A Graphic Representation of Intermediary Metabolism 0. S. RASK The Johns Hopkins University, Baltimore, Maryland
T
HE PROCESSES of life have, since the earliest tlmes, . mterested everybody from the lowly illiterate concerned only with self-preservation to the experimental philosopher searching for causes and effects. It is impossible, therefore, to trace the science of biology to its beginning. However, a few of its relatively modern landmarks can be noted. In 1628 the English physiologist, William Harvey, described the circulation of blood essentially as i t is recognized now. In 1771 Karl Wilhelm Scbeele in Sweden, and in 1774 Joseph Priestly in England, independently of one another, discovered oxygen, and thereby opened the door to the science of biochemistry which was founded in 1790 by the French chemist, Antoine Lauret Lavoisier, in his demonstration that animal heat is the result of oxidation similar to that whicb takes place in a fire. In 1847 the German chemist Justus von Liebig, a "great-grand pupil"' of Lavoisier, recognized organic foods as composed mainly of proteins, fats, and carbohydrates. Since then physiologists and biochemists have demonstrated that in the alimentary tract, proteins are hydrolyzed, that is, digested into amino acids, fats into fatty acids and glycerol, and carbohydrates into monosaccharides, mainly glucose. The purpose of these digestions is to render the foods diffusible through the intestinal wall and absorbable into the blood. In their above hydrolyzed or digested forms, therefore, the foods enter the blood. The succeeding transformations and utilizations of foods constitute intermediary metabolism which is outlined on the accompanying chart. In leaving the alimentary tract, glucose and the amino acids enter the portal blood stream which carries them to the liver for further actions by this organ, as indicated in the chart. Glycerol and the fatty acids are absorbed by a diierent route. They enter the thoracic duct, which carries them directly into the systemic circulation without first entering the liver as glucose and the amino acids do. In studying the chart further, i t will be necessary to consider the digestion products of each food separately. Glucose. In the liver, glucose and glycogen exist together in a very responsive and delicate equilibrium rigidly controlled by the two more or less mutually antagonistic hormones, insulin and epinephrin. As indicated in the chart by the single-tailed arrows, insulin accelerates a general glycogenesis, that is, the
conversion of glucose not only from the portal blood but also from the systemic blood into liver and muscle glycogens. In this manner insulin exercises its wellknown function of lowering blood sugar. As indicated by the double-tailed arrows, epinephrin accelerates glycogenolysis, that is, hydrolysis, of both liver and muscle glycogens, converting the former into glucose and the latter into lactic acid. In converting liver glycogen into glucose, epinephrin exercises its wellknown function of raising the blood sugar level. A post-absorptive blood sugar level of a little less than 100.0 milligrams of glucose per 100.0 milliliters of blood is indicative and diagnostic of a normal balance between the opposing actions of insulin and epinephrm. In pancreatic diabetes the blood sugar level is higher due to the absence of insulin necessary to balance epinephrin. Closely associated with the intermediary metabolism of glucose and a part of it is the chemistry of muscle contraction. As indicated in the chart, the chemistry of muscle contraction (as recognized by some authorities a t the present time) consists of a series of about fourteen reactions by which muscle glycogen is ultimately converted into carbon dioxide and water. In the first two of these reactions two molecules of phosphocreatin yield two molecules of phosphoric acid to adenylic acid whicb is thereby converted into adenylic acid pyrophosphate. In the third reaction the resulting adenylic acid pyrophosphate is decomposed into adenylic acid and two molecules of phosphoric acid, the latter combining with glycogen to form fructose diphosphate. Then follow reactions four to eight (and a t the same time some unnumbered reactions) by which half of the fructose diphosphate is converted into lactic acid while the other half is reconverted into glycogen. Reaction nine is the oxidation of a part of the lactic acid into carbon dioxide and water. Reactions ten and eleven are, respectively, the conversions of the remaining lactic acid into muscle and liver glycogens. Reaction twelve is a reconstitution of adenylic acid pyrophosphate from adenylic acid and two molecules of phosphoric acid. The cycle is completed by reactions thirteen and fourteen which are the reverse of reactions two and one, respectively. This system series of reactions is supposed to be "thrown out of gear" by a liberation of ammonia from adenylic acid or adenylic acid pyrophosphate with the formation of inosinic acid or inosinic acid pyrophos1 Liebig was a pupil of Gay-Lussac; Gay-Lussac w a a pupil of phate, respectively. Berthollet; and Berthollet was a pupil of Lavoisier. 373
I NCREATIC DIABETES /--'-IOTIONAL GLYCOSURU
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GLUCOGENIC
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B FAT DEPOTS
KEY CRTN. CREATtN AA =AOENYLIC ACID P =ORTHOPHOSPHATE RADICAL CONNECTS REACTING SUBSTANCES a CONNECTS REmTION PRODUCTS
EPlNEPHRlN
CARBOHYDRATE OXIDATION RELEASE OF LIVER AMYLASE
M ACTION
FACIUTATES
ACCELERAES
INHlBlTS
DEPRESSES FACILITATES
NOWESSENTIAL INHIBITS
*VERZAR AND CAHN-HOUGET THEORIES
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EPlNEPHRiN ACTION e lNSULIN ACTION
UEXOTHERMIC REACTION
4 ENWTHERUlC REACTWN ACE -ACETONE BODIES WIONS-HCO, ~*.--HPO.
Fat. I n 1915 it was demonstrated by J. F. McClendon a t the University of Minnesota, that the contents of the intestinal tract are not sufficiently alkaline for fatty acids to be formed into soaps and absorbed as such in accordance with the theory of fat absorption proposed by Pfliiger in 1900. Otherwise fat absorption transport and utilization are not well understood. However, interesting theories on fat absorption and mobilization have in recent years been proposed, respectively, by F. Verzar, and T. Cahn and J. Houget. An attempt has been made to represent on the chart a little of f a t metabolism in accordance with these two theories. Verzar's Fat Absorption Theory: The free fatty acids and glycerol are carried through the intestinal wall by the hydrotropic actions of the bile acids. Then in some outer layer of the intestinal wall the fatty acids and glycerol are supposed to encounter phosphocholine and unite with it to form phospholipins. As such, the fats are transported to the fat depots where the phosphocholme is released and returned to the circulation, while the fatty acids and glycerol are formed into triglycerides to constitute depot fat until mobilized. The Cahn-Houget Theory on Fat Mobilization: Cholesterol enters the fat depots and there combines with the fatty acids to form cholesterol esters which are transported to the liver. In the liver the cholesterol esters are decomposed into free fatty acids and cholesterol. The cholesterol is returned to the circulation, whereas the fatty acids are desaturated and a t the same time combined with phosphoglycerol and choline to form desaturated phospholipins. These are then transported to the muscles where phosphoglycerol and choline are released and returned to the circulation, whereas the free desaturated fatty acids undergo beta oxidation to carbon dioxide and water. Amino Acids. Intermediary protein metabolism has been recognized mainly by three manifestations which occur successively, more or less in the following order: 1. A rise in the concentration of amino acids in the blood from a minimal post-absorptive level of about 25.0 milligrams per 100.0 milliliters to a maximum absorption level of about 45.0 milligrams per 100.0 milliliters. 2. A rise in the urea concentration of the blood from a post-absorptive (and presumably endogenous) level of about 20.0 milligrams per 100.0 milliliters to a maximum level of about 40.0 milligrams per 100.0 milliliters. 3. Excretion by the kidneys of nitrogen about equal in quantity to the nitrogen metabolized, about three-fourths of the excreted nitrogen being in the form of urea. It is an interesting and probably important fact that the blood never contains less than around 25.0 milligrams of amino acids per 100.0 milliliters of blood, even during nonabsorption of amino acids or during prolonged periods of very low protein diets. Appar-
ently this concentration of amino acids is essential for certain required, but as yet unknown, characteristics of blood. If so, lower concentrations of amino acids in the blood may be expected to cause disturbances, possibly as serious as those resulting from a lowering, by means of insulin injections, of the blood sugar level to 50.0 milligrams per 100.0 milliliters of blood. In 1922 Folin and Berglund .at Harvard University discovered that the amino acid nitrogen in the blood might double within two hours after the intake of protein, whereas the urea nitrogen in the blood did not begin to rise appreciably until about four hours after the protein intake. By that time the amino acid nitrogen in the blood had begun to fall. This sequence of events in which a rise in the concentration of amino acids in the blood precedes a rise in the concentration of urea in the blood led Folin to conclude erroneously that amino acid deaminization and urea formation are not localized in the liver. However, in 1924 Bollman, Mann, and Magath of the Mayo Clinic in Rochester, Mmnesota, demonstrated conclusively that urea formation is localized very highly if not exclusively in the liver. In 1932 Krebs and Henseleit a t the University of Freiburg published their ornithine-arginine cycle as the chemical mechanism by which the liver deaminates amino acids and releases the removed nitrogen in the form of urea. As indicated in the chart, the products of these reactions are urea and deaminated molecules of amino acids which are ultimately oxidized as fats in the case of those amino acids which are ketogenic, and as glucose in the case of those amino acids which are listed as glncogenic. In the light of the above discoveries of Bollman, Mann, and Magath and of Krebs and Henseleit, i t has become necessary to reinterpret the earlier observations of Folin and Berglund that the amino acids concentration wave in the blood precedes that of urea as indicating that the Krebs-Henseleit ornithine-arginine cycle is a slower process than the process by which amino acids pass out of the alimentary tract and into the blood stream. If so, an increase in the concentration of amino acids in the blood will obviously precede an increase in the urea concentration in the blood as Folin and Berglund observed. Urea is the principal and terminal nitrogen-containing waste product of protein metabolism. During deaminations of absorbed amino acids, nrea is produced a t such a rate that its level in the blood rises to around 40.0 milligrams per 100.0 milliliters. As a result of excretions by the kidneys, this level falls rapidly to about 20.0 milligrams, which is the minimum concentration of urea in the blood prevailing during periods of nonabsorptions of amino acids. This lowest level of urea in the blood may possibly be determined in part by a renal threshold for nrea excretion and partly by urea originating more or less continually in the wear and tear quota. However that may be, the lowest 20.0 milligrams of urea in the blood is mainly if not entirely endogenous and may therefore be so designated. Urea over and above this level may be desig-
nated as exogenous because i t obviously originates in the amino acids supplied by the food proteins. The column on the left side of the chart indicates, in milligrams per 100.0 milliliters, the composition of blood in terms of those constituents usually deter-
mined in a chemical analysis of blood. The chart also indicates (in the muscle and lung areas) the reactions by which the blood transports oxygen to the tissues and carbon dioxide away from the tissues and out by way of the lungs.
