Question

Analyze the specific role of liver in each of the following:
(i). Carbohydrate, lipid and protein metabolism,
(ii). Processing of drugs and hormones,
(iii). Excretion of bilirubin,
(iv). Synthesis of bile salts and
(v) Activation of vitamin D?

Answer 1

Role of liver in :-

1) Carbohydrate metabolism -

Maintain blood glucose homeostasis.

?Stores glucose as glycogen when blood glucose levels are high (Glycogenesis). Performs glycogenolysis and release glucose into blood.

? Gluconeogenesis - Converts amino acids and glycogen to glucose when glycogen stores are exhausted and blood glucose levels are falling down.

Lipid Metabolism :-

?Liver is the primary site for beta oxidation. Beta oxidation is the breakdown of fatty acids to acetyl CoA.

?Converts excess of Acetyl CoA to ketone bodies to release to tissues.

? Synthesis of cholestrol from Acetyl CoA. This cholestrol can be metabolised to bile acids and bile salts to secrete into the bile.

?Forms lipoproteins for transport of fatty acids, fats and cholestrol in blood.

Protein Metabolism :-

? Formation of plasma proteins.

? Deamination of amino acids to convert them to glucose or for ATP synthesis.

?Formation of urea from ammonia for its removal from the body.

?Transamination reactions for synthesis of non essential amino acids in the body.

2) Processing of Drugs and hormones :-

Liver has role in biotransformation. It is a process by which a substance is converted/transformed from one chemical to another in the body.

?Liver metabolizes drugs and hormones by inactivating them and making more soluble so that they can be removed through the kidneys.

?Performs reactions that may result in formation of products which are more active, less active or inactive.

? Processing of bilirubin and excretion of bile pigments into bile.

3) Excretion of bilirubin :-

Bilirubin is insoluble so cannot be excreted out through kidneys. In liver, Bilirubin is conjugated with glucoronic acid by enzyme glucuronyltransferase making it soluble for excretion.

?This bilirubin is deconjugated and metabolized by colonic bacteria to form colourless urobilinogen. Urobilinogen is oxidised to form stercobilin which is removed in feces and urobilin which is removed by kidneys in urine.

4) Synthesis of Bile salts :-

Bile salts are made up of bile acids conjugated with glycine or taurine.

Primary Bile acids (Cholic acid and chenodeoxycholic acid) are synthesized by liver via Cytochrome p450 mediated oxidation of cholesterol.

?Then liver cells conjugate them with either glycine or taurine to form bile salts ( conjugated bile acids).

5) Activation of Vitamin D

The Vitamin D which is obtained from diet or synthesize from skin is biologically inactive. So for its activation, it needs two hydroxylation steps. First in the liver and sesond in the kidney.

?Cholecalciferol (Inactive vitamin D) is converted to Calcifediol by enzyme 25- hydroxycholecalciferol secreted by liver. Then this Calcifediol can be converted to Calcitriol by enzyme 1- hydroxylase secreted by kidney.

Thus liver also helps in activation of Vitamin D.