1.5. Metabolism of phospholipids
Phospholipids are not an essential energy material. They play an important role in the organization of cellular membranes, in formation of lipoprotein complexes, blood coagulation, immunological reactions, in electron transfer in respiratory chain.
Phospholipids under hydrolysis break down into the higher fatty acids, phosphoric acid, nitrogen bases and glycerole. Phospholipase belongs to esterase (hydrolases subclass).
Phospholipase А1 attacks ester bond of phospholipids in position 1. Phospholipase А2 catalyzes hydrolysis of ester bond in position 2, free fatty acid and lysophospholipid are formed. Phospholipase A1 is localized in endoplasmic reticulum, and А2 - in mitochondria. Phospholipase C attacks ester bond in position 3, 1,2-diglyceride and phosphorylated base are formed. Phospholipase D catalyzes elimination of a nitrogen base from phospholipid.
Phospholipases А2 are best studied. They are found in poison of some snakes. The action of phospholipase on membranes leads to significant changes in their functional activity.
The further metabolism of fatty acids and glycerole was desribed earlier. We will consider choline transformations. One of the major reactions is its acetylation:
Acetylcholine participates in the conduction of nerve impulses.
Phospholipids biosynthesis runs intensively in liver, intestinal wall, testes, ovaries, mammary glands, mainly in endoplasmic reticulum of a cell.
During the synthesis of choline- and ethanolamine containing phospholipids with the help of CTP the CDP-choline or CDP-ethanolamine (active form of nitrogen bases) are formed. Then they react with 1,2-diglycerides. Phosphatidylcholine (lecithin) can be formed also from phosphatidylethanolamine. The donor of methyl groups is S-adenosyl methionine.
At synthesis of inositol containing phospholipids with the help of CTP CDP-diglyceride is formed. It is the active form of diglyceride which then reacts with the nitrogen base.
1.6. Essencial fatty acids. Eicosanoids
Linoleic (18:1; 9,12) and linolenic (18:3; 9,12,15) acids can’t be formed in an organism of mammals including humans due to the lack of the corresponding enzymes. These acids are essential as well as arachidonic acid (20:4; 5,8,11,14). In most mammals, arachidonic acid can be formed from linoleic acid. At long absence in nutrition animals lag in growth, characteristic lesions of skin and hair appear.
Eicosanoids is a large group of physiologically and pharmacologically active substances. They are prostanoids (prostaglandins, prostacyclins, thromboxanes) and leukotrienes. Eicosanoids are derived from arachidonic acid which is a part of phospholipids of plasma membranes. Eicosanoids are divided into 3 groups.
Prostaglandins are 20-carbon fatty acids containing a 5-carbon ring and hydroxi- and/or ketogroups. They are synthesized through the cyclooxigenase pathway. They are synthesized practically in all cells, except erythrocytes and lymphocytes. They influence the tone of smooth muscles, affect blood pressure, control transport of ions across membranes, etc.
It is possible to use PG for prevention of fertilization, stimulation of normal childbirth, interruption of pregnancy, prevention of gastric ulcers, or pain relief, treatment of inflammation and regulation of blood pressure, removal of asthma attacks, etc.
Prostacyclins are subspecies of prostaglandins. They are formed in blood vessel walls. They are inhibitors of thrombocytes aggregation.
Thromboxanes are formed in thrombocytes and after entering a bloodstream invoke vasoconstriction and thrombocytes aggregation.
Leukotrienes are synthesised in leucocytes, thrombocytes and macrophages by lipoxygenase pathway. Leukotrienes are mediators of inflammatory reactions; they cause contraction of bronchial muscle, promote contraction of coronary vessels.