Historically, the presence of phosphorus in brain tissue seems to be reported for the first time in 1719 by Johann Thomas Hensing (1683-1726, professor of medicine et the University of Giessen, Deutchland). He chose the brain as an object of study because he recognized that organ as “truly the throne of the soul and the abode of wisdom, from whose nature the former is the recipient of the virtues of health, and the latter of brilliance”. He carried out his notable study of the chemical composition of the brain and published in Latin his results in 1719 (review in: D. B. Tower, Hensing, 1719. An Account of the First Chemical Examination of the Brain and the Discovery of Phosphorus Therein, Raven Press, New York, 1983). Hensing’s analysis of the brain included the “volatiles” (mainly water), solids, and ash. Hensing examined the composition of ash produced after eating at high temperature and revealed the presence of elemental phosphorus. This was an original discovery as phosphorus had been found previously only in excreta and in the ash of vegetable matter. The presence of phosphorus in the brain was especially intriguing, as its light-emission (phosphorescence) suggested relationships with thinking and generation of ideas.
Antoine-François Fourcroy, who was first professor of chemistry at the Royal Veterinary School at Alfort and at the Jardin du Roi, and later Director of the French Museum of Natural History in Paris. He used extractions with aqueous solvents and alcohol, which led him to conclude that the brain consists of “animal pulp” (largely protein), fatty substances that he regarded as “soaps,” and salts, chiefly phosphates of calcium, ammonia, and sodium (Fourcroy AF., “Examen Chimique du Cerveau de Plusieurs Animaux,” Ann. Chim., 1793, 16, 282-322). He made no mention of Hensing’s work in his text. Forteen years later, Fourcroy and Vauquelin reported the discovery of phosphorus in fish roe. Their preliminary experiments showed that the roe residue was strongly acidic after combustion and that elemental phosphorus was condensing on the walls of the distillation tube. In other experiments, they extracted fish roe with alcohol and obtained a “soap-like material” which contained phosphorus (Fourcroy AF and Vauquelin NL., Expériences Chimiques, pour Servir à l’Histoire de la Laite des Poissons, Ann Chim Phys 1807, 64, 5-18).
Ten years after Fourcroy’s studies of brain chemistry, the German chemist Johann Ludwig Jordan undertook to repeat his work (Jordan JL, Zerlegungsversuche mit dem menschlichen Gehirn, Crell’s Ann. Chem., 1803, Part 5, 364-369, and Part 6, 447-451). Anecdotically, the “Crell’s Annalen für Chemie” created in 1778 by the Swiss professor of medicine Lorenz Crell was the first periodic journal focusing primary on chemistry in Europe. Jordan carried out many experiments and he concluded that the brain mass contains water, albumin (protein), sodium, ammonium, and calcium phosphates, and “a characteristic fatty material” which confirmed the conclusions of Fourcroy. Jordan regarded his lipid extract as a distinctly animal product not encountered elsewhere but probably located in the white matter.
The unmistakable presence of phosphorus in lipids extracted from brain with ethanol was revealed in 1811 by Vauquelin. This famous French chemist (1763-1829) which discovered also chromium and beryllium in 1797, was professor in the most famous institutions of his time. Vauquelin took Chevreul in his laboratory at the Muséum d’Histoire Naturelle in 1803 to study organic materials.
After Vauquelin, Jean-Pierre Couerbe, a French chemist from the Bordeaux region, was trained in chemistry at the School of Pharmacy in Paris, and worked in several laboratories. He introduced the use of ether as well as alcohol to extract brain lipids. Moreover, he was the first to analyze their individual constituents. Couerbe was able to separate five constituents, one of which was cholesterol. His elemental analysis of the isolated cholesterol conforms very closely to that of Chevreul. Thus, he demonstrated that it was a normal constituent of the brain. One of the other fractions, soluble in ether but not in alcohol or water, was saponifiable and contained phosphorus (J.-P. Couerbe, “Du Cerveau, Considéré sous le Point de Vue Chimique et Physiologique,” Ann Chim Phys 1834, ser.2, 56, 160-193), and so exhibited the properties of phospholipids. He named this fraction “céphalote” or “brain wax”. Although his elemental analysis of “céphalote” does not agree well with that for lecithin, his practice of characterizing each of his isolated fractions distinguished him as “the first to apply organic analysis to brain-products” (Thudichum J).
Several chemists isolated similar substances from brain in hot alcohol which were named “matière blanche”, “cérébrote”, “acide cérébrique” or “oleophosphoric acid”. Later, another French chemist, Gobley, isolated from egg-yolk and brain a phosphorus-containing lipid and named it “lecithin” (from the greek lecithos, egg-yolk). He showed in 1850 than glycerophosphoric acid could be prepared from lecithin (J Pharm Chim 1850, 17, 401). From all these investigations he proposed a structure for lecithin, including oleic acid, margaric acid, phosphoglyceric acid and choline (Gobley M., J Pharm Chim 1874, 19, 346)
The phospholipid chemistry made considerable progress with Thudichum (1828-1901). He isolated and characterized many phospholipid fractions using only their nitrogen/phosphorus ratio (“Treatise on the chemical constitution of the brain“, Baillière, Tindall and Cox, London, 1884). He characterized “cephalin” (now phosphatidylethanolamine), distinct from lecithin by solubility properties. He isolated ethanolamine from the cephalin fraction but he considered it as a decomposition product of choline. Ethanolamine phospholipids were described later in the Institute of Physiology and Chemistry in Strasbourg, France (Baumann A., Biochem Z, 1913, 54, 30 and Renall M.H., Biochem Z 1913, 55, 296). Thudichum isolated also a phospholipid he named sphingomyelin (from greek sphingein, to bind tight – myelos, marrow) and described its molecular constituents. After alkaline hydrolysis, he successfully obtained its two constituents bases, sphingosine and choline, in addition to phosphoric acid and a fatty acid. From his studies, Thudichum concluded that phospholipids are “the center, life, and chemical soul of all bioplasm“.
In 1927 three well-defined phospholipids had been described: lecithin, cephalin and sphingomyelin. Later, several others were added to that list: phosphatidic acid isolated from cabbage leaves in 1927 (Chibnall A.C., Biochem J 1927, 21, 233), one acetal phosphatide (now plasmalogen) isolated from beef heart in 1939 (Feulgen R.Z., Physiol Chem 1939, 260, 217). Folch made an important scientific contribution in isolating from brain phosphatidylethanolamine, phosphatidylserine and an inositol phospholipid as components of “cephalin” in 1942 and a “diphosphoinositide” in 1949 (Folch J., J Biol Chem 1949, 177, 497). Cardiolipin was also isolated from brain in 1944 (Pangborn M.C., J Biol Chem 1944, 153, 343) .
During a long time, separation of phospholipids was based on their solvent solubilities. In one of his most famous paper, Folch (J Biol Chem 1942, 146, 35) exploited this peculiarity to separate brain cephalin into three fractions containing ethanolamine, serine and inositol.
Advances in our knowledge of phospholipids have depended upon new methods of separation and analysis. In 1936, first appeared the use of a column of aluminum oxide (Thannhauser et al., J Biol Chem 1936, 116, 527), in 1956 the use of chromatography on silica-impregnated paper (Marinetti et al., Biochim Biophys Acta 1954, 14, 374),and around 1960 the use of thin-layer chromatography (Wagner H. et al., Biochem Z 1961, 334, 175).
No general agreement exists on the best way to classify phospholipids but most classifications contain a category for the glycerol-containing phospholipids (Glycerophosphatides, formerly Phosphoglycerides) and one for the sphingolipids (Sphingosyl phosphatides, formerly Phosphosphingolipids). More recently, new class of phospholipids have been described: N-acyl-O-phosphocholineserines found in human and animal tissues and alkylphosphocholines which were obtained by chemical synthesis.
1 – The term glycerophospholipid signifies any derivative of sn-glycero-3-phosphoric acid that contains at least one O-acyl, or O-alkyl or O-alk-1′-enyl residue attached to the glycerol moiety and a polar head made of a nitrogenous base, a glycerol, or an inositol unit.
2 – The term sphingosyl phosphatide refers to lipids containing phosphorus and a long-chain base. Those containing also a glycoside moiety are considered elsewhere.
3 – N-acyl-O-phosphocholineserines have been described for the first time in 2019 in plasma of subjects suffering Niemann-Pick disease type C1 (Sidhu R et al., J Lipid Res 2019, 60, 1410). This new class of phospholipids was also shown to be present in plasma and brain of Nieman-Pick’s cat models. All molecular species of this phospholipid have fatty chain ranging from C14 to C24, the palmitoyl (C16) being the most abundant. All species could be used as biomarker for the Niemann-Pick disease.
N-Palmitoyl-O-phosphocholineserine
4 – Phospholipid-like molecules have been synthesized, the alkylphosphocholines, which have remarkable biological and therapeutic activities. They are phosphocholine esters of aliphatic longchain alcohols differing in chain length, unsaturation and position of the cis-double bond. Hexadecylphosphocholine (miltefosine), described as a new drug in cancer therapy, has been synthesized in 1992 (Eibl H et al., Prog Exp Tumor Res 1992, 34, 1). Chemically, it is the phosphocholine ester of hexadecanol and has similar physical properties as lysolecithin. That compound has been also used in the treatment of both cutaneous and visceral leishmaniasis (LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet]. Bethesda (MD): National Institute of Diabetes and Digestive and Kidney Diseases; 2012-2017 Mar 15).
Palmitoylphosphocholine
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