Phospholipids phosphatidylinositol

The phosphorylated phospholipid, phosphatidylinositol bisphosphate, is present in cell membranes. On hydrolysis by a phospholipase, it produces two products, inositol trisphosphate and diacylglycerol (Figure 11.25), as follows ... 

In Uver, adrenaline binds to the a-receptor, and the hormone-receptor complex activates a membrane-bound phospholipase enzyme which hydrolyses the phospholipid phosphatidylinositol 4,5-bisphosphate. This produces two messengers, inositol trisphosphate (IP3) and diacylglycerol (DAG) (Figure 12.5). The increase in IP3 stimulates release of Ca ions from the endoplasmic reticulum into the cytosol, the effect of which is glycogen breakdown and release into the blood (see Figure 12.5 and Chapter 6). 

Phosphatidylcholine (lecithin) is the most abundant phospholipid in membranes. Phosphatidylethanolamine (cephalin) has an ethanolamine residue instead of choline, and phosphatidylserine has a serine residue. In phosphatidylinositol, phosphatidate is esterified with the sugarlike cyclic polyalcohol myo-inositol. A doubly phosphorylated derivative of this phospholipid, phosphatidylinositol 4,5-bisphosphate, is a special component of membranes, which, by enzymatic cleavage, can give rise to two second messengers, diacylglycerol (DAG) and inositol l,4,5trisphosphate (InsPsi see p.386). 

Type Gq G proteins activate phospholipase C [4]. This enzyme creates two second messengers from the double-phosphorylated membrane phospholipid phosphatidylinositol bisphosphate (PinsP2), i. e., inositol 1,4,5-tris-... 

Whitman, M Downes, C. P Keeler, M Keller, T and Cantley, L. (1988) Type 1 phosphatidylinositol kinase makes a novel inositol phospholipid, phosphatidylinositol 3-phosphate. Nature 33, 644-646. 

In the early 1950s, several hormones were shown to enhance the incorporation of 32Pj into the anionic membrane phospholipid, phosphatidylinositol (for review see Ref. 1). In 1975, Michell [2] noted that agonists which influence inositol lipid metabolism also provoke Ca2+ mobilisation in stimulated cells. It was first thought... 

Activation of phospholipase C leads to cleavage of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate (IP3) and diacylglycerol (DAG). 1P3 promotes release of Ca2+ from storage organelles, whereby contraction of smooth muscle cells, breakdown of glycogen, or exocytosis may be initiated. DAG stimulates protein kinase C, which phosphorylates certain serine- or threonine-containing enzymes. 

Inositol is a hexahydric sugar alcohol. Its main function is in phospholipids phosphatidylinositol constitutes some 5% to 10% of the total membrane phospholipids. In addition to its structural role in membranes, phosphatidylinositol has a major function in the intracellular responses to peptide hormones and neurotransmitters, yielding two intracellular second messengers inositol trisphosphate and diacylglycerol. 

Phospholipids were visualized as blue-colored bands by using Dittmer s reagent, which specifically stains phospholipids. Phosphatidylinositol is completely separated from phosphatidic acid (PA) and phosphatidylserine... 

PIP2 is formed in the cell membrane by the successive phosphorylation of the membrane phospholipid phosphatidylinositol (PI) to phosphatidylinositol 4-monophosphate (PIP) and then to PIP2. PIP2 is readily recycled by specific phosphomonoesterases to PIP and eventually back to PI. This cycle continues at rest until stimulation of surface receptors occurs. Receptor... 

The fatty acid components of phospholipids may vary, and thus cardi-olipin, as well as most other phospholipids, represents a class of molecules rather than a single species. Asa result, a single mammalian cell may contain thousands of distinct phospholipids. Phosphatidylinositol is unusual in tliat it has a nearly fixed fatty acid composition. Stearic acid usucilly occupies the C-1 position and arachidonic acid the C-2 position. 

Other receptor systems are coupled via GTP-binding proteins (G ), which activate phospholipase C. Activation of this enzyme releases the second messengers inositol triphosphate (IP3) and diacylglycerol (DAG) from the membrane phospholipid phosphatidylinositol bisphosphate (P1P2). The IP3 induces release of Ca2+ from the sarcoplasmic reticulum (SR), which, together with DAG, activates protein kinase C. The protein kinase C serves then to phosphorylate a set of tissue-specific substrate enzymes, usually not phosphorylated by protein kinase A, and thereby affects their activity. 

Significant amounts of phosphatidic acid (PA) are formed during processing as a result of hydrolysis of other phospholipids. This can be explained as the effect of phospholipases and hydrothermal treatment during the conditioning of flaked seeds. Cmolik and co-workers (1987) observed an increase in the amount of phospholipids from 0.5% to 15% during conditioning of seed flakes. It was reported that hydratable phospholipids such as the phosphatidylcholines (PC) and phosphatidylethanolamines (PE) assist the removal of non-hydratable phospholipids. Phosphatidylinositol (PI) and PA are considered to be... 

PIP5K produces the versatile phospholipid phosphatidylinositol 4,5-bispho-sphate (PI4,5P(2)). The physiological functions of the various PIP5K isoforms are not yet fully characterized. In an abstract presented at the AACR in 2009, Treinies reported on a novel PIP5K assay developed by CRT Discovery... 

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