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Biochemical mechanisms underlying development and diagnosis of ischaemic disease
1.Describe the role of lipoproteins and lipoprotein receptors in regulating the metabolism of lipids.
Very very basically: lipoproteins take lipids from the liver to the nonhepatic cells/tissues where they
are endocytosed by lipoprotein receptors and are used by the cells. Any unused lipids are then
returned by lipoproteins to the liver. Nonhepatic cells primary source of cholesterol is LDL.
Types of Lipoproteins
 Chylomicron
 Very low-density lipoprotein VLDL
 Intermediate density lipoprotein IDL
 Low-density lipoprotein LDL
 High-density lipoprotein HDL
As you go down the list the
lipoproteins get smaller in size
but higher in density.
What is a lipoprotein?
A lipoprotein is a particle consisting of a core of hydrophobic lipids surrounded by a shell of polar
lipids and apoproteins. Lipoproteins have two roles: they solubilise highly hydrophobic lipids and
they contain signals that regulate the movement of particular lipids into and out of specific target
cells and tissues. The cholesterol within all the various lipoproteins is identical!
The actions of VLDL, IDL, LDL & HDL
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The liver synthesizes fatty acids and cholesterol and packages them for transport in the blood
plasma in VLDLs.
The primary apoprotein is B-100. The liver secretes VLDLs via exocytosis. Like chylomicrons,
VLDLs undergo constant changes in the plasma. First, the nascent VLDL acquires apo C and E
from HDL.
VLDLs bind to the same membrane bound lipoprotein lipases (LPLs) located on adipose and
muscle tissues where the triacylglycerols are hydrolyzed into fatty acids. The fatty acids are
transported into the adipose cell where they are once again resynthesized into triacylglycerols
and stored. In the muscle, the fatty acids are oxidized to provide energy.
As the tissues absorb the fatty acids and monoacylglycerols, the VLDLs progressively shrink
forming IDLs. IDLs can bind to receptors of liver cells where they are absorbed in an analogous
manner to chylomicrons, or they can be further catabolised to form LDLs.
The primary function of the LDL particles is to provide cholesterol to the peripheral tissues. 75%
of LDLs are removed by the liver and 25% reach the periphery. They deliver their cholesterol by
being internalised after coming in contact with the cell surface and by binding to receptors on
cell-surface membranes.
The B-100 protein component of LDL binds to a specific receptor protein on the plasma
membrane of nonhepatic cells. The LDL receptors are localized in coated pits containing clathrin.
The receptor-LDL complex is endocytosed, forming an endocytic vesicle.
These vesicles fuse with lysosomes which contain a wide array of degradative enzymes. The
cholesterol esters component of the receptor-LDL complex are hydrolysed by a lysosomal acid
lipase to form unesterified cholesterol.
The released (unesterified) cholesterol can then be used for membrane biosynthesis or it can be
reesterified for storage inside the cell.
Once uptaken the cellular receptors and Clathrin coated membrane are recycled to the cell
membrane.
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HDL has numerous functions but their main function is to counter the net function of the lower
density lipoproteins. The lower density lipoproteins main function is to distribute lipids and
cholesterol to the peripheral tissues. HDLs are uptaken by the liver by receptor mediated
endocytosis and the cholesterol esters are degraded for use in bile acids or repackaged into
lipoproteins. Thus the overall effect of HDL is to remove cholesterol from the periphery and take
it back to the liver for reuse. HDL functions:
o Accumulate free cholesterol from the cell membranes of peripheral tissues.
o Esterify uptaken cholesterol so it can no longer be transferred to a cell membrane.
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Three separate processes are affected by the released intracellular cholesterol:
1. Cholesterol suppresses cholesterol synthesis within the cell by inhibiting the activity of the
enzyme HMG CoA reductase, which is the rate limiting step in the synthesis of endogenous
cholesterol.
2. Cholesterol activates the enzyme ACAT, favouring esterification and storage of excess
cholesterol.
3. Cholesterol suppresses the synthesis of LDL receptors, thus protecting the cells from
excessive accumulation of cholesterol.
The Mechanism of the Statin Drugs (refer to three processes above)
The statin drugs suppress intracellular cholesterol synthesis by inhibiting the enzyme HMG CoA
reductase. Because there is less intracellular cholesterol this allows for greater synthesis of LDL
receptors. Because there are more LDL receptors, more cholesterol will be taken up by the cells and
therefore there will be less plasma cholesterol. End result: reduction in endogenous production of
cholesterol and a reduction in plasma LDL. Thus decreasing the progression of atherosclerosis and
the complications associated.
2.Describe the mechanisms that result in the deposition of cholesterol in atherosclerotic lesions.
Endothelial cell injury causes increased permeability of the blood vessel wall to plasma lipids (VLDL
and LDL). This factor increases with aged as arteries and veins lose elasticity and harden making
them more susceptible to injury via factors such as hypertension. LDL and VLDL infiltrate the vessel
wall and are oxidised (oxyVLDL and oxyLDL). From this step atherosclerosis begins. The major factor
in reducing the progression of atherosclerosis is to reduce the amount of LDL and VLDL in the
plasma. Low cholesterol diets and Statins reduce this progression. (The mechanisms of statins are
explained above)
The propensity for developing atherosclerosis substantially increases with elevated levels of LDL.
Excess intracellular cholesterol greatly reduces the synthesising of LDL receptors which also
accounts for raised plasma lipid concentration. Whilst in familial hypercholesterolaemia, affected
individuals lack the genes for making LDL receptor proteins. Because their cells cannot take up LDL
from the blood, the blood concentration of these cholesterol-loaded lipoproteins becomes greatly
elevated. Such persons have a very high tendency toward developing atherosclerosis early in life,
and many die in their youth from coronary artery disease.
3.Demonstrate an understanding of the principles that underlie the application of blood chemistry to
the differential diagnosis of specific tissue damage, particularly in the context of ischemic disease of
the myocardium.
Principles connecting blood chemistry to diagnosis of SPECIFIC tissue damage
When cells of a tissue become damaged or necrotic, contents leak into the extra cellular fluid. This
then passes into the plasma (directly or via lymphatics). Therefore, elevated levels of intracellular
components in plasma can infer that tissue damage/necrosis has occurred.
plasma markers can tell us:
WHAT – identify the specific tissue damaged
WHERE – localise the tissue involved
WHEN – Ascertain the interval since the injury?
HOW – How bad is the damage? Is it getting better or worse?
- How is the body responding to the damage?
- Is this response coping with the damage?
Specific Plasma Markers
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ALT: Alanine Transaminase. LIVER. ALT is found predominantly in the liver. High levels of ALT
almost always indicate that there is a problem with the liver.
AST: Aspartate Transaminase. LIVER, CARDIAC MUSCLE, skeletal muscle and kidneys. If plasma
AST levels are raised it usually means there is liver damage (viral hepatisis) or heart damage
(Myocardial infarct)
ALP: Alkaline Phosphatase isoenzymes. BONE, LIVER, Intestinal Mucosa, Placenta, Bile. Increased
Activity is present in all bone disorders accompanied by increased osteoblastic activity. It is also
raised in Primary and Secondary Liver Malignancies. Decreased activity also indicates pathology.
ACP: Acid Phosphatase Isoenzymes. PROSTATE. ACP is concentrated in the prostate at 100 to
300 times.
CK: Creatine kinase: CARDIAC MUSCLE and SKELETAL MUSCLE – mitochondrial enzyme. It has 2
basic subunits CK-M and CK-B which form three ISOENZYMES (Normal range 25-200U/L)
 CK-MM: Derived predominantly from SKELETAL MUSCLE.
 CK-BB: From BRAIN, lung, and many other tissues.
 CK-MB isoenzyme: Principally from MYOCARDIAM
Testing for a myocardial infarction: CK-MB rises 7-8 hrs after onset of symptoms, peaks at 18 hrs
and returns to normal within 48 hrs
LD: Lactate Dehydrogenase. LD is an essential enzyme in glycolysis for the utilisation of glucose.
It permits organisms to generate a temporary oxygen debit by accumulating lactate, which is
oxidised to pyruvate when oxygen becomes available. In 40% of patients with malignant
tumours total LD is raised
 LD1: HEART, erythrocytes, pancreas renal cortex
 LD2: HEART, erythrocytes, lungs, thyroid, spleen, brain, lymph
 LD2 is normally higher than LD1 but when this ‘flips’ or switches it is a diagnostic
sign or AMI (LD1 > LD2)
 LD3:Lung, lymph nodes, spleen, adrenals, thryroid, brain
 LD4: Skeletal muscles, lungs, adrenals
 LD4 and LD5: are elevated with MALIGNANT TUMOURS.
 LD5: LIVER and SKELETAL MUSCLE.
LIVER. LD5 isoenzyme is a very sensitive marker for liver damage including
hepatitis, intoxication, circulatory disturbance and neoplasm.
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SKELETAL MUSCLE. Raised LD5 is indicitive of skeletal muscle disease and
strenous exercise. Should be used in conjunction with CK-MM and Aldolase.
Amylase: PANCREAS. Increased levels of Amylase indicate Pancreatic disorders (pancreatitis,
pancreatic carcinoma); Non-Pancreatic Abdominal Disease (Perforated ulcers of the bowel,
intestinal obstruction, Ruptured ectopic pregnancy); Other (Renal failure, Other carcinomas,
Diabetic Ketoacidosis, Opiates, etc.)
Cholinesterase:
 True cholinesterase: NERVOUS TISSUE
 Pseudocholinesterase: LIVER, HEART MUSCLE and PLASMA
o Decreased PLASMA PSEUDOCHOLINESTERASE indicates: Decreased synthesis
occuring in the liver consistent with liver disease, Severe anaemia, Myocardial
infarction, organophosphate toxicity (insecticide)
GGT: Gamma-Glutamyltransferase. LIVER, KIDNEY. Raised during hepatobilary disease,
alcoholism.
Cardiac Troponin I & Troponin T: CARDIAC MUSCLE SPECIFIC: Released from damaged
myocytes. Problem is that serum abnormalities are not detected until 6-18 hrs after the onset of
symptoms.
Myoglobin: CARDIAC MUSCLE and SKELETAL MUSCLE. It is the earliest new cardiac marker
available. It is a small molecule which psasses rapidly through damaged cell membranes. It can
be detected in serum within 2-3 hours of MI. Peaks around 8-12 hrs but falls to normal range
after 24 hrs. False postives arise from skeletal muscle damage and renal failure (inability to
excrete molecule)
BNP: Brain Naturiuretic Peptide: CARDIAC MUSCLE. Is secreted by the ventricles of the heart in
response to excessive stretching of myocytes in the ventricles. Raised levels give are used in the
diagnosis of heart failure.
Homocysteine: Appears to exert a direct toxic effect on the intima of arteries, besides inducing
oxidation of low-density lipoproteins and predisposing to thrombus formation by activating
platelets and coagulation factors.
Dietary deficiency of folic acid, vitamin B6 (pyridoxine), and vitamin B12 is also associated with
elevation of homocysteine, as are chronic renal failure, hypothyroidism, and some malignancies.
Lowering the serum concentration of homocysteine by administration of folic acid has been
shown to reduce the risk of adverse cardiovascular events in people with homocystinuria.
Note: Isoenzymes are multiple forms of an enzyme family that catalyze the same biochemical
reaction. They usually have the same molecular weight, but may differ in the following properties:
1. Each isoenzyme of a family has a different affinity for substrates and cofactors. Thus the
Michaelis constant and specificity for different substrates may vary.
2. The various isoenzymes of a family may differ in the ability to be inhibited by specific
agents.
3. They may differ in their physical properties, such as heat stability, charge and amino acid
composition.