Lysolecithin synthesis

Cholesterol is formed in the liver (85%) and intestine (12%) - this constitutes 97% of the body s cholesterol synthesis of 3.2 mmol/day (= 1.25 g/day). Serum cholesterol is esterized to an extent of 70-80% with fatty acids (ca. 53% linolic acid, ca 23% oleic acid, ca 12% palmitic acid). The cholesterol pool (distributed in the liver, plasma and erythrocytes) is 5.16 mmol/day (= 2.0 g/day). Homocysteine stimulates the production of cholesterol in the liver cells as well as its subsequent secretion. Cholesterol may be removed from the pool by being channelled into the bile or, as VLDL and HDL particles, into the plasma. The key enzyme in the synthesis of cholesterol is hydroxy-methyl-glutaryl-CoA reductase (HGM-CoA reductase), which has a half-life of only 3 hours. Cholesterol is produced via the intermediate stages of mevalonate, squalene and lanosterol. Cholesterol esters are formed in the plasma by the linking of a lecithin fatty acid to free cholesterol (by means of LCAT) with the simultaneous release of lysolecithin. (s. figs. 3.8, 3.9) (s. tab. 3.8)... 

The triacylglycerols are incorporated into a heterogeneous population of spherical lipoprotein particles known as chylomicrons (diameter, 75-600 nm) that contain about 89% triacylglycerol, 8% phospholipid, 2% cholesterol, and 1 % protein. Phospholipids of the chylomicron arise by de novo synthesis (Chapter 19) or from reacylation of absorbed lysolecithin. Cholesterol is supplied by de novo synthesis (Chapter 19) or is absorbed. The protein apolipoprotein B-48 (apo B-48) forms a characteristic protein complement of chylomicrons and is synthesized in the enterocyte. Synthesis of apo B-48 is an obligatory step in chylomicron formation. Absence of apo B-48 synthesis, as in the rare hereditary disease abetalipoproteinemia,... 

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. 

Lysolecithin, because of its cyto- and possibly myelinolytic effect, has been searched for and found in small quantities in normal white matter and MS plaques [135, 136]. It may represent an intermediary in the synthesis of lecithin [137]. It could be, however, a product of autolysis [138] since it accumulates in rat brain slices on incubation in vitro [139]. It was not, however, found in MS white matter in other investigations [134]. 

---