Enzymes in the Diagnosis of Pathology

Is the field of the measurement of enzyme activity in a biological speciment (blood, urine, tissue), can aids or helps or useful in the diagnosis of diseases, such measurement may also helped for the follow up , assessment of treatment ,prognosis of the disease.

Normally each enzyme is present in the circulation within certain range which is called (normal or reference range ) which represent the net or the balance of the rate of the release of enzyme by its turnover and the rate of its disappearance a phenomena called some time (wear and tear effect ); wear and tears : normal physiological turnover of the cell. Protein k-deg . K-sys. a.a

Enzyme in the blood are either normal or abnormal , where there is disease process the enzyme activity will increase distribution of enzyme in different tissue .Ex : Tissue damage ( as in myocardial infraction ) and infection ( as in hepatitis ) cause release of tissue enzyme into the bloodstream, also in liver disease leading to biliary obstruction causes an increase in some enzymes by blocking their elimination into the bile.

Elevated enzyme activity in the plasma may indicate tissue damage accompanied by increased release of intracellular enzymes.

Levels of abnormal enzymes: ( nonplasma , specific enzymes) These enzymes may be divided into two groups: 1- Secretory enzymes and function outside the body as in digestion ex: ∞Amylase is produced by the pancrease .Its appearance in the blood serum is indicative of pancreatic disorder . The normal serum level is increased in acute pancreatitis during the first 72 hours after the onset of symptoms.

2- Intracellular enzymes include : Alkaline phosphatase : levels of serum enzyme are elevated in osteological disease ( e.g osteogenic sarcoma , rickets) or biliary disorders ( obstructive jaundice cirrhosis). Acid phosphatase : levels are elevated in carcinoma of the prostate.

Creatine phosphokinase (CPK ): This enzyme found primarily in skeletal and cardiac muscle is elevated in blood after myocardial infarction (MI).

CPK is a dimer consisting of one subunit found in the brain (B) and another in muscle (M) . It is found as three isozymes,in normal serum ,95‰f the CPK activity exists as the CPK3 species . Following myocardial infarction the serum CPK2 (MB) species increases specifically.

Serum Transaminases: These are serum glutamic oxaloacetic transaminase (SGOT) and serum glutamic pyruvic trasaminase (SGPT).

The added enzyme , malate dehydrogenase (MDH), catalyzes the reduction of oxaloacetate to malate and the oxidation of NADH .normal values are 5 to 20 mU/ml.

Lactate dehydrogenase : LDH catalyze the interconvertion of lactate and pyrovate in the presence of NADH or NADH2 .it is distributed generally in body cells and fluids.normal value varies with method . any degree of hemolysis must be avoided because the conc. Of LDH within red blood cell is 100 times that in normal serum . Heparin and oxalate may inhibit enzyme activety. Elevated in all conditions acompanied by tissue necrosis . normal values are 125-270 mU/ml.

∞-Hydroxybutyrate dehydrogenase ( ∞-HBDH) Normal values are 96-210mU/ml . actually this enzyme appears to be the same as LDH-1, its richest source in heart muscle, kidney , and erythrocytes . in myocardial infarction ∞-HBDH is usually elevated for a longer time than is total LDH . elavation will continue for 2to 3 weeks after infection .

Change of izoenzyme in MI

The measurement of the serum levels of numerous enzymes has been shown to be of diagnostic significance. This is because the presence of these enzymes in the serum indicates that tissue or cellular damage has occured resulting in the release of intracellular components into the blood. Hence, when a physician indicates that he/she is going to assay for liver enzymes, the purpose is to ascertain the potential for liver cell damage. Commonly assayed enzymes are the amino transferases: alanine transaminase, ALT (sometimes still referred to as serum glutamate-pyruvate aminotransferase, SGPT) and aspartate aminotransferase, AST (also referred to as serum glutamate-oxaloacetate aminotransferase, SGOT); lactate dehydrogenase, LDH; creatine kinase, CK (also called creatine phosphokinase, CPK); gamma-glutamyl transpeptidase, GGT. Other enzymes are assayed under a variety of different clinical situations but they will not be covered here.

The typical "liver enzymes" measured are AST and ALT The typical "liver enzymes" measured are AST and ALT. ALT is particulary diagnostic of liver involvement as this enzyme is found predominantly in hepatocytes. When assaying for both ALT and AST the ratio of the level of these two enzymes can also be diagnostic. Normally in liver disease or damage that is not of viral origin the ratio of ALT/AST is less than 1. However, with viral hepatitis the ALT/AST ratio will be greater than 1. Measurement of AST is useful not only for liver involvement but also for heart disease or damage. The level of AST elevation in the serum is directly proportional to the number of cells involved as well as on the time following injury that the AST assay was performed. Following injury, levels of AST rise within 8 hours and peak 24-36 hours later. Within 3-7 days the level of AST should return to pre-injury levels, provided a continuous insult is not present or further injury occurs. Although measurement of AST is not, in and of itself, diagnostic for myocardial infarction, taken together with LDH and CK measurements the level of AST is useful for timing of the infarct.

The measurement of LDH is especially diagnostic for myocardial infarction because this enzyme exist in 5 closely related, but slightly different forms (isozymes). The 5 types and their normal distribution and levels in non-disease/injury are listed below. LDH 1 - Found in heart and red-blood cells and is 17% - 27% of the normal serum total. LDH 2 - Found in heart and red-blood cells and is 27% - 37% of the normal serum total. LDH 3 - Found in a variety of organs and is 18% - 25% of the normal serum total.

LDH 4 - Found in a variety of organs and is 3% - 8% of the normal serum total. LDH 5 - Found in liver and skeletal muscle and is 0% - 5% of the normal serum total. Following a myocardial infarct the serum levels of LDH rise within 24-48 hours reaching a peak by 2-3 days and return to normal in 5-10 days. Especially diagnostic is a comparison of the LDH-1/LDH-2 ratio. Normally, this ration is less than 1. A reversal of this ration is referred to as a "flipped LDH.". Following an acute myocardial infart the flipped LDH ratio will appear in 12-24 hours and is definitely present by 48 hours in over 80% of patients. Also important is the fact that persons suffering chest pain due to angina only will not likely have altered LDH levels.

CPK is found primarily in heart and skeletal muscle as well as the brain. Therefore, measurement of serum CPK levels is a good diagnostic for injury to these tissues. The levels of CPK will rise within 6 hours of injury and peak by around 18 hours. If the injury is not persistent the level of CK returns to normal within 2-3 days. Like LDH, there are tissue-specific isoenzymes of CPK and there designations are described below. CPK3 (CPK-MM) is the predominant isoenzyme in muscle and is 100% of the normal serum total. CPK2 (CPK-MB) accounts for about 35% of the CPK activity in cardiac muscle, but less than 5% in skeletal muscle and is 0% of the normal serum total. CPK1 (CPK-BB) is the characteristic isoenzyme in brain and is in significant amounts in smooth muscle and is 0% of the normal serum total.

Since most of the released CPK after a myocardial infarction is CPK-MB, an increased ratio of CPK-MB to total CPK may help in diagnosis of an acute infarction, but an increase of total CPK in itself may not. CPK-MB levels rise 3-6 hours after a myocardial infarct and peak 12-24 hours later if no further damage occurs and returns to normal 12-48 hours after the infarct.