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Clinical Chemistry (Enzymes)

Terms in this set (46)

What is an enzyme?
An organic catalyst. All enzymes are proteins. The substance upon which they act is called the substrate.
What is a coenzyme?
An organic cofactor required for an enzymatic reaction. Unlike the enzyme, which is not changed in the reaction, the coenzyme is changed.

Common coenzymes are NAD (nicotinamide adenine dinucleotide), NADH, NADP, and NADPH.
How is the change from NAD to NADH measured in automated analyzers?
By the change in absorbance at 340 nm. NADH absorbs light at 340 nm; NAD does not.
What are activators?
Inorganic cofactors such as Ca++ or Mg++- that are required for some enzyme reactions
What are isoenzymes?
Slightly different forms of an enzyme that catalyze the same reaction. Because of their different molecular structure, isoenzymes can be separated by their electrophoretic mobility, heat stability, or reaction with specific antibodies.
What are 2 ways in which enzymes are measured?
Traditionally, quantitation has been based on the activity of the enzyme on its substrate, either a decrease in the
concentration of the substrate, an increase in the concentration of the end-product, a decrease in the concentration of the coenzyme, or an increase in the concentration of the changed coenzyme.

Today, immunoassay methods are available to measure some enzymes by mass, for example, CK-MB.
What are zero-order kinetics?
When the rate of an enzyme reaction is dependent on the concentration of the enzyme only. This occurs when there
is an excess of substrate. In first-order kinetics, the rate of the reaction is proportional to the substrate concentration. Enzyme assays should be based on zero-order kinetics.
What is the clinical significance of an elevated lipase?
Lipase is increased in acute pancreatilis. It may also be significantly increased in many other conditions (e.g. opiate administration; pancreatic carcinoma; intestinal infarction, obstruction, or perforation; and pancreatic trauma).
What is the substrate for lipase?
Triglycerides. In turbidimetric assays, the rate of clearing of turbidity is related to the amount of lipase in the
serum
What is the clinical significance of an elevated amylase?
Amylase is increased in acute pancreatitis. It may also be significantly increased in many other conditions, such as
opiate administration; pancreatic carcinoma; intestinal infarction, obstruction, or perforation; pancreatic trauma;
mumps; cholecystitis; hepatitis; cirrhosis; ruptured ectopic pregnancy; and macroamylasemia
What is the substrate for amylase?
Starch. It is broken down to sugars.
Why shouldn't EDTA plasma be used for an amylase determination?
Ca++ is needed for the reaction and EDTA chelates Ca++.
How do amylase and lipase values compare in acute pancreatitis?
They both begin to rise at roughly the same time after an attack (within 5-8 hours for amylase, 4-8 hours for lipase),
and both peak around 24 hours. Amylase returns to normal by 3-5 days; lipase remains elevated longer (8-14
days).
Which enzyme is most specific for acute pancreatitis?
Lipase. It is not found is as many tissues.
Are amylase and lipase elevated in chronic pancreatitis?
They may be marginally elevated during attacks the early stages of the disease, but as the disease progresses severe destruction of acinar tissue eventually reduces the amount of enzymes released into the circulation. Then low levels may be seen.
Why might a pleural fluid amylase be ordered?
A normal pleural fluid amylase rules out acute pancreatitis.
What is the major diagnostic significance of elevated alkaline phosphatase (ALP)?
It is elevated with liver and bone disorders.
In which disease do the highest elevations of alkaline phosphatase (ALP) occur?
Paget's disease, a bone disease characterized by excessive bone destruction and unorganized bone repair. Values
10-25 times the upper reference limit are not unusual
When is a physiological increase in serum alkaline phosphatase seen?
During pregnancy due to release of ALP from the placenta and during childhood due to rapid bone growth.
Increased values at these times are expected and Explained by normal physiology. They are not indicative of
pathology.
What is the clinical significance of acid phosphatase (ACP)?
The highest levels of ACP are in the prostate. At one time ACP was used to diagnose prostate cancer, but it has
been replaced by the more sensitive prostate specific antigen (PSA). ACP is rarely tested in serum today, but its
presence in vaginal washings may be significant in rape cases since it indicates the presence of semen. It can be
detected for up to 4 days after intercourse.
With which cl.inical condition are the highest levels of CK seen?
Duchenne's muscular dystrophy
What are some physiologic (nonpathologic) causes for increased total CK?
Intramuscular injections and vigorous exercise
Which cardiac enzyme is most specific?
CK-MB is the most specific cardiac enzyme, but not the most specific cardiac biomarker.

Cardiac troponins are more specific for acute myocardial infarction
Which cardiac biomarkers are currently used to diagnose acute myocardial infarction (AMI)?
Cardiac troponin (TnT or Tnl) and CK-MB, although CK-MB is playing a less important role since the introduction of troponin assays.
How is CK-MB measured?
Immunoassays using monoclonal antibodies have replaced electrophoresis. CK-MB is detectable 4-6 hours after chest pain due to AMI and remains elevated for 2-4 days.
Which cardiac marker is most specific for cardiac damage?
Cardiac troponins. Levels begin to rise within 3-12 hours of myocardial damage. TnT remains elevated for 8-21 days; Tnl for 7-14 days . Cardiac troponins are therefore useful both for early diagnosis and for late diagnosis after CK-MB has returned to normal.
How is cardiac troponin measured?
lmmunoassays using monoclonal antibodies.
Which cardiac marker is most likely to be elevated in a patient who is admitted to the hospital 4 days after a
suspected myocardial infarction?
Cardiac troponins. CK-MB would have returned to normal.
What is the clinical significance of B-type natriuretic peptide (BNP test)?
BNP is a cardiac hormone that is produced by the heart ventricles in response to ventricular volume expansion and pressure overload. It is a predictor of congestive heart failure in patients suffering from shortness of breath. BNP is also used to evaluate risk in patients who present with chest pain. High BNP predicts an increased risk of death or
subsequent heart attack in patients with acute coronary syndromes. The test is an immunoassay that can be
performed at point of care
What is the clinical significance of highly sensitive C-reactive protein (hs-CRP)?
CRP is a marker of inflammation and appears to be associated with increased risk for coronary heart disease,
sudden death, and peripheral artery disease. hsCRP is measured by turbidimetric immunoassays that use an
antibody to an epitope on the CRP molecule. These assays are sensitive to 0.01 mg/dL. hs-CRP should not be
confused with the conventional flocculation assays for CRP, which only detect gross elevations. At this time, hsCRP screening of the entire adult population is not recommended.
Which enzymes are most useful for the assessment of liver function?
Alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), Tglutamyltransferase (GGT), and lactate dehydrogenase (LD). Other liver function tests include total protein,
albumin, and total and conjugated (direct) bilirubin.
Which enzyme is most sensitive for all types of liver disease?
GGT. It is elevated in most cases of liver disease, regardless of cause. Levels are highest with biliary obstruction.
Is AST or ALT more specific for liver disease?
ALT. AST is found in equal amounts in heart, skeletal muscle, and liver
What are the substrates for AST?
Aspartate and
a-ketoglutarate.
What are the substrates for ALT?
Alanine and
a-ketoglutarate
Which disease state bas the highest elevation of AST and ALT?
Acute hepatitis.
Where is LD found?
In all cells of the body. Because of its wide distribution, serum LD elevations occur in a variety of clinical
conditions including myocardial infarction, hemolysis, and disorders of the liver, kidneys, lung, and muscle. In
liver disease, LD does not increase as much as AST and ALT.
What effect does hemolysis have on LD?
It is increased because LD is I 00-150 times more concentrated in the RBCs.
How should specimens for LD be stored and why?
They should be stored at 25°C and analyzed within 48 hours. They should not be refrigerated because LD decreases more rapidly at 4°C than at 25°C.
Which clinical condition results in the highest levels of LD?
Pernicious anemia.
Which enzyme is a sensitive indicator of alcoholism?
GGT
Which enzyme would be helpful in establishing a diagnosis of bone disease?
Alkaline phosphatase.
Which enzymes would be helpful in establishing a diagnosis of muscle disorders
CK, AST, LD, and aldolase.
What is the clinical significance of a deficiency of glucose-6-phospbate dehydrogenase (G-6-PD)?
Inherited deficiencies ofG-6-PD can lead to hemolytic anemia following exposure to certain drugs. G-6-PD is in the RBCs, so the specimen required is whole blood.
What is the clinical significance of low pseudocholinesterase levels?
Low levels are seen following exposure to insecticides and nerve gases and in patients with hypersensitivity to the muscle relaxant succinylcholine.
What is the advantage to using enzymes as reagents?
Enzymatic assays are very specific. There are fewer false positives than with colorimetric assays.
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