The following points highlight the four main types of enzymes. The types are: 1. Lactate Dehydrogenases 2. Transaminases 3. Creatine Kinase (CK or CPK) 4. Carbonic Anhydrase.
Enzyme: Type # 1. Lactate Dehydrogenases:
It is a glycolytic enzyme involved in the conversion of lactic acid to pyruvic acid and vice versa by the help of the coenzyme NAD.
It exists in five different isozyme forms e.g., LDH1, LDH2, LDH3, LDH4 and LDH5.
Source:
Found in heart muscle as well as in many other tissues. It is liberated to the blood stream during myocardial injury.
Concentration:
The normal level of serum LDH is 200-425 units/100 ml. or 90-200 I.U./liter.
Diagnostic Importance:
i. The rise of serum LDH is not so prompt as that of SCOT and the peak is not reached until the fourth or fifth day but the level may remain abnormally high for 6 to 12 days.
ii. In case of suspected myocardial infarction, the absence or elevation of serum LDH is more significant in ruling out the disease.
iii. In hepatitis, there is an insignificant rise in LDH; but a great rise in LDH is seen in untreated pernicious anemia.
iv. Analysis of an overnight eight-hour urine specimen for lactate dehydrogenase activity is of great value in the diagnosis of serious diseases of kidney and bladder. Elevated findings are found, particularly in carcinoma of these organs.
v. An occasional high value is found in non- carcinomatous lesions of the kidney and bladder.
Enzyme: Type # 2. Transaminases:
The enzymes which are involved in transamination reactions in the living organisms are said to be transaminases.
Examples are:
Glutamate oxaloacetate transaminase (GOT).
Glutamate aspartate transaminase (GPT).
The transaminases are widely distributed in nature.
They also exist in isozyme forms.
Source:
GOT is present in highest amounts in heart muscle, skeletal muscle, brain, liver, and kidney. Traces of these enzymes normally escapes into the blood stream.
Concentration:
The normal level of SGOT is 8-40 Karmen units/dL.
The normal level of SGPT is 0-40 Karmen units/dL.
Diagnostic Importance:
i. When even a small portion of heart muscle is actually injured by occlusion of a branch of the coronary arteries an appreciable amount of GOT escapes into the blood stream and can be recognized by a sharp rise in the serum concentration.
ii. In myocardial injury, a rise in SGOT concentration may occur in a few hours after onset of symptoms. The peak level is a high as 160 units within 48 hours and returns to the normal level within 3 to 5 days.
iii. In myocardial infarction, the rise in SGPT is not so high as that of SGOT. But in liver disease SGPT is greatly increased.
iv. In viral hepatitis, SGPT is always higher than SGOT; but in cirrhosis and hemolytic jaundice SGOT is higher than SGPT.
Enzyme: Type # 3. Creatine Kinase (CK or CPK):
This enzyme is involved in the formation of phosphocreatine from creatine and ATP (adenosine triphosphate). It is frequently referred to as creatine phosphokinase (CPK). It exists in isozyme forms.
Source:
i. It is found in greatest amount in skeletal muscle.
ii. It is present in appreciable amounts in cardiac muscle and in brain.
iii. Small amounts are present in the lung, thyroid and adrenals.
iv. It is apparently not present in liver and kidney and is not detectable in erythrocyte. Hence, serum levels are not affected by hemolysis.
Concentration:
The normal level in serum is 12 to 99 U/I for males and 10 to 66 U/I for females. (by Rosalki method.)
Diagnostic Significance:
i. There is a marked increase in serum creatine kinase levels, up to 50 times the normal, in patients suffering from the Deuchenne type of muscular dystrophy. There is a small elevation in this enzyme activity in patients suffering from certain other forms of muscular dystrophy like limb-girdle and myotonic types.
ii. Enzyme levels are normal in muscular atrophy resulting from neurological disorders or associated with hyperthyroidism.
iii. Increased levels are also found in muscle trauma, in polymyositis, in MC Ardle’s syndrome and after severe exercise.
iv. The activity of this enzyme is increased within 12 hours after the onset of an acute myocardial infarction. The level returns to normal in 3 or 4 days.
v. Increased levels of it can also occur in individuals with hypothyroidism, in patients with diabetic ketoacidosis, after pulmonary infarction, in the presence of convulsive disorders and in malignant hyperthermia.
Enzyme: Type # 4. Carbonic Anhydrase:
This is an enzyme which is associated with the hemoglobin in the red cells (never in the plasma).
It has been isolated in highly purified form and shown to be a zinc-protein complex.
Source:
i. Small amounts of carbonic anhydrase are found in muscle tissue, in the pancreas and in spermatozoa.
ii. Much larger quantities occur in the parietal cells of the stomach.
Function:
a. It specifically catalyzes the removal of CO2 from H2CO3.
The reaction is reversible:
CO2 + H2O ⇋ H2CO3
b. In the stomach, it is involved in the secretion of hydrochloric acid:
H2CO3 ⇋ H+ + HCO3
c. Since it occurs in the tubules of the kidney, it is also involved in hydrogen ion secretion.