Difficulty: Intermediate | Prerequisites: Enzymology fundamentals (enzyme structure, kinetics, inhibitors), basic liver and bone pathology
This set of notes covers the four enzymes you will encounter most often in clinical chemistry liver and bone panels: AST, ALT, ALP, and GGT. Each enzyme has a characteristic tissue distribution, and elevations in serum point to specific kinds of organ damage. Understanding which enzyme goes up, how high, and in what pattern is the core diagnostic skill these lessons build. You should be comfortable with the enzymology fundamentals (covered in the companion study notes) before working through this material, since the assay methods all depend on coupled enzymatic reactions and absorbance measurement at 340 nm.
AST and ALT are transaminases used to evaluate liver disease; ALT is more liver-specific, while AST also rises in cardiac and skeletal muscle injury. ALP is elevated in hepatobiliary obstruction and bone disorders. GGT is a sensitive marker for liver and biliary disease and helps confirm whether an elevated ALP is hepatic or skeletal in origin. Each enzyme has its own reference range, specimen-handling requirements, and pattern of elevation that points to different diagnoses.
Aspartate aminotransferase (AST)
A transaminase enzyme that catalyses the transfer of an amino group between aspartate and alpha-keto acids. Older terminology: serum glutamic-oxaloacetic transaminase (SGOT or GOT). Think of it as a marker that rises when liver, heart, or muscle cells are damaged.
Alanine aminotransferase (ALT)
A transaminase that catalyses the transfer of an amino group from alanine to alpha-ketoglutarate, forming glutamate and pyruvate. Older terminology: serum glutamic-pyruvic transaminase (SGPT or GPT). In simple terms, this is the more liver-specific of the two transaminases.
Alkaline phosphatase (ALP)
A group of enzymes that catalyse the hydrolysis of phosphomonoesters at an alkaline pH, liberating inorganic phosphate from organic phosphate esters. Think of it as a marker that goes up in both liver obstruction and bone diseases.
Gamma-glutamyltransferase (GGT)
An enzyme that transfers the gamma-glutamyl residue from gamma-glutamyl peptides to amino acids, water, and other small peptides. In simple terms, a very sensitive liver enzyme used to confirm that an ALP elevation is coming from the liver rather than bone.
Pyridoxal phosphate
The active form of vitamin B6 that serves as a coenzyme for both AST and ALT reactions. Think of it as the helper molecule both transaminases need to function.
Karmen method
The coupled enzymatic reaction method used to measure AST activity. Uses malate dehydrogenase (MDH) as the indicator reaction and monitors the change in absorbance at 340 nm as NADH is oxidised to NAD+.
Cholestasis
Reduced or stopped bile flow. Bile builds up in the liver rather than draining to the small intestine, and bile acids eventually enter the bloodstream. In simple terms, a blockage or slowdown in bile leaving the liver, which causes ALP to rise sharply.
Hemolysis
The rupture of red blood cells, releasing their contents into the surrounding fluid. A significant source of error in AST and ALP measurements because these enzymes are concentrated in erythrocytes.
Paget's disease
A bone disease in which the body's normal bone-recycling process is disrupted. New bone tissue replaces old bone abnormally, causing bones to become fragile and misshapen. Produces some of the highest ALP elevations seen clinically.
AST is widely distributed in human tissue. The highest concentrations are found in cardiac tissue, liver, and skeletal muscle. Smaller amounts are present in the kidney, pancreas, and erythrocytes.
AST is used to evaluate hepatocellular disorders. The degree of elevation helps suggest the diagnosis:
Condition | Approximate AST Elevation |
|---|---|
Viral hepatitis | Up to 100x upper reference limit |
Infectious mononucleosis | Up to 20x upper reference limit |
Skeletal muscle disorders | Up to 8x upper reference limit |
Cirrhosis | Up to 4x upper reference limit |
Pulmonary emboli/embolism | Up to 3x upper reference limit |
In acute myocardial infarction (AMI):
AST rises within 6 to 8 hours
Peaks at 18 to 24 hours
Returns to normal within 4 to 5 days
AST is not used to diagnose AMI, but awareness of the AST pattern is useful when ruling out concurrent liver damage in a patient with myocardial infarction.
The assay uses a coupled enzymatic reaction:
AST catalyses the transfer of an amino group between L-aspartate and 2-oxoglutarate, producing oxaloacetate.
Oxaloacetate reacts with NADH in the presence of malate dehydrogenase (MDH) to form malate and NAD+.
The rate of NADH oxidation is measured at an absorbance of 340 nm.
Optimal pH: 7.3 to 7.8.
In some assays, LDH is included in the reagent to convert endogenous pyruvate to lactate during the lag phase.
Hemolysis gives falsely elevated results because erythrocytes contain AST.
AST is stable in serum for 3 to 4 days at refrigerated temperatures.
5 to 30 U/L at 37 degrees C.
ALT is found in high concentrations in the liver. It is considered the more liver-specific enzyme of the transferases.
Hepatocellular disorders (hepatitis, cirrhosis) produce higher ALT levels than intra- or extrahepatic obstruction.
ALT is more specific for liver disease than AST.
ALT does not exhibit a significant increase in muscular dystrophy.
ALT is not affected in cases of pulmonary emboli or acute pancreatitis.
This specificity for liver disease is a frequent exam point: if the question asks which transaminase is more liver-specific, the answer is ALT.
The typical assay is a coupled enzymatic reaction:
ALT catalyses the transfer of an amino group from alanine to alpha-ketoglutarate, forming pyruvate and glutamate.
Lactate dehydrogenase (LD) is used as the indicator enzyme, reducing pyruvate to lactate while simultaneously oxidising NADH.
The change in absorbance at 340 nm is directly proportional to ALT activity.
Optimal pH: 7.3 to 7.8.
Pyridoxal phosphate acts as its coenzyme.
ALT is stable for 3 to 4 days at 4 degrees C.
ALT is relatively unaffected by hemolysis (a useful contrast to AST).
6 to 37 U/L at 37 degrees C.
Highest concentrations of ALP are found in the liver, bone, intestines, spleen, kidney, and placenta. Within the liver, ALP is found on cell surfaces in sinusoidal and bile canalicular membranes. In bone, it is found on osteoblasts.
ALP is elevated in two main categories of disease: hepatobiliary disease and bone disorders with osteoblastic involvement.
Hepatobiliary disease:
In hepatobiliary disorders, ALP levels are increased due to obstructive disease and rise more significantly than ALT and AST.
In biliary tract obstruction, cholestasis induces ALP synthesis, causing levels to rise 3 to 10 times the upper reference limit.
In hepatitis and cirrhosis (hepatocellular conditions), ALP rises up to 3 times the upper reference limit.
Bone disorders:
Paget's disease produces some of the highest ALP elevations. The disease disrupts normal bone recycling, and the pelvis, skull, spine, and legs are most commonly affected.
Healing bone fractures also increase ALP levels.
Physiological elevations (not pathological):
ALP is normally higher in children than adults because of active bone growth.
ALP is normally higher in pregnant women because the placenta is a source of ALP.
Decreased ALP:
Seen in hypophosphatasia, caused by lack of the ALP bone isoenzyme, characterised by insufficient bone calcification.
ALP catalyses the hydrolysis of phosphomonoesters at an alkaline pH, liberating inorganic phosphate from an organic phosphate ester with the concomitant production of an alcohol.
Hemolysis may cause slight elevations because ALP is approximately six times more concentrated in erythrocytes than in serum.
Activity in serum increases approximately 3 to 10% on standing at 25 degrees C or 4 degrees C for several hours. ALP must be run as soon as possible after collection.
Values may be 25% higher following ingestion of a high-fat meal.
Adults: 50 to 115 U/L at 37 degrees C
Children aged 4 to 15 years: 54 to 369 U/L at 37 degrees C (much higher because of bone growth)
GGT activity is found primarily in the kidney, brain, prostate, pancreas, and liver. Despite its wide tissue distribution, clinical applications of the GGT assay are confined mainly to evaluation of liver and biliary system disorders. GGT serves as an additional liver function test.
The most widely accepted substrate is gamma-glutamyl-p-nitroanilide. The gamma-glutamyl residue is transferred to glycylglycine, releasing p-nitroaniline, a chromogenic product with a strong absorbance at 405 to 420 nm. The reaction can be used as a continuous-monitoring or fixed-point method.
GGT is a sensitive marker for liver and biliary disease. Its principal clinical value is in helping to determine whether an elevated ALP is hepatic or skeletal in origin:
If both ALP and GGT are elevated, the source is likely hepatobiliary.
If ALP is elevated but GGT is normal, the source is likely bone.
GGT is stable, with no loss of activity for 1 week at 4 degrees C.
Hemolysis does not interfere with GGT levels because the enzyme is lacking in erythrocytes. This is a notable contrast to AST and ALP.
Male: 6 to 55 U/L at 37 degrees C (0.1 to 0.9 microkat/L)
Female: 5 to 38 U/L at 37 degrees C (0.1 to 0.6 microkat/L)
Values are lower in females, presumably because of suppression of enzyme activity by oestrogenic or progestational hormones.
This table summarises the clinically significant enzymes from the lecture material. Use it for rapid revision.
Enzyme | Abbreviation | Primary Clinical Association |
|---|---|---|
Acid phosphatase | ACP | Prostatic carcinoma |
Alanine aminotransferase | ALT | Hepatic disorder |
Aldolase | ALD | Skeletal muscle disorder |
Alkaline phosphatase | ALP | Hepatic disorder, bone disorder |
Amylase | AMS | Acute pancreatitis |
Angiotensin-converting enzyme | ACE | Blood pressure regulation |
Aspartate aminotransferase | AST | Myocardial infarction, hepatic disorder, skeletal muscle disorder |
Cholinesterase | CHE | Organophosphate poisoning |
Chymotrypsin | CHY | Chronic pancreatitis insufficiency |
Creatine kinase | CK | Myocardial infarction, skeletal muscle disorder |
Elastase-1 | E1 | Chronic pancreatitis insufficiency |
Glucose-6-phosphate dehydrogenase | G-6-PD | Drug-induced haemolytic anaemia |
Glutamate dehydrogenase | GLD | Hepatic disorder |
Gamma-glutamyltransferase | GGT | Hepatic disorder |
Glutathione-S-transferase | GST | Hepatic disorder |
Glycogen phosphorylase | GP | Acute myocardial infarction |
Lactate dehydrogenase | LDH | Myocardial infarction, hepatic disorder, haemolysis, carcinoma |
Lipase | LPS | Acute pancreatitis |
5'-Nucleotidase | NTP | Hepatic disorder |
Pseudocholinesterase | PChE | Organophosphate poisoning, genetic variants, hepatic disorder, suxamethonium sensitivity |
Pyruvate kinase | PK | Haemolytic anaemia |
Trypsin | TRY | Acute pancreatitis |
Triacylglycerol lipase | TRY | Triacylglycerol acylhydrolase |
Students often assume AST is liver-specific. It is not. AST is also elevated in cardiac tissue damage, skeletal muscle disorders, and even pulmonary embolism. ALT is the more liver-specific transaminase.
Students frequently think hemolysis affects all enzyme assays equally. In fact, hemolysis falsely elevates AST and slightly elevates ALP, but does not interfere with GGT (because GGT is lacking in erythrocytes) and has minimal effect on ALT.
It is a common mistake to assume that elevated ALP always indicates liver disease. ALP also rises significantly in bone disorders (Paget's disease, healing fractures) and physiologically in children and pregnant women. GGT is the confirmatory test: if GGT is also elevated, the source is hepatobiliary; if GGT is normal, the source is likely bone.
Students sometimes confuse the older and newer enzyme names. AST = SGOT = GOT. ALT = SGPT = GPT. Both sets of terminology appear on exams.
Know which enzyme is more liver-specific (ALT) and which is more widely distributed (AST). This distinction is tested frequently.
Memorise the reference ranges for all four enzymes and the measurement temperature (37 degrees C).
Be able to interpret patterns: high AST with high ALT = hepatocellular damage; high ALP with high GGT = hepatobiliary obstruction; high ALP with normal GGT = bone disorder.
Know the AST timeline in AMI: rises in 6 to 8 hours, peaks at 18 to 24 hours, returns to normal in 4 to 5 days.
Know which enzymes are affected by hemolysis (AST and ALP) and which are not (GGT and ALT).
Remember that ALP reference ranges are much higher in children (54 to 369 U/L) than adults (50 to 115 U/L).
GGT reference ranges differ by sex: lower in females because of hormonal suppression of enzyme activity.
True or False: ALT is more specific for liver disease than AST. (True)
Fill in the blank: The coenzyme used by both AST and ALT is ______. (pyridoxal phosphate)
True or False: Hemolysis interferes with GGT measurement. (False, GGT is lacking in erythrocytes)
Fill in the blank: ALP levels are normally higher in children because of active ______. (bone growth)
True or False: An elevated ALP with a normal GGT suggests a hepatobiliary source. (False, it suggests a bone source)
Q: A patient presents with elevated AST, elevated ALT, and normal ALP. What is the most likely category of liver disease?
A: Hepatocellular disease (e.g. hepatitis, cirrhosis), where transaminases rise more than ALP.
Q: A patient has markedly elevated ALP and elevated GGT but only mildly elevated AST and ALT. What does this pattern suggest?
A: Hepatobiliary obstruction (cholestasis), where ALP and GGT rise more significantly than the transaminases.
Q: Why are GGT reference ranges lower in females than males?
A: Oestrogenic and progestational hormones suppress GGT enzyme activity.
Q: A blood sample is haemolysed. Which of the four enzymes (AST, ALT, ALP, GGT) will be most affected?
A: AST will be most affected (falsely elevated). ALP may be slightly elevated. ALT is relatively unaffected. GGT is not affected at all.
Q: What is the clinical utility of measuring GGT alongside ALP?
A: GGT helps determine whether an elevated ALP is of hepatic or skeletal origin. If both are elevated, the source is likely hepatobiliary. If ALP is elevated but GGT is normal, the source is likely bone.
Q: A child aged 8 has an ALP of 300 U/L. Is this abnormal?
A: Not necessarily. The reference range for children aged 4 to 15 is 54 to 369 U/L, which is much higher than the adult range because of active bone growth.
Q: What wavelength is monitored in both AST and ALT assays, and what is being measured?
A: 340 nm. The assays monitor the rate of NADH oxidation to NAD+.
This material connects directly to the enzymology fundamentals covered in the companion study notes (enzyme structure, Michaelis-Menten kinetics, inhibitors). The coupled enzymatic reaction assays used for AST and ALT rely on the kinetics principles covered there.
AST and ALT patterns are also relevant to hepatology and gastroenterology topics, particularly viral hepatitis, cirrhosis, and biliary obstruction. ALP and GGT connect to bone physiology and metabolic bone disease topics.
The concept of isoenzymes, covered in the enzymology notes, becomes clinically important here: ALP bone isoenzyme vs. ALP liver isoenzyme, and CK-MB vs. CK-MM are examples of how isoenzyme identification helps pinpoint the tissue of origin.
AST, SGOT, GOT, aspartate aminotransferase, serum glutamic-oxaloacetic transaminase, ALT, SGPT, GPT, alanine aminotransferase, serum glutamic-pyruvic transaminase, ALP, alkaline phosphatase, GGT, GGTP, gamma-glutamyltransferase, pyridoxal phosphate, Karmen method, coupled enzymatic reaction, NADH, 340 nm, hepatocellular, hepatobiliary, cholestasis, Paget's disease, hemolysis, liver function tests, bone markers, clinical chemistry, CHEM 2, clinical chem 2 lab, transaminases, reference ranges, enzyme activity units