CLINICAL ENZYMOLOGY

Measurements of the activity of enzymes in plasma are of value in the diagnosis and management of a wide variety of diseases.

 Small amounts of intracellular enzymes are present in the blood as a result of normal cell turnover. When damage to cells occurs, increased amounts of enzymes will be released and their concentrations in the blood will rise.

 However, such increases are not always due to tissue damage.  Other possible causes include:  increased cell turnover  cellular proliferation (e.g. neoplasia)  increased enzyme synthesis (enzyme induction)  obstruction to secretion

 One international unit is the amount of enzyme that will convert one micromole of substrate per minute per litre of sample and is abbreviated as U/L.  Katal (catalytic activity) is defined as the number of mole of substrate transformed per second per litre of sample.

Disadvantages of enzyme assays  A major disadvantage in the use of enzymes for the diagnosis of tissue damage is their lack of specificity to a particular tissue or cell type. Many enzymes are common to more than one tissue.

 Second, some enzymes exist in different forms (isoforms)  Individual isoforms are often characteristic of a particular tissue: although they may have similar catalytic activities

Alkaline phosphatase (ALP)  Normal serum level of ALP is U/L or mmol/(hour  L).  This enzyme present in high concentrations in the  liver  bone (osteoblasts)  placenta  intestinal epithelium. 

 The causes of an increase in plasma ALP activity  Physiological increases are been in pregnancy, due to the placental isoenzyme, and in childhood (when bones are growing), due to the bone isoenzyme.

ACID PHOSPHATASE (ACP)  It hydrolyses phosphoric acid ester at pH between 4 and 6.  ACP is secreted by prostate cells, RBC, platelets and WBC.  Normal serum value for ACP is U/L or mmol/(hour  L).

 ACP total value is increased in prostate cancer and highly elevated in bone metastasis of prostate cancer.  ACP is therefore an important tumour marker.  ACP is present in high concentration in semen, a finding which is used in forensic medicine in investigation of rape.

Aminotransferases  Two aminotransferases are used in diagnosis and management: aspartate aminotransferase (AST)  and alanine aminotransferase (ALT).

ASPARTATE AMINO TRANSFERASE (AST)  It is also called as serum glutamate- oxaloacetate transaminase (SGOT).  Normal serum level of AST is U/L or ( mmol/(hour  L))  It is significantly elevated in myocardial infarction. It if moderately elevated in liver diseases.

Measurement of cardiac enzyme levels  Measure cardiac enzyme levels at regular intervals, starting on admission and continuing until the peak is reached or until 3 sets of results are negative. Biochemical biomarkers are useful for both diagnosis and prognostication

Laboratory Diagnosis of Myocardial Infarction

ALANINE AMINO TRANSFERASE (ALT)  It is also called as serum glutamate-pyruvate transaminase (SGPT).  Normal serum level of ALT is 5-30 U/L or ( mmol/(hour  L))

 Very high values (100 to 1000 U/L) are seen in acute hepatitis, either toxic or viral in origin.  Both ALT and AST are increased in liver diseases, but ALT >AST.  Moderate increase (25 to 100 U/L) may be seen in chronic liver disease such as cirrhosis, and malignancy in liver.

 Ritis coefficient (AST/ALT) in normal conditions is  1.33 ± 0,42.

LACTATE DEHYDROGENASE (LDH) (LD)  Normal value of LDH in serum is U/L.  Values the upper range are generally seen in children. Strenuous exercise will slightly increase the value. LDH level is 100 times more inside the RBC than in plasma, and therefore minor amount of hemolysis will result in a false-positive test.

 This enzyme exists in body tissues as a tetramer. Two monomers, H and M, can combine in various proportions with the result that five isoenymes of LD are known.  So five combinations of H and M chains are possible; H4, H3M, H2M2, M3H and M4

 The iso-enzymes are usually separated by cellulose acetate electrophoresis at pH 8.6.  Lactate dehydrogenase isoenzymes (as percentage of total):  LDH %  LDH %  LDH %  LDH4 8-16%  LDH %

 Increase in total LDH level is seen in  hemolytic anemias,  hepatocellular damage,  muscular dystrophy,  carcinomas,  leukemias,  and any condition which causes necrosis of body cells. 

Creatine kinase (CK)  Normal serum value for CK is U/L for males and U/L for females.  CK is a dimer; each subunit has a molecular weight of 40,000. The subunits are called B for brain and M for muscle. Three isoenzymes, BB, MM and MB, occur.  Normally CK2 (MB) iso-enzyme is only 5% of the total activity.

Causes of an increased plasma creatine kinase activity.  often >10 x ULN  polymyositis  rhabdomyolysis (e.g. trauma, malignant hyperpyrexia)  Duchenne muscular dystrophy  myocardial infarction  5-10 x ULN  following surgery  skeletal muscle trauma  severe exercise  myositis  carriers of Duchenne muscular dystrophy  usually <5 x ULN  physiological (Afro-Caribbeans)  hypothyroidism  drug (statin) treatment

NUCLEOTIDE PHOSPHATASE (NTP)  It is a marker enzyme for plasma membranes and is seen as an ecto- enzyme (enzyme present on the cell membrane).  Normal NTP level in serum is 2-10 U/L. It is moderately increased in hepatitis and highly elevated in biliary obstruction.

GAMMA GLUTAMYL TRANSFERASE (GGT)  It is seen in liver, kidney, pancreas, intestinal cells and prostate gland.  In the body it is used in the synthesis of glutathione.  Normal serum value of GGT is U/L in male and 5-30 U/L in female.

 This value is moderately increased in infective hepatitis and prostate cancers. The GGT level is highly elevated in alcoholism, obstructive jaundice and neoplasm's of liver.

PROSTATE SPECIFIC ANTIGEN (PSA)  It is produced from the secretory epithelium of prostal gland. It is normally secreted into seminal fluid  Normal value is 1 -5 µg/L. It is very specific for prostate activity. Values between 4-10 µg/L is seen in benign prostate enlargement; but values above 10 µg/L is indicative of prostate cancer.

 Interest in this enzyme derives largely from the fact that it hydrolyzes a muscle- relaxant drug, widely used in anaesthesia, called succinylcholine (scoline). Occasionally, patients are found in whom the effect of this drug, which paralyzes respiration, persists for several hours after it has been administered (scoline apnoea). Many of these patients have an abnormal cholinesterase activity.

GLUCOSE-6-PHOSPHATE DEHYDROGENASE  This is an important enzyme in the hexose monophosphate shunt pathway of glucose.  Normal value of GPD in RBC is U/10 12 cells.  It is mainly used for production of NADPH. It has a special role in the RBC metabolism.

AMYLASE  This splits starch to maltose. It is activated by calcium, chloride and fluoride ions.  It is produced by pancreas and salivary glands  Normal serum value is U/L, (12-32 g/(hour× L)).

LIPASE  It will hydrolyse triglyceride to β- monoglyceride and fatty acid.  The enzyme is present in pancreatic secretion.  Normal serum range is U/L.

Enolase  It is a glycolytic enzyme. Neuron- specific enolase (NSE) is an iso- enzyme seen in neural tissues and Apudomas. NSE is a tumour marker for cancers associated with neuro- endocrine origin, small cell lung cancer,neuroblastoma,pheochromocyt om, medullary carcinoma of thyroid, etc.  Upper limit of NSE is 12 μg/ml.