Diagnostic Enzymes

Diagnostic enzymes

Serum enzymes used to detect tissue damage, assess organ function, and monitor disease progression

Factors influencing serum enzyme levels

Cell damage or necrosis, tissue mass, enzyme induction or inhibition, clearance rate, hemolysis, specimen handling, age, and pregnancy

Hemolysis effect on enzymes

Causes falsely elevated intracellular enzymes such as LD and AST due to red blood cell rupture

Enzyme induction

Increased enzyme synthesis caused by drugs or alcohol leading to elevated serum levels

Clearance rate of enzymes

Rate at which enzymes are removed from circulation affects serum concentration

Continuous enzyme monitoring

Preferred method that measures reaction rate over time and confirms linearity

Endpoint enzyme measurement

Single measurement after stopping the reaction at a fixed time and is less accurate than continuous methods

AST (Aspartate Aminotransferase)

Enzyme found in liver, cardiac muscle, skeletal muscle, and red blood cells and is elevated in liver disease, myocardial injury, and muscle damage

ALT (Alanine Aminotransferase)

Primarily a liver enzyme that is elevated in hepatocellular injury and is more liver-specific than AST

Clinical value of ALT

Preferred enzyme for detecting liver disease due to higher specificity

Creatine Kinase (CK)

Enzyme involved in energy metabolism found in skeletal muscle, cardiac muscle, and brain

CK-MM isoenzyme

Isoenzyme found primarily in skeletal muscle and elevated in muscle injury and rhabdomyolysis

CK-MB isoenzyme

Isoenzyme associated with cardiac muscle and elevated in myocardial injury

CK-BB isoenzyme

Isoenzyme found in brain tissue and rarely measured clinically

CK isoenzyme separation methods

Electrophoresis and immunochemical mass assays

Lactate Dehydrogenase (LD)

Enzyme present in most tissues that catalyzes the conversion of lactate to pyruvate

LD isoenzyme structure

Tetramer composed of H and M subunits forming five isoenzymes

LD1 and LD2

Isoenzymes found in heart, red blood cells, and kidney and historically associated with myocardial infarction

LD3

Isoenzyme associated with lung, pancreas, and spleen tissue

LD4 and LD5

Isoenzymes associated with liver and skeletal muscle

LD and malignancy

General elevation of all LD isoenzymes seen in many cancers

LD analytical error

Hemolysis falsely increases LD activity and cold exposure decreases LD activity

Alkaline Phosphatase (ALP)

Enzyme that hydrolyzes phosphomonoesters at alkaline pH and is found in liver, bone, intestine, placenta, spleen, and kidney

Liver ALP isoenzyme

Elevated in hepatobiliary obstruction such as gallstones

Bone ALP isoenzyme

Elevated in Paget’s disease, osteomalacia, fractures, hyperparathyroidism, and normal bone growth

Placental ALP isoenzyme

Elevated during pregnancy

Intestinal ALP isoenzyme

Associated with intestinal disease and commonly found in individuals with blood types B and O

ALP isoenzyme separation methods

Electrophoresis, heat inactivation, selective chemical inhibition, and immunoassay

Gamma-Glutamyl Transferase (GGT)

Enzyme found in liver, kidney, pancreas, brain, and prostate and elevated in hepatobiliary disease

Clinical use of GGT

Used to differentiate liver-related ALP elevation from bone disease and is elevated in chronic alcoholism

Amylase

Enzyme that breaks down carbohydrates and is sourced from the pancreas and salivary glands

Amylase clinical significance

Elevated in acute pancreatitis, mumps, intra-abdominal events, and after opiate administration

Macroamylasemia

Condition where amylase binds to immunoglobulins causing persistent elevation without pancreatic disease

Lipase

Pancreatic enzyme that breaks down triglycerides into fatty acids and glycerol

Lipase in pancreatitis

More specific and longer-lasting marker of acute pancreatitis than amylase

Pseudocholinesterase (CHE)

Enzyme produced in the liver that cleaves choline esters and has an unclear physiologic function

Decreased cholinesterase levels

Seen in liver disease, organophosphate poisoning, and after succinylcholine administration

Glucose-6-Phosphate Dehydrogenase (G-6-PD)

Red blood cell enzyme involved in NADPH production that protects cells from oxidative damage

G-6-PD deficiency

Sex-linked disorder that predisposes patients to hemolytic anemia when exposed to oxidant drugs or fava beans

Apoenzyme

Protein portion of an enzyme without required cofactors

Holoenzyme

Active enzyme consisting of an apoenzyme plus its required cofactors

Prosthetic group

Non-protein component tightly bound to an enzyme and essential for activity

Cofactor

Non-protein substance, often a metal ion, required for enzyme function

Coenzyme

Organic molecule such as NADH that participates directly in enzymatic reactions

Activator

Substance that increases enzyme activity, commonly a metal ion

Denaturation

Loss of an enzyme’s three-dimensional structure resulting in decreased or absent activity

First-order enzyme kinetics

Reaction rate depends on substrate concentration at low substrate levels

Zero-order enzyme kinetics

Reaction rate is independent of substrate concentration when enzyme saturation is reached

Michaelis-Menten curve

Graph of reaction velocity versus substrate concentration used to describe enzyme kinetics

Lag phase

Initial phase where enzyme-substrate complexes are forming

Linear phase

Phase where product formation is proportional to enzyme concentration

Substrate depletion phase

Phase where reaction rate decreases due to reduced substrate availability

Lineweaver-Burk plot

Double-reciprocal plot of 1/V versus 1/[S] used to calculate Km and Vmax

Advantage of Lineweaver-Burk plot

Provides a linear relationship that allows easier and more accurate determination of Vmax