Introduction to Clinical Enzymology

Vocabulary-based flashcards covering the basics of clinical enzymology, enzyme classifications, and their diagnostic utilities in various diseases.

Enzyme

A protein or RNA produced by living cells that is highly specific and highly catalytic to its substrates, allowing chemical reactions to occur efficiently under required conditions.

Pseudocholinesterase

An enzyme primarily present in the circulation that performs its physiological role directly in the blood.

Alanine transaminase (ALT)

A cytosolic enzyme found in high concentrations in the liver; marked plasma elevations are an index of acute hepatocytic damage such as viral or toxic hepatitis.

Oxidoreductases (EC 1)

Enzymes that catalyze redox reactions ($A_{red} + B_{ox} \rightarrow A_{ox} + B_{red}$) and are categorized into oxidase and reductase.

Transferases (EC 2)

Enzymes that catalyze the transfer or exchange of certain groups among some substrates ($A-B + C \rightarrow A + B-C$).

Hydrolases (EC 3)

Enzymes that accelerate the hydrolysis of substrates ($A-B + H_2O \rightarrow A-OH + B-H$).

Lyases (EC 4)

Enzymes that promote the removal of a group from a substrate to leave a double bond or catalyze its reverse reaction (synthase).

Isomerases (EC 5)

Enzymes that facilitate the conversion of isoisomers, geometric isomers, or optical isomers ($A-B-C \rightarrow A-C-B$).

Ligases (EC 6)

Enzymes that catalyze the synthesis of two molecular substrates into one molecular compound using energy release ($A + B + ATP \rightarrow A-B + ADP + P_i$).

Translocases (EC 7)

Enzymes that catalyze the movement of ions or molecules across membranes or their separation within membranes.

Functional plasma enzymes

Enzymes that perform physiologic functions in blood and are present at all times in the circulation of healthy individuals.

Non-functional plasma enzymes

Enzymes that do not perform physiological functions in blood; their presence is due to routine cell destruction or tissue damage from injury or disease.

Macroenzymes

Large enzyme complexes, such as in macroamylasaemia, that can limit the diagnostic utility of enzyme findings.

Aspartate Aminotransferase (AST)

An enzyme with cytosolic and mitochondrial locations found in the heart, liver, and skeletal muscle; higher plasma levels than ALT signify more severe hepatocytic injury due to its mitochondrial origin.

Lactate Dehydrogenase (LDH)

An enzyme comprising five isoenzymes (LDH1 to LDH5) where $LDH_1$ is found in cardiac cells and $LDH_2$ levels are normally higher than other isoenzymes.

CK-MB

An isoenzyme of creatine kinase found in cardiac and skeletal muscle; a ratio of $CK-MB:CK \ge 2.5\%$ is highly suggestive of myocardial infarction (MI).

Glycogen Phosphorylase BB

An isoenzyme and newer cardiac marker that rises within $1-3\,hrs$ of ischaemia; it is heart muscle specific due to the blood-brain barrier.

Acid Phosphatase (ACP)

An enzyme derived from the prostate, bone, liver, RBCs, and platelets; the prostatic isoenzyme is tartrate-labile, while erythrocytic ACP is inhibited by cupric ions.

Amylase

An enzyme from the pancreas (P-amylase) and salivary gland (S-amylase) that is markedly elevated in acute pancreatitis and Diabetic Ketoacidosis (DKA).

Lipase

A pancreas-derived enzyme more specific than amylase with a longer half-life ($7-14\,days$), useful for evaluating late presenting cases of acute pancreatitis.

Angiotensin Converting Enzyme (ACE)

An enzyme sourced from the pulmonary endothelium used as a diagnostic marker for sarcoidosis.

Tryptase

A marker derived from mast cells used to aid in the diagnosis of local and systemic anaphylaxis.

Gamma Glutamyl Transferase (GGT)

An enzyme occurring in the liver, kidneys, and prostate; it is induced by drugs like anticonvulsants and alcohol, showing marked elevations in cholestatic liver disease.

Alkaline Phosphatase (ALP)

An enzyme group from bone, liver, and placenta; its activity is physiologically higher in children than adults and very high during pregnancy.

5' Nucleotidase

A cell membrane enzyme that is markedly elevated in biliary tract obstruction and unaffected by bone diseases.