12. Enzymes and cardiac markers Flashcards
Why are enzymes measured?
- to identify diseases that occur as a result of abnormalities in enzyme concentration or function (e.g. inherited metabolic diseases)
- to detect tissue damage
- markers of therapeutic response or drug toxicity
- As reagents for measurement of other substances
Other causes of increased enzymes other than tissue injury?
Increased synthesis and decreased clearance
How can measuring enzymes show tissue damage?
- Enzymes are primarily found intracellularly
- However, small amounts of these intracellular enzymes are detected in plasma as a result of normal cell turnover
- Level of intracellular enzymes in the plasma will increase following tissue injury
- This is because the membranes become leaky in scenarios of tissue damage
If there is severe injury (necrosis), what will leak out into the circulation?
cytosolic and subcellular enzymes will leak out into the circulation
Why is measuring enzymes TIME-crucial?
- Level of intracellular enzymes in the plasma will increase following tissue injury.
- This is because the membranes become leaky in scenarios of tissue damage. If there is more severe injury (necrosis), then the cytosolic AND subcellular enzymes will leak out into the circulation.
- Therefore, measuring enzymes is TIME-crucial.
- Release of cytosolic enzymes happens first, followed by release of sub-cellular enzymes.
Are enzymes highly specific?
Relatively few enzymes are highly cell specific, most others are widely distributed in many organs and tissues
Some enzymes may occur as different iso-enzymes. Individual iso-enzymes are characteristic to certain tissues. How do you check whether a raised enzyme has come from a particular tissue?
You may try checking whether another enzyme released by the same tissue has also been raised (e.g. ALP and GGT)
What organs is alkaline phosphatase present in high concentrations in?
Liver, bone, intestines and placenta
What are pathological increases in ALP mostly due to?
Liver and bone diseases
What is an increase in bone diseases associated with?
Increased osteoblastic activity
If GGT is raised with ALP, what does this imply?
Implies that the ALP is coming from the liver
What can be used to separate out the isoenzymes?
Electrophoresis (bone specific ALP immunoassay is now available)
What are the causes of raised ALP?
Physiological causes include pregnancy - 3rd trimester (from placenta), and childhood - growth spurts.
Pathological causes more than 5 x upper limit of normal include: bone (Paget’s disease, osteomalacia) and liver (cholestasis, cirrhosis).
Pathological causes less than 5 x upper limit of normal include: bone (tumours, fractures, osteomyelitis), liver (infiltrative disease, hepatitis). NOTE: ALP is NOT increased in osteoporosis unless there is a fracture
Where is amylase produced?
Amylase is secreted by exocrine pancreas
In what condition is there high serum amylase activity?
In acute pancreatitis
What is the level of amylase in acute pancreatitis?
It is usually 10 x the upper limit of normal
Apart from amylase, what else is measured in acute pancreatitis?
Pancreatic lipase is also often measured because it is a good marker of acute pancreatitis
The salivary glands also produce an amylase isoenzyme. What is this raised in?
Can be raised in parotitis
What else apart from acute pancreatitis can increases in amylase be seen?
The salivary glands also produce an amylase isoenzyme (it can be raised in parotitis). Increases in amylase may also be seen with other causes of acute abdomen.
What is the most widely used marker of muscle damage?
Creatine kinase
There are three forms of creatine kinase, what are the two types of subunits which form the dimers?
M (muscle) and B (brain)
What are the three forms of creatine kinase?
CK-MM - skeletal muscles; CK-MB - cardiac muscle; CK-BB - brain
What is the activity of CK-BB in the brain?
The activity is minimal even in severe brain damage (very small quantities)
Which form of CK is responsible for almost the entire normal plasma activity?
CK-MM
Is CK routinely separated into isoenzymes?
when CK is measured it is NOT routinely separated into isoenzymes
What can statin-related myopathy range from?
Statin-related myopathy can range from myalgia to rhabdomyolysis
What are the risk factors for statin-related myopathy?
- Polypharmacy (in particular fibrates and cyclosporin and other drugs metabolised by CYP3A4)
- high dose
- genetic predisposition
- previous history of myopathy with another statin
- vitamin D deficiency (increased risk of stain intolerance)
What are other causes of raised CK, other than statin-related myopathy?
Muscle damage due to any cause, myopathy (e.g. Duchenne muscular dystrophy), myocardial infarction, severe exercise, physiological (Afro-Caribbeans)
Give an example of enzymes being used as a marker of therapeutic response or drug toxicity:
Measurement of TPMT activity is encouraged before starting patients on a thiopurine (e.g. azathioprine, 6-mercaptopurine)
Give an example of enzymes being used as reagents for measurement of other substances:
Glucose oxidase is used to measure glucose in plasma
What is the time course of cardiac enzyme elevations?
A. Myoglobin rises 0-1 days, peaks at 1.6 multiples of the AMI cutoff limit.
B. Cardiac troponin rises 0-8 days (peaks day 2), peaks at 50 multiples of the AMI cutoff limit.
C. CK-MB rises 0-4 days (peaks day 1), peaks at 4 multiples of the AMI cutoff limit.
D. Cardiac troponin rises 0-2 days (peaks day 1), peaks at 0.8 multiples of the AMI cutoff limit. Troponins are much better markers. Myoglobin rises very quickly however it is NOT specific to cardiac muscle.
Which enzymes were previously used to determine myocardial infarction?
CK, AST, LDH. The different enzymes will rise at different times. None of these enzymes are measured anymore.
Which enzyme is a cytosolic enzyme and is the first thing to be released in an MI?
Myoglobin
Where is CK-MB found?
Within the mitochondria and nucleus
Where are troponins present?
Present within the contractile apparatus of the cardiac muscle. There is also a free cytosolic pool of troponins.
How are troponins released?
Initially, troponins from the free cytosolic pool will be released. Later on, as the contractile bundles break down, there will be increased release of troponins.
When do troponins rise?
Rises at 4-6 hours post MI
When do troponins peak?
12-24 hours post MI
How long do troponins remain elevated?
For 3-10 days
When should troponin be measured?
It should be measured 6 hours and again at 12 hours after the onset of chest pain.
How can you be almost certain that a patient has not had an MI?
After 12-24 hours if there is no rise in troponin
What is the diagnostic criteria for acute MI?
EITHER 1 or 2.
- Typical rise and gradual fall in troponin or more rapid rise and fall in CK-MB with at least one of the following: ischaemic symptoms, pathological Q waves on ECG, ECG changes indicative of ischaemia, coronary artery intervention.
- Pathological findings of an acute MI
Which cardiac biomarkers aid decisions regarding thrombolysis?
None of the current cardiac biomarkers rise quickly enough to aid decisions regarding thrombolysis (therefore it is decided based on clinical findings and ECG findings)
What are the biomarkers in cardiac failure?
Natriuretic peptides ANP (atria) and BNP (ventricles)
What biomarker can be measured to assess ventricular function?
BNP
What biomarker can be used to exclude HF in a clinical setting?
BNP
What do most assays measure?
ACTIVITY rather than mass
What is one international unit (U) of enzyme activity defined as?
One international unit (U) of enzyme activity is defined as the quantity of enzyme that catalyses the reaction of 1 micromol of substrate per minute
What assay conditions is activity dependent on?
Temperature and pH. • NOTE: reference ranges may differ between laboratories (because the conditions may be different)
