T H E LAW O F PROBABILITY APPLIED TO T H E FORMATION O F FATS FROM CARBOHYDRATES1 BY EDGAR J. WITZEMANN
I n considering the possible mechanism of the formation of fatty acids in living organisms there are certain outstanding fundamental facts which must be taken into account. Five important facts of this kind follow: ( I ) The fats in plants are almost altogether synthesized from carbohydrates. The same is largely true in most animals. (2) The carbohydrates involved are largely hexoses or their polymers, since these are most abundant. I n animals glucose and glycogen alone are involved so far as is known. ( 3 ) The fatty acids occurring in plants and animals are almost solely composed of those acids having an even number of carbon atoms in their chains. (4) The fat stores are composed almost entirely of acids having long chains with eighteen carbon atoms. (5) Oleic acid is the principle unsaturated acid occurring in vegetable and mineral fats. The other unsaturated acids are quantitatively of isolated or much less importance. Taking the above facts into account two general types of hypotheses have been developed concerning the chemical mechanism of the formation of fatty acids. A. They are built up mainly from short carbon chains (less than six). B. They are built up mainly from units of six carbon atom chains. I n the first hypothesis ( 3 ) above, namely that the acids contain an even number of carbon atoms, has received undue emphasis. Most of the meagre direct chemical data has been marshalled in support of this type of hypothesis. Thus the 1 Contribution from the Otho S. A. Sprague Memorial Institute, Rush Medical College, Chicago, Ill.
Edgar J . Witzewtam
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recent theory of Magnus-Levy1 is based on the intermediate formation of acetaldehyde in order to explain the formation of butyric, caproic and acetic acids in liver autolysis. The sugar is thought to decompose into lactic acid which in turn gives carbon dioxide, hydrogen and acetaldehyde. Buchner and Meisenheimer,2 Nencki,? H. St. Raper2 and Euler5 have advocated somewhat similar ideas, which were extended in some respects, especially by Euler. Ida Smedley6 pointed out the possible importance of the condensation of acetaldehyde with pyruvic acid thus, for example : ( I ) CH3.COH CH3COC02H + CH3CHOHCH&OC02H, (2) CH3CHOHCHzCOCOsH 0 --+ CHSCHOHCH~CO~HCOS, (3) CHaCHOHCHzC02H - HzO + CH3CH : CHCOzH. Although the above types of hypotheses all represent reactions that can or do take place they fail in important respects to account for certain facts. For instance, why should the greater portion of fats be made up of (218 acids if acetaldehyde and pyruvic acid are intermediate compounds in fheir formation? Why should not intermediate short chain acids especially occur more largely if they are intermediate stages in the formation of the typical CIS acids? I n emphasizing the fact that fatty acids occurring in nature contain only an even number of carbon atoms, the other fundamental facts given above are disregarded. These facts may be graphically summarized by the statement that six carbon atom sugars give rise t o eighteen carbon atom fatty acids. This statement gives emphatic emphasis to the arithmetical relation of the number of carbon atoms in the compound transformed and in the final product. Emil Fischer' has utilized this fact clearly in his hypothetical interpreta-, tion of the formation of fats from sugar.
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Arch. Anat. Phys., Phys. Abt., 1902,365. Ber. deutsch. chem. Ges., 43, 1773 (1910). Ibid., IO, 1033 (1877). 4 Proc. Chem. SOC.,23, 235 (1907); Jour. Physiol., 32, 216 (1906). Pflanzenchemie, 11, z I z ( I 909). 6 Zentr. Physiol., 26, 915 (1912); Jour. Physiol., Dec. (1912). 7 Untersuchung iiber Kohlenhydrate und Fermente, I I O (1884-1903). 1
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“In order to derive stearic and oleic acids, which occur combined with glycerol in most fats, from sugar, it is only necessary to assume that three molecules of the latter are joined through their aldehyde groups as occurs with formaldehyde in the synthesis of sugar. Then a molecule of 18 carbon atoms would result in which only a transposition and removal of oxygen is necessary in order to produce these acids.” * * “For palmitic acid with 16 carbon atoms, which also occurs abundantly in fats, the same explanation would be adequate if one molecule of hexose and two molecules of pentoses which occur so abundantly in plants were brought together; moreover, i t is also possible that it is formed from a system with 18~carbon atoms by splitting.” * * * “If the sugar molecule is not completely broken up in the formation of fat, which I consider improbable, one may expect that the sugars with different carbon content will give rise to differently constituted fats.” Little or no laboratory experimental data is a t hand t o support this hypothesis but some of the biological data seems ummistakably clear in its support. The transformation of sugars and carbohydrates of unripe oil seeds into fat1 appears to be quite direct. The results of Gierke’s experiments2 give even clearer support. He found in guinea pigs that fat tissue which normally is free from glycogen on the second and third days, when they are heavily fed on carbohydrates, contains much glycogen on the eighth day. In fasting or in extended feeding periods the glycogen disappeared from the fat cells by the fourteenth day. However, this hypothesis, even in the absence of definite chemical data, commends itself for its simplicity and i t was therefore of interest to learn whether there are any other considerations that can be enlisted in its support. If fatty acids are built up two carbon atoms a t a time, as suggested by the first hypothesis, i t follows that since the over~~
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Cf. for instance F. Czapek: Biochemie der Pflanzen, 2nd Ed., p 742, et seq. Verh. d. pathol. Ges., 1906, 182.
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Edgar J . Witzemann
whelming proportion of all fatty acids found everywhere are CIS acids, there must be a reason for the synthesis proceeding to this point and then stopping just there. Such a reason could be supplied by a sufficiently sharp change in some physical property that regulates the synthesis. But such a sharp change is not known. I n fact the fatty acid series is one of the homologous series in organic chemistry most noted for regular and gradual transition in physical properties on moving up or down the series. In the absence of such a change or discontinuity in properties the synthesis of fatty acids by the addition of two carbon atoms at a time becomes amenable to the law of probability for any given set of chemical conditions. Thus if we accept the statistical fact of CISacids being in the preponderance in fats these acids become our “bull’s eye” in discussing their formation from this point of view. Accurate information as to the quantitative occurrence of the various fatty acids is not available, but a qualitative estimate on the basis of such information as is obtainable from the handbooks, etc., gives a curve such as that represented by the solid line in Fig. I . Now, on connecting up the points which represent the relative abundance of the various acids instead of obtaining a smooth descending curve on both sides of C18, the curve shows prominences at CI2, C24, etc., which do not conform to the typical probability curve. If, however, the prominences are connected, as in the dotted line in the figure the typical probability curve is obtained. Moreover, the number of carbon atoms at the prominences are all divisible by six and this suggests the direct relation to the parent hexose sugars provided by Emil Fischer’s hypothesis. Accordingly, the data when assembled and judged in terms of the law of probability, appear to be against the first type of hypothesis and to support the second hypothesis. Moreover by this view of it it becomes possible to explain why the major portion of fatty acids should have eighteen carbon atoms. If these acids are built up six carbon atoms a t a time the first complex Clz is still sufficiently soluble either as the disaccharide or as the CI2 acid to react rapidly with
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another C6 unit to give CISeither as the trisaccharide or the CI8 acid. But in both cases the CIScompound is much less soluble, and presumably less reactive, than the smaller Cg or Clz units so that higher acids would not be formed rapidly from CI8units. Here we have a sufficiently large change in physical properties when dealing with 6 carbon units to easily account for the non-formation of large amounts of fatty acids containing more than eighteen carbon atoms.
Fig. I
The smaller occurrence of the intermediate acids, such as Cl0, CI4, CI6, etc., may be due to the fact that they are formed from higher unsaturated acids by loss of two or more carbon atoms or by such syntheses as those involved in the first hypothesis. The apparent anomalously large occurrence of palmitic acid (C,,) may be accounted for by the known fact mentioned above that oleic acid undergoes a transforma-
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Edgar J . I/Vitzevnawn
tion a t the CY- and O-carbon atoms to give palmitic acid and another product. This type of reaction occurs for instance in the treatment of oleic acid with strong alkali. I n conclusion, it needs merely to be said that even the qualitative application of the law of probability appears to rationalize the problem of the mechanism of the formation of fatty acids from sugars in an interesting way. If true, the suggestion herein given constitutes a different biological application of the probability law from that known in biology as Quetelet’s Law of Fluctuating Variation.
