Lipids site of synthesis

Smooth endoplasmic reticulum (SER) is site of lipid synthesis and drug metabolism... 

The endoplasmic reticulum (ER) is folded and provides a large surface area. It is the roadway of the cell and allows for transport of materials through and out of the cell. There are two types of ER. Smooth endoplasmic reticulum contains no ribosomes on their surface. This is the site of lipid synthesis. Rough endoplasmic reticulum has ribosomes on their surface. They aid in the synthesis of proteins that are membrane bound or destined for secretion. 

To determine which organelles were involved in lipid synthesis, plant cell organelles (mitochondria, microsomes, peroxisomes, plasmalemma, nuclei) were isolated in vitro and fed with various radioactive precursors ([ C]ace-tate or [ C]acetyl-CoA, [ K ]malonate or [ K ]malonyl-CoA, [ P]phos-phate, [ C]glycerol or [ K]l]glycerophosphate, [ C]choline or [ CJethanola-mine, etc.). The biosynthetic capacities of each organelle were thus evidenced (see Mazliak, 1975, 1977) in an effort to locate the sites of lipid synthesis in plant cells. The following points appeared clearly from the experiments ... 

In mammals the introduction of new double bonds into mono- and polyunsaturated fatty acids exclusively occurs in the carboxyl end and is never directed toward the terminal methyl-group. Therefore no transition of fatty acids belonging to the linoleic acid family into those of the linolenic acid type has been observed. This has been shown by means of terminally labeled synthetic polyunsaturated fatty acids (Stoffel 1961, Klenk 1964). The complete enzyme system for polyunsaturated fatty acid synthesis is arranged on the cytoplasmic membranes. In view of the importance of polyunsaturated fatty acids for the structure of glycero-phospholipids, it is interesting to mention the acyl-transferases catalyzing the acylation of the j8-position of lysolecithin, lysophosphatidic acid and L-a-glycero-phosphate. These and other enzymes of phospholipid biosynthesis are located in the cytoplasmic reticulum, which therefore appears to be the main site of lipid synthesis of the cell. 

Acyl carrier protein (ACP) plays a central rOle In lipid metabolism, serving as both a component of plant fatty acid synthetase (1) and as a substrate/cofactor for complex lipid biosynthesis (2). The protein has been purified from a number of plant sources and Its amino acid sequence determined for the protein from both barley leaf (3) and spinach leaf (4) material. Both of these two previously mentioned sources of ACP have two detectable forms of the protein (5-6) whilst in seed material only one form has been detected (5). ACP has been shown, using immunological techniques, to be a developmentslly regulated protein In maturing soy bean seeds. The activity of the protein appearing just prior to lipid accumulation (7). Despite the Importance of this protein in lipid metabolism and the fact that seeds are a major site of lipid synthesis there Is no reported literature on the characterization of ACP from seed material. The present study was aimed at a detailed characterization of ACP from rape (Brassica napus ) seed ... 

The liver is a major metaboUc organ. It is the location of the urea cycle, purine synthesis and is the main site of lipid synthesis. It is also the site of KB synthesis, though it is unable to utilize KBs for energy as it lacks the enzymes 3-ketoacyl CoA transferase or succinyl CoA transferase (also known as thiophorase). Approximately 10 per cent of the liver s mass is glycogen, compared with 1—2 per cent of the mass of muscle. However, as there is far more muscle than liver in the body, about two-thirds of the body s glycogen by weight is stored in muscle. 

In eucaryotic cells, glycolysis, gluconeogenesis and fatty acid synthesis takes place in the cytosol, while the Krebs cycle is isolated within mitochondria glycogen is made in glycogen granules, lipid is synthesized in the endoplasmic reticulum and lysosomes carry on a variety of hydrolytic activities. As in procaryotic cells, ribosomes in the cytosol are the site of protein synthesis. 

After cholesterol is formed, it can be converted to other steroids of widely varying physiological function. The smooth ER is an important site for both the synthesis of cholesterol and its conversion to other steroids. Most of the cholesterol formed in the liver, which is the principal site of cholesterol synthesis in mammals, is converted to bile acids, such as cholate and glycocholate (Figure 21.30). These compounds aid in the digestion of lipid droplets by emulsifying them and rendering them more accessible to enzymatic attack. 

The liver is the site of the synthesis of plasma phospholipids and lipoproteins. Rats maintained on a diet deficient in choline often develop fat deposits in liver tissue. How could choline deficiency be related to this aberration in lipid metabolism ... 

There are a number of different of levels at which control of fatty acid biosynthesis is exerted, and these may involve regulation of the rate of synthesis and of the proportions of particular fatty acids synthesised. Liver, rather than adipose tissue, is the main site of lipid biosynthesis in birds two thirds of the total lipid biosynthesis in the domestic fowl occurs in the liver (Saadoun LeClerq, 1983). Diet has a greater effect on hepatic lipid biosynthesis than it has on that in other tissues of the body (Saadoun Leclercq, 1987). Most studies on control of the rate of lipid biosynthesis have been based on the liver, but those on the control of the type of fatty acid synthesised have used the uropygial gland. 

Effect of Insulin and Diabetes on Adipose Tissue. For a long time, the role of adipose tissue in metabolism was underestimated. It is now clear that the adipose tissue cell is more than a site of lipid storage. It is also an actively metabolizing cell—performing lipid synthesis and breakdown—with well-developed... 

Membranes from different sources have different concentrations of proteins, carbohydrates, and lipids (Table 2). Major differences between bacterial and mammalian membranes can be seen (Kom, 1968) however, it is the purpose of this review whenever possible to deal exclusively with mammalian systems. Among mammalian membranes, myelin represents the least complex system with respect to enzymatic or biosynthetic activity. Other mammalian membranes exhibit many functional and enzymatic properties, and it is therefore not surprising that these membranes are in a state of flux and that turnover of the components may be seen (Arias et al., 1969 Baker and Humphreys, 1972 Ferber, 1971 Warren and Click, 1968 Siekevitz et al., 1967 Haddad et al., 1977 Doyle and Baumann, 1979). Click and Warren (1969) have even demonstrated protein synthesis in isolated surface membrane vesicles from mouse fibroblasts, indicating sites of protein synthesis within the membrane structure. 

The type II cells which are present at the alveolar surface are the exclusive site of surfactant synthesis. In these cells the necessary proteins and lipids are made on the endoplasmic reticulum from where they are assembled into lamellar bodies. Surfactant is then released from the latter by exocytosis. 

LIPOPROTEINS. Blood plasma lipoproteins are prominent examples of the class of proteins conjugated with lipid. The plasma lipoproteins function primarily in the transport of lipids to sites of active membrane synthesis. Serum levels of low density lipoproteins (LDLs) are often used as a clinical index of susceptibility to vascular disease. 

LBPs are likely to have conventional roles in the energy metabolism and transport of lipids in nematodes for membrane construction, etc. Many parasitic helminths have deficiencies in the synthesis of some lipids and so their lipid acquisition, transport and storage mechanisms clearly need to be specialized and therefore pertinent to the host-parasite relationship (Barrett, 1981). From a practical point of view, lipid transporter proteins may also be important in the delivery of anthelmintic drugs to their target most anthelmintics are hydrophobic and if they do not distribute to their site of action within the parasites by simple diffusion across and along membranes, then the parasite s own carrier proteins may be involved. 

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