enzymes tend to transform the purin bodies to other members of the group. Thus, guanase hydrolyzes guanin with the formation of ammonia and xanthin; adenase hydrolyzes adenin into ammonia and hypoxanthin; hypoxanthin passes by oxidation into xanthin, which, in turn, through the agency of xanthinoxidase is transformed into uric acid. Finally, the uricolytic enzymes transform uric acid into allantoin, though this destructive power seems to have been largely or wholly lost in human beings. Guanin and adenin preëxist in nucleic acid. Among the products of hydrolysis, xanthin and hypoxanthin are apt to be produced. change may also be brought about by the enzymes in the tissues. Such Caffein is the familiar constituent of coffee beans (0.8-1.7 per cent), cocoa beans (0.1-0.8 per cent), cocoanuts (1.2 per cent), and tea leaves (2-5 per cent). Two and one-half to 5 per cent is present in guarana, the roasted fruit of Paullinia, which is eaten in South America. Theobromin is present to the extent of 1.5-2.4 per cent in cocoa beans and in smaller amounts in Kolo nuts and tea leaves. It is not present in coffee beans. Theophyllin has been isolated from the extracts of tea. Because this ring is a portion of the purin structure, it has been suggested that the pyramidins are purin decomposition products, but such proves not to be the case. At least two of them preëxist in nucleic acid, but it is probable that uracil is formed from cytosin during the hydrolysis. LECITHOPROTEINS.-Lecithoproteins have lecithin or a phospholipin as the prosthetic group. Their existence in cytoplasm is probable but they have not yet been isolated. The globulin vitellin is believed to exist in egg yolk either wholly or in part as lecithovitellin; likewise, a lecithoprotein has been described as present in the gastric mucosa. DERIVED PROTEINS.-These are the transformed proteins, including proteins that have undergone decomposition, or synthesized compounds resembling such products. Primary Protein Derivatives are certain transformed proteins that have not been changed to the state of hydrolytic decomposition. PROTEANS.-These are the first products of the action of acids, enzymes or water. They are insoluble in water. METAPROTEINS.-The proteins in this group are formed by the further action of acid or alkalies and include acid albumin and alkali albumin. COAGULATED PROTEINS.-These are the insoluble modifications produced by the action of heat and alcohol. The coagulated egg white is a familiar example. Secondary Protein Derivatives present varying stages of hydrolytic decomposition. PROTEOSES.—These proteins are soluble in water, not coagulated by heat and precipitated by saturation of their solution with ammonium sulphate. The individual members are designated by their particular name, e.g., albumoses, globuloses, gelatoses, etc. PEPTONES. These products of further hydrolysis are soluble, noncoagulable by heat and are not precipitated by saturation of their solution with ammonium sulphate. They are somewhat more diffusible and give the biuret reaction, though with a decidedly more pinkish hue than the native proteins or even the proteoses. PEPTIDS.—These are relatively simple compounds of the amino-acids, either products of hydrolytic decomposition or of synthetic origin, whose exact composition is known. The amino-acids combine through the amino and carboxyl groups. Peptids may or may not give the biuret reaction. They are designated di-, tri-, tetra-, penta-peptids, etc., according to the number of amino-acids that enter into their composition. Where this is large, the designation polypeptid is employed. NITROGENOUS EXTRACTIVES.-More or less closely related to the amino-acids of proteins are certain nitrogenous substances that are frequently found associated with them and which, together with nonprotein amino-acids, go to make up what were formerly known as nitrogenous extractives. AMIDS.-These, found in both animals and plants, are derivatives of the fatty acids in which the hydroxyl of one or more carboxyl group is replaced by an amino (NH2) group. Thus asparagin of plants is an amid of aspartic acid. BETAIN. This compound, which occurs in both animals and plants, is a trimethylamin derivative of acetic acid. GUANIDIN.—These derivatives, notably methylguanidin of meat, the diamino-acid arginin and creatin, a constituent of meat, are of wide ocThese substances must all be recognized as food constituents in small quantities. currence. CARBOHYDRATES Of the constituents of an ordinary mixed diet, the carbohydrates are usually the most abundant, and, from the standpoint of cost, the most economical. They are found more especially in the various vegetable substances and are of exceedingly varied structure and composition, as a rule containing in the molecule 6 atoms of carbon (or a multiple of 6) in combination with hydrogen and oxygen in the proportion to form water as shown in the following formulæ, grape sugar CH12O6, cane sugar, C12H22O11, etc. The carbohydrates include the sugars which are aldehyd or ketone alcohols having the following common properties: (a) they have a sweet taste, (b) optically active, (c) may or may not reduce alkalin metallic solutions, (d) and yield characteristic crystalline compounds with phenylhydrazin. Classification of Carbohydrates.-Those containing multiples of three carbon atoms are fermentable by yeast, and yield alcohol. Others when heated with strong acids yield characteristic derivatives, i.e., the pentoses yield furfurol; the hexoses yield levulinic acid. We recognize the following classification according to whether one, two or more sugar molecules enter into the formation of the molecule: Monosaccharids, Disaccharids, Polysaccharids. THE MONOSACCHARIDS.-Under this classification belong the grape sugar group, comprising those which cannot be split by hydrolysis into sugars of lower molecule weight. They are white, odorless, sweet, crystallizable substances, readily soluble in water, sparingly soluble in alcohol, insoluble in ether. Like all aldehyds and ketones they are easily oxidized, acting as reducing agents. They are unaffected by digestive enzymes, and if not attacked by bacteria in the digestive tract, they are absorbed and enter the blood current unchanged. In order to show in its name that a substance belongs to the carbohydrates, the ending ose is used to distinguish these bodies from the members of other groups. According to the number of carbon atoms present, the monosaccharids are again subdivided into classes called, viz.: Trioses C3H6O3-glycerose an alcoholic aldehyd. Pentoses C5H1005-arabinose, ribose, rhamnose and xylose. Nonoses C,H1809-mannonose, glucononose. The monosaccharids are converted into corresponding alcohols by nascent hydrogen, and conversely, the alcohols into monosaccharids by the loss of hydrogen. The more abundant ones are readily susceptible to alcoholic fermentation and are utilized for the production of glycogen in the animal body and in the maintenance of the normal glucose content of the blood (5). PENTOSES.—These substances, C5H1005, occur in many foods and vegetables as complex carbohydrates known as pentosans. When taken into the body pentosans are split off by hydrolytic cleavage, and pentose is excreted in the urine; considerable pentosans are found in the urine of persons addicted to the use of morphin. Pentoses are of great importance, as they are constituents of certain nucleic acids and are contained in the nuclei and cytoplasm of cells (6). As pentosans they are widely distributed throughout the vegetable kingdom, where, particularly as xylan and arabin, they form a considerable proportion of the building material for the human economy. Pentosans are found in the juice of a large variety of green plants, being more abundant in the morning than in the evening. They They occur to a considerable extent in the seeds of cereals and legumins, straws, grasses, etc. Water-free cassava contains 3.96 per cent, and eleven kinds of gum reveal its presence. They are not very digestible. The pentoses are converted into furfurol during the manufacture of alcohol. Experiments have been made to ascertain if the pentoses might be taken in diabetes as a substitute for other sugars; the results have shown that although the pentoses are soluble, they are not readily digested or assimilated by human beings, and that the portion which is absorbed readily passes into the urine. HEXOSES. These substances, C6H12O6, are the most important of the aldoses and ketoses, both from a practical and physiological standpoint. The chief examples are dextrose and levulose, but there are two groups: (a) aldoses comprise dextrose, mannose, galactose, gulose, formose, acrose, talose; (b) ketoses comprise levulose and sorbinose. Dextrose or glucose is also called "grape sugar" and "diabetic sugar." This is physiologically the most important sugar occurring in an impure state in sweet fruits, such as grapes, also in seeds, roots and honey. It is also obtained from other carbohydrates by hydrolysis either by acids or enzymes, and thus becomes the principal form of the carbohydrate after the food enters into the animal economy. In the animal economy of a healthy individual, the glucose of the blood is constantly being burned and replaced. In diabetes, the system loses to a greater or less degree the power to burn glucose; it then accumulates in excessive amounts in the blood, from which it escapes through the kidneys. A temporary loss of glucose in the urine may occur as the result of feeding large quantities of sugar at a time. This condition is known as alimentary glycosuria. Under ordinary circumstances in a healthy individual any surplus of glucose absorbed from the digestive tract is converted into glycogen and stored in the liver. Levulose, fructose, CH12O6, or fruit sugar, occurs with glucose in sweet fruits and honey. It resembles glucose in most of its chemical and physical properties, but does not crystallize from an aqueous solution. It may, however, be obtained in white silky needles from an alcoholic solution. It is met with generally as a thick syrup, is about as sweet as cane sugar and turns the plane of polarized light to the left; it is formed together with glucose by the action of dilute mineral acids or ferments on cane sugar(7). Fructose may occur in normal blood, but probably only in insignificant quantities. It serves, like glucose, for the production of glycogen; and the fructose which enters the body either through being eaten as such or as the result of the digestion of cane sugar is mainly changed to glycogen on reaching the liver, so that it does not enter largely into the blood or the general circulation. It may be readily understood, however, that fructose should be converted in the liver into glycogen, which on hydrolysis yields glucose. The sugar of fruit is usually an admixture of dextrose and levulose, and is called invert-sugar. It is uncrystallizable and forms granular masses in dried fruit-it consists of 4 parts of levulose and 3 parts of hydrated dextrose, some of which arise by inversion of saccharose. Galactose, CH12O6, is not found free in nature, but results together with glucose from the hydrolysis of milk sugar, either by acids or by digestive enzymes, and appears to have the same power as glucose and fructose to promote the formation of glycogen in the animal body. It is also formed together with dextrose when gum arabic is boiled with dilute sulphuric acid. Galactose does not ferment with yeast. The anhydrids of galactose, known as galactans, occur quite widely distributed in plant products. The galactans of certain legumes are found to be readily digested. |