1. Amino Acid Pool
The amount of free amino acids distributed throughout the body is called amino acid pool. Plasma level for most amino acids varies widely throughout the day. It ranges between 4–5 mg/dl. Following a protein containing meal, the amino acid levels rise to ( mg / dl).

2. Tissue Amino acids
The amino acids are transported into tissues actively. Pyridoxal-P (B6-P) is one of the requirement for this active transport. Tissue uptake is also favoured by hormones:
• Insulin, growth hormone and testosterone favour the uptake of amino acids by tissues (anabolic hormones).
• Estradiol stimulates selectively their uptake by uterus.
•Epinephrine and glucocorticoids: Stimulate the
uptake of amino acids by the Liver.

3. Sources of amino acid pool
1.Dietary protein
2.Breakdown of tissue proteins
3.Biosynthesis of amino acids in liver (except essential amino acid).

4. Fate of amino acid pool.

5. Protein Turnover
All the body proteins except collagen are in a constant state of degradation and resynthesis. About 1-2% of total body proteins are degraded and resynthesized every day
Nitrogen Balance
Nitrogen balance means the difference between nitrogen intake and nitrogen loss.

6. Nitrogen Intake
1-Dietary protein, every 100 gram proteins contain 16 gram nitrogen.
2-Traces of inorganic nitrogen in the form of nitrates (NO3 - ) and nitrites (NO2-)
Nitrogen loss
1-In urine in the form of non-protein nitrogenous substances as urea, uric acid, creatine, creatinine and ammonia
2-In stools in the form of digestive juices
3-In sweat in the form of urea

7. Positive Nitrogen Balance
means that nitrogen intake is more than nitrogen loss. It occurs in:
Growing children
Pregnancy
Convalescence from wasting diseases
Negative Nitrogen Balance
means that nitrogen loss is more than nitrogen intake. It occurs in:
Diabetes mellitus
Fever
Starvation
Wasting diseases
Nitrogen equilibrium means that nitrogen intake equals nitrogen loss. It occurs in healthy adults on an adequate diet

8. Essential Amino Acids
its cannot be synthesized by the organism but must be supplied in the diet usually combined in proteins.
Features of Essential Amino Acids
The eight essential amino acids (or indispensable amino acids) are: valine, leucine, iso-leucine, threonine, methionine, phenylalanine, tryptophan and lysine ( to remember one may use Mattvillphly). Exclusion of any one of these essential amino acids leads to( –ve) N-balance manifesting as loss of weight, fatigue, loss of appetite and Nervous irritability

9. Non-essential Amino Acids
Dispensable amino acids can be synthesized in the body.
Semi-essential Amino Acids
These are growth promoting factors since they are not synthesized in sufficient quantity during growth (arginine & histidine). They become essential in growth children, pregnancy, and lactating women.

10. Dissimilation of Amino Acid (N-Catabolisim of Amino Acid)
*transamination
*Deamination(oxidative or nonoxidative deamination)
*Transdeamination
* NH3 transport
*formation of urea.

11. Transamination
*Transamination means transfer of amino group from α-amino acid to α-keto acid with formation of a new α-amino acid and a new α-keto acid.
*The liver is the main site for transamination.
*All amino acids can be transaminated except lysine, threonine, proline and hydroxy proline.
*All transamination reactions are reversible.
*It is catalyzed by aminotransferases (transaminases). It needs pyridoxal phosphate as a coenzyme.

13. Role of pyridoxal phosphate in transamination
Pyridoxal phosphate acts as an intermediate carrier for amino group.
Pyridoxal phosphate accepts the amino group from amino acid to form. pyridoxamine phosphate, which in turn gives the amino group to a-keto acid.
Examples of transaminases
Alanine transaminase
Aspartate transaminase
Glutamate transaminase

14. Clinical significance of serum transaminases
Transaminases are intracellular enzymes.
Their levels in blood plasma are low under normal conditions. ALT (GPT) is present mainly in the cytoplasm of liver cells.
AST (GOT) is present in both cytoplasm and mitochondria in liver, heart and skeletal muscles.
-Any damage to these organs will increase the level of transaminases in blood

15. -In liver diseases, there is an increase in both serum ALT (SGPT) and AST (SGOT) levels. In acute liver diseases, e.g. acute viral hepatitis, the increase is more in SGPT
-In chronic liver diseases, e.g. liver cirrhosis the increase is more in SGOT.
-In heart diseases, e.g. myocardial infarction, there is an increase in SGOT only.
-In skeletal muscle diseases, e.g. myasthenia gravis, there is an increase in SGOT only.

16. Deamination
Deamination means the removal of amino group from a-amino acid in the form of ammonia with formation of a-keto acid
The liver and kidney are the main sites for deamination
Deamination may be oxidative or non oxidative

17. Oxidative deamination It is catalyzed by one of the following enzymes:
Oxidative deamination
It is catalyzed by one of the following enzymes:
1- L-amino acid oxidases
2-D-amino acid oxidases

19. B-Non-oxidative deamination
It is catalyzed by one of the following enzymes:
1-Dehydrases
This enzyme deaminates amino acids containing hydroxyl group e.g. serine, homoserine and threonine.It needs pyridoxal phosphate as coenzyme.

21. 2-Desulfhydrases
This enzyme deaminates sulpher containing amino acids e.g. cysteine and cystine. It needs pyridoxal phosphate as a coenzyme.

22. 3-Deamination of Histidine
3-Deamination of Histidine