Cephalins hydrolysis

Under suitable conditions all of the ester (and ether) linkages of a phosphatide can be hydrolyzed. What organic compounds would you expect to obtain from the complete hydrolysis of (see Table 23.5) (a) a lecithin, (b) a cephalin, and (c) a choline-based plasmalogen [Note Pay particular attention to the fate of the o ,/3-unsaturated ether in part (c).]... 

It was thought that phosphatidic acid formation might lead to the formation of the phospholipids, lecithin and cephalin. At this time direct incorporation of phosphatidic acids into the cationic phospholipids has not been demonstrated, but it is possible that the known precursors of lecithin, a,/8-diglycerides, are formed by hydrolysis of the phosphate esters of phosphatidic acids. The formation of the cationic lipids requires the following steps. 

Phospholipids A lipid containing phosphorus which on hydrolysis yields fatty acids, glycerin, and a nitrogenous compound. Lecithin cephalin and sphingomyelin are the best known examples. 

The phospholipases hydrolyze lecithin, cephalin, and other phospholipids. The products of the hydrolysis vary according to the bonds which are attacked, but only two phospholipases will be discussed here. The reader is referred to Zeller s review for a complete discussion of these enzymes. 

Earlier methods employed for the resolution of the phospholipid fraction of tissue depend primarily on the different solubility characteristics of its components. Kirk (46) extracted the precipitate obtained from the alcohol-ether extract of tissues by treatment with acetone and magnesium chloride with moist ether which dissolves only lecithin and cephalin and leaves sphingomyelin as an insoluble residue. Determination of choline in the ether-soluble fraction after the hydrolysis was a measure of the lecithin present and cephalin was calculated by difference from the phosphorus determinations. 

Addition of absolute alcohol to the ether solution has been used to precipitate cephalin (15,18,32,46,97). Artom and Fishman (8) estimated the lecithin and sphingomyelin in chloroform extracts from the choline content after hydrolysis, and sphingomyelin separated as the reineckate from a separate aliquot of the chloroform extract. Cephalin was calculated as the difference between the total and the choline-containing phospholipids. It should be noted that Sinclair and Dolan (82) have demonstrated that the ether-insoluble fraction of the phospholipids is simply a portion of all the phospholipids present and that the percentage of the material insoluble in ether is a function of the amount of magnesium chloride used in the separation of the phospholipid fraction of the tissue. Furthermore, sphingomyelin prepared as the ether-insoluble residue has since been found to be contaminated with hydrolecithin (91,92) which is also insoluble in ether. 

The relative resistance of sphingomyelin to alkaline hydrolysis has been used to some extent as a method for estimating the sphingomyelin content of tissues (75) and of lecithins and cephalin (37,45,103). Lecithin and cephalin are quantitatively hydrolyzed into acid-soluble phosphorus compounds by normal KOH at 37 C. within 24 hours. Thus, the difference between the total phospholipid phosphorus and the saponifiable phosphorus represents the amount of sphingomyelin phosphorus. 

The hydrolysis of lecithins and cephalins always produces, in addition to the oc-glycerophos-phoric acid, some of the -isomer, which has the phosphoric acid at the middle C atom. This observation led to the belief that there are both - and S-lecithins (and cephalins), but it is now recognized that the jS-form is an artifact, a rearrangement product, and that the natural glycero-phosphatides are derived without exception from the a-glycerophosphoric acid. 

Enzymic hydrolysis of aminoethylphosphoric acid in the tissues is presumably followed by utilization of the inorganic phosphate for synthesis of lecithin and of demethylation of lecithin to form cephalin. The aminoethylphosphoric acid normally occurring in the body tissue may be a product of catabolism of cephalin. 

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