM. These results suggest that the calmodulin antagonists used activate the base-exchange reactions only in the presence of Ca2+ without changing the affinity of each free base to base-exchange enzymes. The activation of the base-exchange reactions was not due to the increase in free Ca2+ caused by the drug-induced inhibition of Ca2+ binding to platelet membranes.

N-Acylation of ethanolamine phospholipids in canine myocardium

N-Acylethanolamine phospholipids, which are found in infarcted but not in normal canine myocardium, were produced by preparations of normal myocardial tissue incubated in the presence of millimolar concentrations of Ca2+. Biosynthetic activity from endogenous substrates was associated with both microsomal and mitochondrial fractions and exhibited an alkaline pH optimum. The time course of N-acylethanolamine phospholipid synthesis and degradation was followed after pulse labeling of myocardial ethanolamine phospholipids with [1,2-14C]ethanolamine. Enzymic N-acylation of both phosphatidylethanolamine and lysophosphatidylethanolamine was demonstrated by incubating these substrates with homogenates of myocardial tissue. Neither free fatty acids nor acyl-CoA derivatives served as acyl donor but some of the constituent fatty acids of exogenous phosphatidylethanolamine were recovered among the amide-linked fatty acids of the N-acylethanolamine phospholipids. N-Acylation may thus occur by the transfer of O-acyl groups catalyzed by a Ca2+ -dependent transacylase. Since N-acylethanolamine phospholipids are precursors of the biologically active N-acylethanolamines, their biosynthesis may constitute an important injury-induced metabolic event aimed at the protection of ischemic myocardial tissue.

On the biosynthesis and metabolism of N-acylethanolamine phospholipids in infarcted dog heart

Minced tissues from both infarcted and apparently normal areas of canine myocardium 6 h after ligation of the left descending branch of the coronary artery were incubated with [1,2-(14)C]ethanolamine. In each case substantial amounts of radioactivity were incorporated into both phosphatidylethanolamine and N-acylphosphatidylethanolamine. Incubation of homogenates from the same tissues with N-[1-(14)C]palmitoyl phosphatidylethanolamine yielded labeled N-palmitoyl ethanolamine. The data support the concept that N-acylethanolamines, pharmacologically active compounds which accumulate in infarcted myocardium, are produced by N-acylation of ethanolamine phospholipids followed by phosphodiesterase action.