DIAGNOSTIC ENZYMES AND THEIR CLINICAL IMPORTANCE

Enzymes are substances which catalyse metabolic and biochemical reactions. They are also known as biological catalysts. They enable metabolic and biochemical reactions to occur by lowering the activation energy of such reactions. The following are examples of diagnostic enzymes viz,lactatedehydrogenase(LDH),
creatinekinase(CPK),alkaline phosphatase,aspartate aminotransferace(AST),
alanine aminotransferace(ALT),acid phosphatase,glucose-6-phosphate dehydrogenase
and so on.



It is pertinent to note that the rationale for assaying enzyme activities is based on the premise that changes in activities reflect changes that have occurred in aspecific tissue or organ. Lactate dehydrogenase is an important enzyme involved in glucose metabolism.It is a tetramer(has 2 different subunits).It has five isoenzymes which are designated LDH1(HHHH),LDH2(HHHM),LDH3(HHMM),LDH4(HMMM) and LDH5(MMMM) respectively.Isoenzymes catalyse similar reaction but migrate differently when subjected to electrophoresis.Hence,lactate dehydrogenase activity is found in cardiac muscles and red blood cells( i.e LDH1,LDH2 and LDH4),skeletal muscle(LDH5),Liver (LDH5),kidney and brain (LDH3).Hence,increased LDH activities can be used to detect myocardial infarction,leukaemia,liver cirrhosis,tumours of the central nervous system.



Creatinekinase(CPK) exists as a dimeric enzyme with three isoenzymes CPK1,CPK2, and CPK3.The subunits are designated BB,MB, and MM respectively.Total creatinekinase activity shows an increase following myocardial infarction.It is pertinent to note that it is only when CPK2(i.e MB subunit) contribute more than 6% of the total creatinekinase activity that creatinekinase is implicated in myocardial infarction.
Alkaline phosphatase is an enzyme which transport phosphate groups across cell membranes of organs.It is found in the osteoblasts(bone cells),bile canali ( in the liver),small intestinal epithelium,proximal tubule(kidney) and placenta.Increase in alkaline phosphatase occurs mainly in bone diseases such as Paget disease,Osteogenic sarcoma,liver diseases and bilary tract.



Aspartate aminotransferase (AST) is an enzyme involved in transamination reactions. Transamination reactions produce keto and amino acids simultaneously in tissues where they occur.The assay of serum AST is particularly useful in the diagnosis of myocardial infarction.Elevated AST levels is indicative of damage to the myocardium. Although AST activity is still used in the diagnosis of myocardial infarction,it has been replaced to some extent by the assay of total creatinekinase activity(especially the isoenzyme CPK2).



Serum aminotransferase activities are sensitive indicators of parenchymal liver damage.Both alanine and aspartate aminotransferases have been implicated in infective hepatitis,infectious mononucleosis and hepatocellular damage.Exposure to toxic substances such as carbontetrachloride poisoning ,therapeutic substances such as chlormphenicol, cephalosporins and paracetamol overdose have also resulted in elevated AST and ALT activities.



The serum barely detectable glucose-6-phosphate dehydrogenase activity .An elevated activity is indicative of myocardial infarction .Red blood cells contain 120 – 240 units per 1012 cells. Defeciency of this enzyme can result in hemolytic anaemia through the use of antimalarials such as primaquine, aromatic compounds ,it has also been associated to Favism (resulting from Fava beans consumption).



Acid phosphatase is found in the prostate, red blood cells, small amounts are found in bone, liver, spleen and pancreas. The assay of serum acid phosphatase is mostly used to detect prostatic carcinoma and to monitor the treatment.