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SUGGESTIONS TO TEACHERS OF BIOLOGICAL CHEMISTRY.
I. THE CLASSIFICATION OF LIPINS VICTOR E. LEVINE, DEPARTMENT OF BIOLOGICAL CHEMISTRY AND NUTRITION. SCHOOL OP MEDICINE. CREIGHTON UNIVERSITY, OMAHA, NEBRASKA
The classifications of carbohydrates and proteins have been well worked out and have proven quite satisfactory. Not so with the classification of lipins, for in this field much confusion of terms yet occurs. Thus the term lipin and lipoid are used interchangeably by many authors. Since the tentative classification of Rosenbloom and Gies' no further attempt has been made in the classification of the lipins. The classification now proposed divides the lipins into two main divisions: the true lipins and the lipoids. In the true lipins are included all ether soluble or alcohol-ether soluble compounds, which yield on hydrolysis fatty acid and alcohol. In the lipoids are to be found all ether soluble or alcohol-ether soluble compounds of biologic occurrence, which are not esters holding in combination fatty acid and alcohol, such as the sterols and the essential oils. The true lipins, following closely the nomenclature for proteins, are subdivided into simple lipins, conjugated lifiins and derived lipins. The conjugated lipins are further grouped into classes which bear close analogy to those in the conjugated proteins. The chromolipins, phospholipins, and glycolipins find similar prototypes in the chromoproteins, pbosphoproteins, and glycoproteins. The derived lipins, like the derived proteins, represent hydrolytic decomposition products. The close resemblance in the nomenclature followed for protein and for lipin is of obvious advantage to the student. The Classification of Lipins The term "lipin" is applied to a class of biologic substances soluble in ether or in an alcohol-ether mixture. The lipins may be divided into two main groups: true lipins and lipoids.
True Lipins The true lipins may be classified into three groups: simple lipins, conjugated lipins and derived lipins. I. SIMPLE LIPINS
(Compounds which yield on hydrolysis, fatty acid and glycerol or fatty acid and a monohydric alcohol of high molecular weight.) a Fats-Neutral esters of glycerol and fatty acid which are solid at 20' C. 6 Fatty oils-Neutral esters of glycerol and fatty acid which are liquid at 20' C. 1 Drying oils-These harden on exposure to light and air. Example: linseed oil. J. Ro~enhloomand W. J. Gies, Biochemical BdIeLin, 1, 51 (1911-12).
2 Semi-drying oils-These thicken slowly on exposure to light and air. Example: cottonseed oil. 3 Non-drying oils-These remain liquid on exposure to light and air. Example: olive oil. c Waxes-Esters of the higher alcohols such as cetyl alcohol, myricyl alcohol, etc., with iatty acids. Example: beeswax, carnnuba wax. 11. CONJUGATED LIPINS (Compounds which yield on hydrolysis, not only fatty acid and alcohol but some other complex such as sulphuric acid, phosphoric acid, monosaccharide, amino acid or some organic base like choline or neurine.) a Phospholipins (Phosphatides)-These yield on hydrolysis, fatty acid, glycerol. phosphoric acid and some organic base like choline, neurine, etc. Enample: lecithin; cephalin; sphingomyelin. 2 h Glycolipins-These yield on hydrolysis, fatty acid, glycerol and a monosaccharide such as glucose or galactose. Example: cerebrin, phrenosin, kerasin. c Glycophospholipins-These yield on hydrolysis besides fatty acid and glycerol, phosphoric acid and also a monosaccharide. Example: jecorin. d Sulpholipins (Su1phatides)-These yield on hydrolysis, fatty acid, glycerol and sulphuric acid. Example: protagon. e Sulpho-phospholipins-These yield on hydrolysis, fatty acid, glycerol, sulphuric acid and phosphoric acid. Example: sulpho-phospholipin of lung. f Amindipins-These yield on hydrolysis amino acid, fatty acid and glycerol. Example: bregenin. g Proteolipins-These are lipins in conjugation with protein. Proteolipins, in which the prosthetic group is a higher fatty acid, are easily made artificially although their existence in biologic tissue is still doubtful. Proteolecilhin is a type or proteolipin found in cytoplasm and in limiting membrane. The prosthetic group is lecithin or same other phospholipin. None of this type has thus far been isolated. h Chromolipins-These are colored organic compounds soluble in the usual lipin solvents. Example: yellow pigment of egg yolk, of butter f a t and of xanthomas, of staphylococcus pyogenes aureus and of staphylococcus pyogenes citreus. 111. DERIVED LIPINS
(Compounds other than phosphoric acid, sulphuric acid, amino acid or monusaccharide, obtained as a result of the decomposition of lipins.) a Fatty Acids-Example: oleic acid, linoleic acid, palmitic acid, stearic acid. h Alcohols-Example: glycerol, myricyl alcohol. c Organic BasesExamplcs: choline, neurine, oxyethylamine, onyamino-hutyric acid.
Lipoids (Compounds not esters of fatty acid and alcohol, but which are closely The phospholipins have been customarily subdivided into groups varying in the number of atoms oi nitrogen and phosphorus contained. The recent work of Levene and also of MacLeanQpoint to the fact that only three lipins--lecithin, cephalin and sphingomyelin-have a definite composition and that the others are mixtures. In view of this finding, an elaborate sub-grouping of phospholipins seems unwarranted. P. A. Levene, Physiological Reuicius, 1, 327 (1921).
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associated with lipins and resemble them in their solubility in ether or alcohol-ether.) The lipoids may be subdivided into two groups: I. Sterols-These are alcohols, solid a t the ordinary temperature, readily soluble in ether and in chloroform, and easily crystallized. Example: cholesterol, phytosterol, stercorin, coprosterin. 11. Essential Oils-These are volatile, oily and generally odoriferous substances of varied chemical nature, heing aldehydes, acids, terpenes, alcohols, etc. Example: oil of cloves, turpentine, oil of wintergreen.
The acids entering into the composition of lipins may be conveniently divided into groups which follow: A. Straight-Chained or Aliphatic Acids. I.
Saturated Acids. n
h
Monocarhoxylic Acids. 1 Saturated fatty acids of the general formula C.H%-lCOOH. Example: palmitic acid (n = 15). stearic acid (n = 17). 2 Hydroxy acids of the general formula G H d 0 H ) C O O H . Example: lanopalmic acid (n = 15) in wool fat; cocceric acid (n = 30) in cochineal wax. 3 Dihydroxy acids of the general formula CIH~-X(OH)~COOH. Example: dihydroxy stearic acid in = 17) in castor oil, lanoceric acid (n = 29) in wool fat. Dicarbovylic acids of the general formula C,Hm(COOH)2. Example: japanic acid (n = 20) in Japan wax.
11. Unsaturated Acids.
n Unsaturated acid3 with one double bond of the general formula C,H2,-,COOH. Example: oleic acid in = 17). h Unsaturated acids with one double bond and hydroxy group of the general formula CnHn-n(OH)COOH. Example: ricinoleic acid in = 17). c Unsaturated acids with two double bonds of the general formula C,HI,-.COOH. Example: linoleic acid (n = 17). d Unsaturated acids with three double bonds of the general formula CnHln-sCOOH. Example: linolenic acid (n = 17). e Unsaturated acids with four double bonds of the general formula C,H2,-v COOH. Example: clupadonic acid (n = 17) in Japanese sardine oil, isamic acid (n = 13) in a vegetable from the French Conga, therapic acid (n = 17) in cod-liver oil. B. Cyclic Acids. Example: hydnacarpic acid and chaulmwgric acid found in chaulmwgra oil.
Both acids have the same molecular formula as linoleic acid. Instead of absorbing four atoms of bromine, chaulmoogric acid absorbs only two. The second unsaturated linkage occurs in cyclic formation.
