sakai-Clinical Enzymology Flashcards
Why is an increase of normally intracellular enzymes in the blood an indication of increased cell damage or even cell death?
Are these former intracellular enzymes considered functional serum enzymes or
are they considered nonfunctional serum enzymes?
The normal turnover of cells does not lead to a large increase of intracellular
enzymes in the blood. However, when cells get damaged, then a leakage of large
amounts of respective intracellular enzymes into the blood can occur.
These enzymes are nonfunctional plasma enzymes, as they are not meant to be
active or have a function in the blood plasma.
Is the measured concentration of injury markers at a specific time a direct
indication of how much enzyme leaked out into the blood? Explain!
Why does creatine kinase (CK) peak in serum earlier than LDH?
The concentration of injury markers at a specific time is not a direct indication of
how much enzyme leaked out into the blood. The momentary concentration results
from both processes, leakage into the blood and removal from the blood.
CK peaks in serum earlier than LDH as CK diffuses faster into serum, it has a
smaller molecular weight and it has a shorter half life in blood than LDH.
What are isozymes? Why can they often be separated by electrophoresis?
Isozymes are a group of enzymes that catalyze the same reaction.
But their amino acid composition and overall charge is different from each other
and that is why they often can be separated by electrophoresis.
Also, some isozymes contain different subunits which are cell specific.
Which reaction is catalyzed by creatine kinase? Is this reaction reversible? How
many subunits are found in creatine kinase?
In general, a kinase is an enzyme that phosphorylates something using ATP.
Creatine kinase is an enzyme that phosphorylates creatine to creatine phosphate.
The reaction is reversible, and when creatine phosphate and ADP are used, ATP
can be formed due to the fact that creatine phosphate is an energy rich molecule.
[This is very special, please recall that the reactions of most kinases are irreversible
like for example: protein kinases, hexokinase, glucokinase etc.]
Creatine kinase consists of 2 subunits, they can be identical or different from each
other. The subunits are named M and B leading to CK-MM, CK-MB or CK-BB.
Why is serum CK-MB (CK-2) an indicator for myocardial infarction? What is the
percentage of CK-MB in heart cells? What is the percentage of the other isozyme in
heart cells? Describe the creatine kinase isozymes for skeletal muscle and for the
brain!
During myocardial infarction, many heart cells die and a large amount of CK is
released into the blood.
The heart is special, as it has a high percentage of CK-MB of about 30 %. The
other isozyme found in the heart has the composition CK-MM which represent
about 70% of total CK in the heart.
CK-MM is mostly found in highest percentage in skeletal muscle (98%) and CK-
BB is mostly found in the brain and intestinal smooth muscle.
Why do you find after myocardial infarction not only an increase of serum CK-MB but also of serum CK-MM?
The heart has CK-MM (70 %) and CK-MB with about 30 %.
After an MI, both forms are found in the blood.
[due to the dilution factor in blood, serum CK-MB represents above 3-5% of total
serum CK]
Why would it be an advantage to calculate the % of serum CK-MB related to total
serum CK?
The percentage of serum CK-MB of total serum CK is an indicator of the severity
of the MI.
About 3 % serum CK-MB of total serum CK and above can be an indicator for acute myocardial infarction [together with other signs of MI and increased cardiac troponins]
Is serum creatine kinase an injury marker for liver damage? Explain!
Creatine kinase is not found in the liver, and it is not a liver injury marker.
Is myoglobin a specific marker for MI? Why is serum myoglobin often analyzed
together with serum CK-MB and serum cardiac troponins cTnI or cTnT? Which
method is used for analysis?
Myoglobin in the blood is not a specific marker for MI, as it is also elevated during
general muscle damage.
However, it is an early injury marker, and together with serum CK-MB and serum cardiac specific troponins, it can be measured for early MI recognition. ELISA tests are available for myoglobin, CK-MB and cTnI
Are cardiac troponins enzymes? Why are troponins I and T, but not troponin C, used as a cardiac injury marker? Are serum troponins used as early MI marker or as late MI marker? Explain!
Troponins are proteins but not enzymes.
Troponin C does not have an isoform that is specific for the heart, but the cardiac isoforms cTnI and cTnT are measured as MI injury marker. They have the advantage that they are early markers and can also even be measured after several days or a week following a large MI.
Compare the time frame of peaks of serum CK-MB to total serum LDH after
myocardial infarction!
Following myocardial infarction, serum CK-MB peaks after about 24 hrs and LDH
peaks later after 48 hrs.
Serum CK-MB and serum LDH can be measured before the time of their peaks in serum.
When serum LDH peaks, serum CK-MB and total serum CK levels are mostly already back to normal baseline.
How many subunits are found in LDH? How many different kinds of subunits? Why
can the different isozymes be used as markers for MI or skeletal muscle damage or
even for hepatitis?
LDH has 4 subunits, and we find 2 different kinds of subunits, H and M.
The heart muscle contains mainly LDH-1 (H4) and LDH-2 ( H3M) with a high LDH- 1 to LDH-2 ratio. RBCs contain also mainly LDH-1 and LDH-2, but with a low LDH-1 to LDH-2 ratio. The skeletal muscle and the liver contain mainly LDH-5 (M4)
Why do you find at admission after an MI a serum LDH pattern with low
LDH- 1/LDH-2 ratio and after 24 hours after myocardial infarction a change towards
a high LDH-1/LDH-2 ratio (referred to as flip)?
At admission after myocardial infarction, the serum LDH pattern represents still the
pattern found in RBC which shows mainly LDH-1 and LDH-2 with a low LDH-1 to
LDH-2 ratio.
RBCs are commonly destroyed and the LDH leaks into the blood and represent the normal baseline. After 24 hrs following an MI, the total LDH in the blood is not very much increased, but when the isozymes are analyzed, we find a change to a high LDH-1 to LDH-2 ratio. This ratio is found normally intracellular in the heart muscle, and this“flip” in serum is indication of MI where the heart cell death leads to serum LDH with the specific heart composition.
Does cardiac defibrillation (cardioconversion) lead to leakage of CK-MB as found
during MI into the serum? Discuss! Read the MCQ in Lippincott’s page 68.
CK-MB in serum can be an indicator of cell death of heart cells.
Defibrillation is adjusted so that no heart damage should occur, and therefore CK-MB from heart cells should not increase in serum from this procedure. [Sometimes the chest muscles get damaged, and this leads to an increase of CK- MB from those muscles. The chest muscles have more CK-MB than normal skeletal muscle, but still much less than heart muscles. Damage of chest muscles is not enough to lead to an increase in large amounts of serum CK-MB as is seen after an MI. ]
Which two aminotransferases are often tested in serum as liver injury marker? Where
in the cell do you find ALT and where AST?
Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are often
measured in serum as possible liver injury marker. These enzymes are not specific for
the liver, and actually aspartate aminotransferase was measured in the past as heart
injury marker.
Alanine aminotransferase is more specific than AST as it is found in particular high concentration inside liver cytosol and is used together with other enzymes (or bilirubin) as liver injury marker.
ALT is found in cytosol, AST is found in both cytosol and mitochondria.
Increased serum levels of AST and ALT and a ratio of AST/ALT ratio above 2
can indicate, that the liver mitochondria are damaged.
This can happen due to alcohol abuse.
