The fats

Front matter

22102080458

MONOGRAPHS ON BIOCHEMISTRY

EDITED BY

Preface

There are many who study Physiology without appreciating how far the chemistry of the fats has been advanced by technical and scientific chemists. At the same time, there may be chemists, versed in the chemistry of fats, who do not appreciate all that this subject may mean to the biologist. This small book, which is aimed to reach both these classes of readers, and so runs the risk of missing both, must be considered to have attained its object if it proves useful to either. For the field of biochemical work to which it refers needs workers, who may be either physiologists who have trained themselves chemically, or chemists who are alive to the legitimate aspirations of biology.

J. B. L.

I I iLk A

T I.'

H*

* .

Contents

CHAPTER PAGE

Introduction . - - i

I. A. The Fatty Acids 4

Introduction

There is a large group of substances widely distributed through the animal and vegetable kingdoms which have one property in common, that they contain in their chemical constitution and are mainly composed of the radicals of fatty acids, principally and generally only the higher ones. These compounds with but few well-attested exceptions form a group by themselves that does not overlap the borders of the other groups of substances, the proteins and carbohydrates, which together with them constitute the material in which the phenomena of life are manifested. They are at least as sharply defined a group as either of these, and they hardly can be said to have less physiological significance. And yet there is no one comprehensive name to include them all. The term fat has a precisely defined chemical meaning which restricts its application to a certain number only of the substances of this group, and yet many of these substances that are in the strict chemical sense fats have on account of their physical properties to be spoken of as oils, while some are generally known as waxes : of the compounds, on the other hand, that are not in the strict sense fats, some have had assigned to them the term “ wax,” a term for which also a chemical definition has been devised that does not correspond to the ordinary meaning of the word, while for a large class, the physiological importance of which is daily coming more into evidence, no better general name has been proposed than “ lipoids,” which is at once a cloak for ignorance and an indefinable limbo into which any one can thrust anything of which he knows little or nothing, including often what is not a compound of any fatty acid at all.

Of these compounds of fatty acids occurring in nature the commonest are esters of the triatomic alcohol glycerol. These are fats when they are solid at the ordinary temperature and oils when they are liquid. Such esters may contain one, two or three fatty acid radicals, the mono-, di- or triglycerides as they are respectively called ; for instance the esters formed by butyric acid with glycerol

Of these the a form must clearly be capable of occurring in two stereoisomeric optically active modifications : — are : —

I, the a and /3 monobutyrins — a\ v-/r:i . kj n

i

I^CHa. OH

CHo. OH

I

2. the a/3 and the aa dibutyrin —

{CH2.OOC.C3H- jCH2.OOC.C3H7

CH.OOC.C3H, andaaicH.OH

I I

CH2.OH (^CHa . OOC . C3H7

the form being also capable of stereoisomerism : —

3. tributyrin —

CH,. OOC . C3H7 CH'. OOC . C3H7 CHa. OOC . C3H7

But in one and the same diglyceride or triglyceride molecule the fatty acid radicals may be not all the same as in the, above instances ; there may be two different radicals in a diglyceride and two or even three different ones in a triglyceride ; and then other stereoisomeric modifications become possible in the case of the aa diglycerides and also of the triglycerides.

When it is remembered that glycerides of a very large number of fatty acids occur in nature, it is clear that the number of arrangements of these in glycerol esters must be large indeed.

Again, when a glyceride contains less than three fatty acid radicals the place of the third or possibly of the second and third may be taken by an acid of a different kind. In lecithine, for instance, there are two fatty acid radicals, and the third hydroxyl group of the glycerol is coupled by an ester union with phosphoric acid. And there are many other compounds in which there is reason for supposing that phosphoric acid and fatty acids are coupled with the same glycerol molecule. Since phosphoric acid is a tribasic acid, the acid hydroxyl groups which are not engaged by the glycerol in such a compound are free to enter into combination with other groups, alcohols or bases ; as, for instance, in lecithine, with the alcoholic base choline.

Then, besides glycerol, there are several other alcohols that are found associated with fatty acids, and some of these are very widely distributed, if only in relatively small amounts.

Lastly, it must be remembered that fats and oils as they occur in nature are never single chemical substances, seldom indeed entirely composed of glycerides. In any fat or oil there is a mixture of several glycerides with varying quantities of cholesterol or substances related to it and often free fatty acids or soaps as well.

These considerations show that the number of compounds of fatty acids which have to be dealt with in biological chemistry, and for which we have no single comprehensive name, is very large ; and also that the precise constitution of any one such compound may, even when it is not complicated by the presence of phosphoric acid, be difficult to determine, whereas, where this complication does exist, the difficulty is bound to be very greatly increased. In fact at the present time these more complicated compounds of the fatty acids are among the most obscure of those with which animal or vegetable chemistry has to deal, more obscure and indefinite even than many of the proteins.

INTRODUCTION

The plan adopted in the following biochemical study of the compounds of fatty acids is first to review the properties and characteristics of the various fatty acids that are found in animals or plants, then those of the alcohols and their fatty acid esters. In the second section the methods in use in the laboratory for obtaining and determining the amount of compounds of fatty acids present in any given material will be dealt with, principally from the point of view of the student of animal physiology. In the third section the methods that have served hitherto for the purpose of examining and determining the character of the compounds obtained will be described, and finally, in the fourth section some points will be discussed in the physiology of the compounds dealt with in the previous sections.

NOTE ON TERMINOLOGY.

The following terms will be used in these pages : —

Phospholipines^ to denote compounds of fatty acids that contain phosphorus and nitrogen.

Galactolipines^ to denote compounds of fatty acids that contain nitrogen and galactose.

Lipines, to denote compounds of fatty acids containing nitrogen but no phosphorus or carbohydrate group.

The phospholipines correspond to the phosphatides of Thudichum, and include lecithine, kephaline, cuorine and sphingomyeline, and other still less completely defined substances.

The galactolipi?!ts correspond to the cerebrins, cerebrosides or cerebrogalactosides of various authors.

The lipines correspond to the amido lipotides and cerebrin acids of Thudichum, and include certain other substances described by others.

The practice of dividing up the “ phosphatides ” into mono- and diamino mono- or di-phosphatides has less to be said for it than at first sight appears. The resulting names convey no more than is conveyed when the ratio N ; P is given ; they cannot be remembered unless this ratio is remembered. Objections to the use of the term phosphatides for any of these substances are given in a later section.