Q.- Give a brief account of the steps of synthesis of Very Low density lipoprotein (VLDL). High light the clinical significance of impaired VLDL synthesis.
Answer- There are striking similarities in the mechanisms of formation of chylomicrons by intestinal cells and of VLDL by hepatic parenchymal cells (Figure -1), perhaps because—apart from the mammary gland—the intestine and liver are the only tissues from which particulate lipid is secreted.
Steps of synthesis-
1) Protein synthesis- The major apoprotein of VLDL, Apo B100 is synthesized in the rough endoplasmic reticulum. VLDL particles are stabilized by two lipoproteins apo B-100 and apo E (34 kd). Apo B-100, one of the largest proteins known (513 kd), is a longer version of apo B-48. Both apo B proteins are encoded by the same gene and produced from the same initial RNA transcript. In the intestine, RNA editing modifies the transcript to generate the mRNA for apo B-48, the truncated form.
Newly secreted VLDL contains only a small amount of apolipoproteins C and E, and the full complement is acquired from HDL in the circulation.
2) Lipid synthesis and formation of lipoprotein –In the fully fed state, apo B-100 is synthesized in excess of requirements for VLDL secretion and the surplus is destroyed in the liver (Figure-2). During translation of apo B-100, microsomal transfer protein-mediated lipid transport enables lipid to become associated with the nascent polypeptide chain.
The liver is a major site of triacylglycerol and cholesterol synthesis. Triacylglycerols and cholesterol in excess of the liver’s own needs are exported into the blood in the form of very low density lipoproteins (d<1.006 g cm-3).
After release from the ribosomes, these particles fuse with more lipids from the smooth endoplasmic reticulum, producing nascent VLDL.
3) Glycosylation and release of VLDL- After addition of carbohydrate residues in golgi apparatus, VLDL particles are released from the cell by reverse pinocytosis. VLDL are secreted into the space of Disse and then into the hepatic sinusoids through fenestrae in the endothelial lining.
Figure-1- Showing the steps of synthesis of VLDL in the liver cell, (RER- Rough endoplasmic reticulum, SER- Smooth endoplasmic reticulum, G- Golgi apparatus, N- Nucleus, S- Space of Disse and VLDL- very low density lipoprotein)
Clinical Significance
Fatty liver (steatosis)
It is an abnormal accumulation of certain fats (triglycerides) inside liver cells.
Hepatic triacylglycerol synthesis provides the immediate stimulus for the formation and secretion of VLDL. Impaired VLDL formation or secretion leads to nonmobilization of lipid components from the liver, resulting in fatty liver.
Causes of fatty liver- Imbalance in the rate of triacylglycerol formation and export causes fatty liver. For a variety of reasons, lipid—mainly as triacylglycerol—can accumulate in the liver. Extensive accumulation is regarded as a pathologic condition. When accumulation of lipid in the liver becomes chronic, fibrotic changes occur in the cells that progress to cirrhosis and impaired liver function.
Fatty livers fall into two main categories-
A) More synthesis of Triglycerides or
B) Defective VLDL synthesis (Metabolic block)
A) More synthesis of Triglycerides-Triglycerides are synthesized in excess due to more availability of Fatty acid and glycerol. The fatty acids used are derived from two possible sources: (1) synthesis within the liver from acetyl-CoA derived mainly from carbohydrate (perhaps not so important in humans) and (2) uptake of free fatty acids from the circulation. The first source is predominant in the well-fed condition, when fatty acid synthesis is high and the level of circulating free fatty acids is low. As triacylglycerol does not normally accumulate in the liver under this condition, it must be inferred that it is transported from the liver in VLDL as rapidly as it is synthesized and that the synthesis of apo B-100 is not rate-limiting. Free fatty acids from the circulation are the main source during starvation, the feeding of high-fat diets, or in diabetes mellitus, when hepatic lipogenesis is inhibited.
Thus high carbohydrate diet stimulates de novo fatty acid synthesis by providing excess of Acetyl CoA and high fat feeding provides more flux of fatty acids from the diet that can be esterified to provide excess triglycerides.
B) Defective VLDL synthesis The second type of fatty liver is usually due to a metabolic block in the production of plasma lipoproteins, thus allowing triacylglycerol to accumulate.
Theoretically, the lesion may be due to-
(1) A block in apolipoproteins synthesis-
Causes- can be-
a) Protein energy Malnutrition
b) Impaired absorption
c) Presence of inhibitors of endogenous protein synthesis e.g.- Carbon tetra chloride, Puromycin, Ethionine etc. The antibiotic puromycin, ethionine (α-amino-γ-mercaptobutyric acid), carbon tetrachloride, chloroform, phosphorus, lead, and arsenic all cause fatty liver and a marked reduction in concentration of VLDL (Figure-2). The action of ethionine is thought to be caused by a reduction in availability of ATP due to its replacing methionine in S-adenosylmethionine, trapping available adenine and preventing synthesis of ATP.
d) Hypobetalipoproteinemia- Defective apo B gene can cause impaired synthesis of apo B protein.
(2) A failure in provision of phospholipids that are found in lipoproteins-
a) A deficiency of choline, which has therefore been called a lipotropic factor can cause impaired formation of phosphatidyl choline (Lecithin),a glycerophospholipid (Figure-2)
b) Choline is formed by methylation from ethanolamine, with S-Adenosyl Methionine acting as a methyl group donor. Methionine deficiency can cause impaired choline synthesis and thus fatty liver besides other clinical defects.
c) Deficiency of essential fatty acids- can also lead to impaired Phospholipid synthesis
(3) Impaired Glycosylation- Orotic acid causes fatty liver; it is believed to interfere with glycosylation of the lipoprotein, thus inhibiting release, and may also impair the recruitment of triacylglycerol to the particles. In conditions of orotic aciduria (disorder of pyrimidine nucleotide biosynthesis), fatty liver can be observed (Figure-2)
4) Impaired secretion of VLDL- oxidative stress is a common cause for membrane disruption of lipoprotein. The action of carbon tetrachloride probably involves formation of free radicals causing lipid peroxidation (Figure-2). Some protection against this is provided by the antioxidant action of vitamin E-C, beta carotene and selenium in the supplemented diets.
Figure-2- Showing the biochemical basis of fatty liver disease. Imbalance in the rate of triacylglycerol formation and export causes fatty liver.
Clinical conditions causing fatty liver-Clinically fatty liver is of two types-
1) Non alcoholic fatty liver- Fatty liver (with or without fibrosis) due to any condition except alcoholism is called nonalcoholic steatohepatitis (Macro vesicular steatosis).
Causes of nonalcoholic steatosis or NAFLD are
(The biochemical basis for each condition has been explained above)
2) Alcoholic fatty liver- Alcoholism leads to fat accumulation in the liver, hyperlipidemia, and ultimately cirrhosis. The fatty liver is caused by a combination of impaired fatty acid oxidation and increased lipogenesis, which is thought to be due to changes in the [NADH]/[NAD+] redox potential in the liver, and also to interference with the action of transcription factors regulating the expression of the enzymes involved in the pathways.
Oxidation of ethanol by alcohol and aldehyde dehydrogenase leads to excess production of NADH (Figure-3)
Figure-3- Showing steps of metabolism of alcohol
A) Effect of excess NADH – More triglyceride synthesis
1) The NADH generated competes with reducing equivalents from other substrates, including fatty acids, for the respiratory chain, inhibiting their oxidation and causing increased esterification of fatty acids to form triacylglycerol, resulting in the fatty liver.
2) Oxidation of ethanol, leads to the formation of acetaldehyde, which is oxidized by aldehyde dehydrogenase, producing acetate. Acetate is converted to Acetyl coA and there is more fatty acid synthesis.
3) Accumulation of NADH causes more formation of Glycerol-3-P (shift of equilibrium of reaction), that can be used for the synthesis of triglycerides.
B) Improper apo- protein synthesis – Malnutrition is a common finding in chronic alcoholism There is less availability of essential amino acids.
C) Impaired Phospholipid synthesis-Due to malnutrition there is less availability of essential fatty acids and choline leading to defective Phospholipid synthesis.
D) Impaired secretion of VLDL- chronic alcohol consumption is associated with oxidative stress that can cause impaired VLDL secretion.
Thus multiple factors are responsible for alcoholic fatty liver disease
Lipotropic agents- Agents such as choline, Inositol, Methionine and other essential amino acids, essential fatty acids, anti oxidant vitamins, vitamin B12, folic acid and synthetic antioxidants which have the apparent effect of removal of fats from the liver cells, and thus prevent the formation of fatty liver are called lipotropic agents.
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