Formation of urea
It is also an important aspect of protein metabolism. Urea is formed in the liver from ammonia, amino groups and CO2 in the presence of ATP and some enzymes.
The amino groups unite with CO2 to form urea which is taken from the liver to the excretory organs from where it is eliminated from the body in the form of urea.
Urea formation as explained by Krebs and Henseleit involves various steps collectively known as Ornithine Cycle.
Metabolism of Fats:
Fat is hydrolysed into glycerol and fatty acid which are either oxidized to release energy or stored in the body cells known as adipose tissues. The normal liver contains about 4% of lipids and plays an important role in the metabolism of fat.
(i) Oxidation of Glycerol:
Glycerol converts into glycerol phosphate and dihydroxy acetone phosphate which may either be converted to glycogen or oxidized to C02 and H20 through glycolytic pathway.
(ii) Oxidation of Fatty Acids:
Muscles of the body normally oxidize the glucose (glycogen) as their energy fuel, but fatty acids may also be oxidized when glucose (glycogen) level falls below normal. Fatty acids on their oxidation, form acetyl CoA or ‘active acetate’ which enters the citric acid cycle the end products of fatty acid (acetyl CoA) are CO2, H2O and ATP which is a high energy molecule.
Metabolism of Cholesterol:
Cholesterol is of major significance due to its relationship with many physiologically active steroids, sex-hormones, adrenal cortex hormones and bile salts.
It is normally present in blood to the extent of 150 to 250 mg per 100 ml and 1.5 to 2.0 gm is synthesized daily in the human body. Cholesterol is synthesized in the body from acetyl which is formed either from fatty acids or from the carbohydrates through pyruvate.