By the term endogenous protein metabolism is meant the disintegration of those proteins which already exist as components of living cells (tissue proteins). The term exogenous protein metabolism implies the breakdown of food proteins which do not exist as parts of the cell protoplasm.
The classical view of protein metabolism, originally proposed by Folin, is now challenged. Folin’s view was the metabolic patterns of exogenous (dietary) and endogenous (body) protein metabolism differ, i.e., the end products of endogenous protein metabolism are uric acid, creatine and neutral sulphur whereas urea is the end product of exogenous protein metabolism.
However isotopic experiment reveals that the body proteins are in a constant state of turnover and the body proteins are continually broken down and replaced by new proteins synthesized from dietary amino acids. The replacement of protein is rapid in plasma, liver, kidneys and intestinal tract and slow in haemoglobin, muscle and skin.
Dynamic State of Amino Nitrogen and Proteins of the Body:
It has been established by the work of Schoenheimer and others that the amino nitrogen of amino acids (except lysine) is distributed to other amino acids of different tissues and reversibly it is withdrawn from those amino acids to the amino acids which contained amino nitrogen by the process of deamination and reamination.
Protein Storage (Labile Protein):
Nitrogen excreted for the first few days after protein starvation was greater and then becomes more or less constant. The disorganized protein present in liver, thymus, prostate, seminal vesicle, alimentary tract, pancreas, spleen and kidneys, etc., are drawn upon to meet the need of the body and these proteins are called labile proteins. These are utilized for the synthesis of other proteins and may be oxidized to gain energy when required.
End Products of Protein Metabolism:
The nitrogen released from amino acid (protein) catabolism is excreted from the body in different form which varies in different species of animals. The considerable amount of nitrogen excreted through the urine of men, mammals and amphibians, etc., is urea. The birds and reptiles excrete nitrogen as uric acid mainly. The man also excretes nitrogen of purine bases as uric acid. Creatine excretion is exclusively from tissue protein breakdown. Neutral sulphur excretion does not indicate the nature of dietary or tissue protein breakdown.
Due to dynamic state of amino nitrogen of amino acid and differential pattern of metabolism of labile protein, the excretion of different nitrogenous metabolic end product does not always give real metabolic picture of protein metabolism. But some clue may be obtained if the standardized experiments were done e.g., increased creatine excretion, may indicate increased tissue protein, i.e., specially muscle protein catabolism going on. Increased uric acid excretion may be due to increased purine catabolism. Increased urea excretion may indicate dietary protein catabolism.
Brief Life History of the End Products of Exogenous Protein Metabolism:
From Folin’s experiment, although challenged, it appears that the end products of exogenous protein metabolism are urea, ammonia, inorganic sulphate and 50% of the total uric acid excreted.
The brief life history of these products is mentioned below:
Chemistry – CO(NH2)2.
(a) From deamination of amino acids (mainly from exogenous sources),
(b) From salts like ammonium carbonate, lactate, etc., taken in diet or as drug, and
(c) From the amino acid arginine.
It breaks down into urea and ornithine. The end product of metabolism of these bodies is urea.
Method of Formation:
Site of Formation:
The liver is main site.
Amount in Blood:
20-40 mgm (average 30 mgm) of urea present per 100 ml of blood. Almost equally distributed in plasma and corpuscles.
Functions Served by Urea Formation:
Urea formation helps to maintain the reaction of blood constant. Because, in it, one acid (carbonic acid) and two molecules of ammonia remain neutralized.
An adult taking a normal mixed diet excretes urea through urine an average of 30 gm daily (2% if total urine volume be 1,500 ml). 80% of urinary nitrogen is excreted in the form of urea.
(a) From deamination of amino acids, both exogenous and endogenous. Although deamination takes place chiefly in the liver, recent observations indicate that ammonia is also formed in the kidneys. Ammonia formed in the liver is converted into various substances. Kidneys can deaminate amino acids normally. The amount increases during acidosis and falls in alkalosis, and
(b) Certain ammonium salts taken in food or as drug, e.g., ammonium chloride.
Fate and Functions of Ammonia:
(a) Form Ammonium Salts. The purpose served is to keep the blood reaction constant,
(b) Ammonia may be utilized for the synthesis of amino acid, uric acid, nucleoproteins and other nitrogenous compounds.
With a mixed diet in an adult man the total daily output is about 0.7 gm. Ammonia nitrogen constitutes about 2-4% of total urinary nitrogen.
Relation with Blood Reaction:
In acidosis more ammonium salts will be formed. Reverse changes will take place in alkalosis.
Significance of Variations:
Although ammonia is an end product of exogenous protein metabolism, yet its amount in the urine is determined by the relative proportion of acids and bases in the body. In conditions of acidosis it rises, in alkalosis it falls. Ammonia coefficient is a reliable guide to the condition of acidosis or alkalosis of the body.
They are greatly produced from dietary proteins in the body. Consequently, they may be taken as the end products of exogenous protein metabolism. (The ethereal sulphates have a different life history altogether and has been discussed under ‘Sulphur Metabolism’).
It is an index of both endogenous and exogenous protein metabolism.
Brief Life History of the End Products of Endogenous Protein Metabolism:
From dietetic experiments in dogs it has been found that creatinine and neutral sulphur remain absolutely unchanged. So they are solely of endogenous origin. In this connection, the compound creatine, although not shown in this experiment, but when present in urine, should be regarded as derived from endogenous protein metabolism, because it is the precursor of creatinine.
Thus these compounds are wholly endogenous. Half of uric acid is of endogenous origin and the other half is of exogenous origin. The life history of neutral sulphur has been discussed under Sulphur Metabolism and of uric acid under Uric Acid Metabolism.
A brief summary of the life history of creatine and creatinine is given below:
Methyl guanidoacetic acid.
Total Amount in the Body:
90-120 gm in adult, 98% of it is present in the striated muscle as creatine phosphate. Skeletal muscles contain about 0.5% creatine. It is also found in heart (about half to the amount in skeletal muscles, i.e., 0.25%), testes, brain and uterus, specially during pregnancy.
Amount in Blood:
It is present in blood about 10 mgm per 100 ml and remain mostly in the red cells. As it is present inside the red cells, it is not filtered. Hence, it is usually not present in the urine.
Origin and Formation of Creatine:
(a) Creatine synthesis requires three amino acids, viz., arginine glycine and methionine (as S-adenosyl methionine),
(b) The compound guanidoacetic acid is an intermediate step in the synthesis of creatine,
(c) The methyl group of creatine is derived from methionine,
(d) The stages in creatine synthesis appear to be as follows- two organs, i.e., kidneys and liver, are also involved for the complete synthesis of creatine.
In kidneys, glycine and arginine react where amidine group (-CNHNH,) of arginine is transferred to glycine with the formation of guanidoacetic acid (glycocyamine) by the enzyme transamidinase. Transamidinase enzyme is present only in kidneys and pancreas. But this enzymatic reaction mostly takes place in the kidneys.
Methylation of guanidoacetic acid takes place, in the liver, because the liver contains the enzyme guanidoacetic methyl transferase. Guanidoacetic acid is converted into creatine with the help of amino acid, methionine (active form) in presence of enzyme guanidoacetic methyl transferase and glutathione (GSH).
When methyl group of methionine is transferred to guanidoacetic acid to form creatine (methyl guanidoacetic acid), the methionine is converted into S-adenosyl homocysteine. Activation of methionine takes place by ATP when methionine is converted into S-adenosyl methionine. Recently, it has been shown that in mammals both transamidiantion and methylation reactions, involved in the synthesis of creatine, take place in the pancreas.
Effects of Creatine Feeding:
If creatine is ingested in small amounts (up to 1 gm daily), none is found in the urine, but in moderate amounts (up to 5 gm daily) a little is excreted as creatine and the rest is stored. But if large amounts (20 gm) of creatine be taken, the major part (15 gm) is excreted as such, another part (4.5 gm) is retained and a small part (0.5 gm) is excreted as creatinine in the urine. This shows that creatine is not a waste product. It is useful and there is a store for it in the body.
Until this reservoir is filled up, no creatine will appear in the urine. Creatine synthesis is dependent on kidney transamidinase activity. The kidneys were thought to be the only site of the transamidinating enzyme until recently. However, recent studies have indicated that the pancreas may play a unique role in the synthesis of creatine within the mammalian body.
Interrelation with Creatinine:
These two compounds are closely interrelated. They are readily interconvertible while in solution. Creatinine is anhydride of creatine having one molecule of water less. Acid medium favours the formation of creatinine, whereas alkaline medium favours the formation of creatine. But in vivo creatinine cannot be converted into creatine, although the reverse is the rule.
Fate and Functions:
i. Creatine is converted into creatine phosphate (phosphagen) which takes an essential part in the chemical changes underlying muscular contraction. Creatine, when given in moderate amounts by mouth, disappears completely in the body and nothing appears in the urine. This is supposed to be due to its conversion into creatine phosphate and subsequent storage in the muscles.
ii. Creatine certainly has some function in tissues other than muscles but its nature is not known.
ii. Creatine is the precursor of creatinine.
Excretion of Creatine:
Creatine is not generally present in the urine of normal adult males. But it may be excreted abnormally.
Its excretion in the urine is determined by the following factors:
Up to the age of puberty it is constantly present in the urine of both sexes. It has been suggested to be due to an increased production of creatine, induced in some unknown way, by the activity of growth impulse. It may also be due to a lower capacity of the undeveloped muscles for creatine storage. There is a third possibility in that the children possess less power to convert creatine into creatinine.
After puberty it is found intermittently in healthy a female which is not related to menstruation.
It is constantly present during pregnancy. It rises to a maximum of 1.5 gm daily after confinement and is probably derived from the involuting uterus. The sex difference of creatine excretion cannot be properly explained. That increased creatine excretion is not due to the less muscular development in females is proved by the fact that it occurs even in women who are highly trained physically. That sex has something to do here is supported by the observation that creatinuria is common in eunuchs. It may be easily induced in old people (naturally with diminished sex functions) by administration of small amount of creatine.
High protein and low carbohydrate diets increase creatine excretion. The former acts by stimulating tissue metabolism due to its high specific dynamic action. The latter acts indirectly by the absence of its sparing effects upon the breakdown of tissue protein.
v. Increased Tissue Breakdown:
In any condition that increases the breakdown of tissues, specially of striated muscles, as in starvation, prolonged diabetes mellitus, hyperthyroidism, fevers and other wasting diseases which increase the basal metabolic rate, the creatine excretion is increased. In certain diseases of muscles (myopathy) where muscles undergo degeneration, a large amount of creatine is excreted.
In such conditions 90% or more creatine appears in an unchanged form in the urine even when it is given by mouth in small amount. This is said to be due to a lower storage capacity of the muscle. It is also probable that in this disease (i.e., myopathy) the reversible enzyme reaction, by which the broken creatine phosphate becomes re-synthesised in the muscle, is absent.
It is the anhydride of creatine.
It is mostly formed from breakdown of creatine phosphate in the body. This process is not catalysed by any enzymes and is irreversible. Creatine-labelled with isotopic 15N gives creatinine containing same isotopic 15N.
Site of Formation:
A large amount in the muscle.
Effects of Creatinine Feeding:
When orally administered nearly 80% is promptly excreted in the urine. Hence, it is considered to be a waste product. It is a no-threshold substance. It is filterd by the glomeruli and is also actively secreted by the tubular cells in the urine. Amount in blood: Normally it is present about 0.7-2.0 mgm per 100 ml. This level is very constant and it is considered to be pathological when its value increases about 2 mgm. Creatinine is also found in bile, sweat and in secretion of stomach and intestine.
About 1.2 -2.0 gm in adult males and 0.8-1.5 gm in adult females are excreted in 24 hours. The amount excreted is remarkably constant for a particular individual. It is related to the muscle bulk and is higher in muscular persons. This stands in great contrast with creatine excretion, which bears no relation to muscular development. The excretion increases during work and exercise but is immediately followed by a fall, so that the daily output remains constant.
Significance of Variation:
Creatinine represents the waste products of creatine metabolism and it arises in the body from the spontaneous breakdown of creation phosphate. It serves practically no function in the body apparently. As its excretion is not related with food protein so its variations in the excretion indicate some of the metabolic disorders. Appearance of creatinine in urine is known as creatinuria when a small amount of creatine is also excreted along with creatinine.
The creatine value gradually decreases as the maturity is advanced. Its excretion increases in fevers, starvation, on a carbohydrate-free diet and in diabetes mellitus. It may increase due to excessive tissue destruction releasing creatine or due to failure of creatine being properly phosphorylated. So creatinine excretion is independent of food proteins and is to be considered as an index of endogenous protein metabolism.