Plasma enzymes in diagnosis ( clinical enzymology )
Dr. Wajdy J. MajidAmylase Amylase (molecular weight 45 kDa) breaks down starch and glycogen to maltose. It is present at a high concentration in pancreatic juice and in saliva and may be extracted from other tissues, such as the gonads, Fallopian tubes, skeletal muscle and adipose tissue. Being of relatively low molecular weight, it is excreted in the urine. Estimation of plasma amylase activity is mainly requested to help in the diagnosis of acute pancreatitis, in which the plasma activity may be very high.
However, it may also be raised in association with other intraabdominal and extra-abdominal conditions that cause similar acute abdominal pain; thus a high result is not a specific diagnostic marker for acute pancreatitis If the plasma amylase activity fails to fall after an attack of acute pancreatitis, there may be leakage of pancreatic fluid into the lesser sac (a pancreatic pseudocyst); although C-reactive protein (CRP) may be a more useful marker of resolving uncomplicated acute pancreatitis ,
Causes of raised plasma amylase activity :– Acute pancreatitis, (may be greater than 5–10 times)– Severe glomerular impairment,– Diabetic ketoacidosis,– Other acute abdominal disorders like :• perforated peptic ulcer• acute cholecystitis• intestinal obstruction• abdominal trauma• ruptured ectopic pregnancy.
- Salivary gland disorders : • mumps • salivary calculi – Miscellaneous causes:• acute alcoholic intoxication,• macroamylasaemia,• Ectopic production from tumour.Macroamylasaemia :In some patients, high plasma amylase activity is due to low renal excretion of a macroenzyme form, despite normal glomerular function. The condition is symptomless and it is thought that the enzyme is bound to IgA, giving a complex of molecular weight about 270 kDa. This harmless condition may be confused with other causes of hyperamylasaemia .
Alkaline phosphatase : The ALPs are a group of enzymes that hydrolyze organic phosphates at high pH. They are present in most tissues but are in particularly high concentration in the osteoblasts of bone and the cells of the hepatobiliary tract , intestinal wall , renal tubules and placenta In adults, plasma ALP is derived mainly from bone and liver in approximately equal proportions; the proportion due to the bone fraction is increased when there is increased osteoblastic activity that may be physiological .
Causes of raised plasma alkaline phosphatase activity :Physiological : – During the last trimester of pregnancy, the plasma total ALP activity rises due to the contribution of the placental isoenzyme.– In preterm infants, plasma total ALP activity is up to five times the URL in adults, and consists predominantly of the bone isoenzyme.– In children, the total activity increases by about two to five times during the pubertal bone growth. There is a gradual increase in the proportion of liver ALP with age.
Bone disease:– Rickets and osteomalacia ,– Paget’s disease of bone (may be very high),– Secondary malignant deposits in bone,– Osteogenic sarcoma (only if very extensive),– Primary hyperparathyroidism with extensive bone disease (usually normal but may be slightly elevated),– Secondary hyperparathyroidism. Liver disease:– Intrahepatic or extrahepatic cholestasis ,– space-occupying lesions, tumours, granulomas and other causes Inflammatory bowel disease : the gut ALP isoenzyme (ulcerative colitis) . Malignancy : bone or liver involvement or direct tumor production.
Possible causes of low plasma alkaline phosphatase activity : A low plasma ALP concentration is less usual, but may be caused by the following : Arrested bone growth: – Achondroplasia, – Hypothyroidism, – Severe vitamin C and vitamin B12 deficiency. Hypophosphatasia : an autosomal recessive disorder, associated with rickets or osteomalacia.
Causes of raised plasma aspartate aminotransferase activities :_ Artefactual: due to in vitro release from erythrocytes if there is (haemolysis)_ Physiological: during the neonatal period (about 1.5 times the upper adult reference limit)._ Marked increase (may be greater than 5–10 times the upper reference limit or URL):– Circulatory failure with ‘shock’ and hypoxia,– Myocardial infarction,– Acute viral or toxic hepatitis._ Moderate to slight increase (usually less than five times ) :– Hepatic steatosis [fatty liver or non-alcoholic fatty liver disease (NAFLD)],– cirrhosis (may be normal sometimes),– infectious mononucleosis (due to liver involvement),– cholestatic jaundice,– malignant infiltration of the liver (may be normal),– skeletal muscle disease,– after trauma or surgery (especially after cardiac surgery),– severe haemolytic episodes (of erythrocyte origin),– certain drugs.Note that AST is not specific for hepatic disease.
Alanine aminotransferase : Alanine aminotransferase ( GPT) is present in high concentrations in liver and, to a lesser extent, in skeletal muscle, kidney and heart. Causes of raised plasma alanine aminotransferase activities: _ Marked increase (may be greater than 5–10 times URL) :– circulatory failure with ‘shock’ and hypoxia,– acute viral or toxic hepatitis._ Moderate to slight increase (usually less than five times URL):– Hepatic steatosis (fatty liver or NAFLD),– cirrhosis (may be normal sometimes),– infectious mononucleosis (due to liver involvement),– liver congestion secondary to congestive cardiac failure,– cholestatic jaundice,– certain drugs.Note that ALT is more specific for hepatic disease than AST.
Lactate dehydrogenase Lactate dehydrogenase catalyses the reversible interconversion of lactate and pyruvate. The enzyme is widely distributed in the body, with high concentrations in cells of cardiac and skeletal muscle, liver, kidney, brain and erythrocytes; measurement of plasma total LDH is therefore a non-specific marker of cell damage.Causes of raised plasma total lactate dehydrogenase activity :_ Artefactual: due to in vitro haemolysis or delayed separation of plasma ._ Marked increase (may be greater than 5–10 times ):– Circulatory failure with ‘shock’ and hypoxia,– Myocardial infarction .– Some haematological disorders: in blood diseases such as megaloblastic anaemia, acute leukaemias and lymphomas . In cases of lymphoma LDH can be used as a tumor marker. _ Moderate to slight increase (usually less than five times URL):– viral hepatitis,– malignancy of any tissue,– skeletal muscle disease,– pulmonary embolism,– infectious mononucleosis,– certain drugs.
Creatine kinase CPK : Creatine kinase is most abundant in cells of cardiac and skeletal muscle and in brain, but also occurs in other tissues such as smooth muscle. Isoenzymes of creatine kinase Creatine kinase consists of two protein subunits, M and B, which combine to form three isoenzymes, BB (CPK-1) MB (CPK-2) MM (CPK-3) _ CK-MM is the predominant isoenzyme in skeletal and cardiac muscle and is detectable in the plasma of normal subjects. _ CK-MB accounts for about 35 per cent of the total CK activity in cardiac muscle and less than 5 per cent in skeletal muscle; its plasma activity is always high after myocardial infarction. _ CK-BB is present in high concentrations in the brain and in the smooth muscle of the gastrointestinal and genital tracts. Increased plasma activities may occur during parturition. Although they have also been reported after brain damage, for example trauma or cerebrovascular accident .
Causes of raised plasma creatine kinase activities_ Artefactual: due to in vitro haemolysis ._ Physiological:– neonatal period (slightly raised above the adult URL),– during and for a few days after parturition . _ Marked increase (may be greater than 5–10 times URL):– dermatomyositis and polymyositis– ‘shock’ and circulatory failure,– myocardial infarction– muscular dystrophies– rhabdomyolysis (the breakdown of skeletal muscle)Rhabdomyolysis can be defined as an acute increase in plasma CK concentration greater than 10 times the upper limit of normal.
_ Moderate to slight increase (usually less than five times URL):– Infections, for example viral,– Muscle injury , after surgery , and even after intramuscular injection.– Physical exertion – there may be a significant rise in plasma activity after only moderate exercise, muscle cramp or following an epileptic fit.– Hypothyroidism– Alcoholism (possibly partly due to alcoholic myositis),– Malignant hyperpyrexia,– Certain drugs, for example statins, ciclosporin, cocaine, Severe muscle breakdown results in grossly elevated plasma CK concentrations, sometimes up to 100 000 U/L. This can be due to trauma, sever exertion , alcohol, heat, electrolyte disturbances and drugs such as statins. Causes of low plasma creatine kinase activityThis is unusual but may include cachetic states associated with reduced muscle mass, for example alcoholism, undernutrition and patients in intensive care.
Lipase Sometimes, when it is difficult to interpret plasma amylase results, it may be more useful to measure plasma lipase This enzyme is also derived from the pancreas but is more specific for pancreatic pathology. lipase has a longer half-life than amylase and therefore may be more useful in the diagnosis of late-presenting acute pancreatitis.Acid phosphatase : Acid phosphatase (ACP) is found in cells of the prostate, liver, erythrocytes, platelets and bone. The main indication for estimation was to help diagnose prostatic carcinoma and to monitor its treatment. Estimation has been largely replaced by the measurement of plasma prostate-specific antigen (PSA), a protein derived from the prostate, this test is more specific and sensitive for diagnosis and monitoring treatment .Causes of raised plasma acid phosphatase activity :– artefactually raised following rectal examination and urinary catheter, due to pressure on prostatic cells,– disseminated carcinoma of the prostate.– Haemolysed specimen .– Paget’s disease of the bone,– some cases of metastatic bone disease, – polycythaemia.
ɤ-Glutamyl transferase ɤ -Glutamyl transferase occurs mainly in the cells of liver, kidneys, pancreas and prostate. Plasma GGT activity is usually higher in males than in females.Causes of raised plasma ɤ -glutamyl transferase activity :-Induction of enzyme synthesis, without cell damage, by drugs or alcohol. ( anticonvulsants, phenobarbital and phenytoin) and alcohol induce proliferation of the endoplasmic reticulum.- Cholestatic liver disease : when changes in GGT activity usually parallel those of ALP. -Hepatocellular damage, such as that due to infectious hepatitis; measurement of plasma aminotransferase activities is a more sensitive indicator of such conditions.Very high plasma GGT activities, out of proportion to those of the aminotransferases, may be due to:_ Alcoholic hepatitis,_ Induction by anticonvulsant drugs or by alcohol intake,_ Cholestatic liver disease,_ Hypertriglyceridaemia,_ Fatty liver.
MYOCARDIAL INFARCTION
It is one of the largest killers in Western urbanized societies , Diagnosis is usually made on the clinical presentation and ECG findings and confirmed by the characteristic changes in plasma enzymes activities or troponin levels. Cardiac enzymes Creatine kinase ( CPK ) Lactate dehydrogenase (LDH) or hydroxybutyrate dehydrogenase(HBD) Aspartate aminotransferase (AST) These were used diagnostically, although they are now rarely assayed in the diagnosis of acute myocardialAll plasma enzyme activities (including that of CKMB) may be normal until at least 4 h after the onset of chest pain due to a myocardial infarction; Blood should not be taken for enzyme assay until this time has elapsed. If the initial plasma CK activity is normal, a second sample should be taken about 4–6 h later. A rise in the plasma CK activity supports the diagnosis of an infarction. The simultaneous measurement of plasma CK-MB activity, which is shown to exceed about 5 per cent of the total CK activity, may occasionally help in early diagnosis; A raised plasma CK-MB activity or concentration alone is not diagnostic of an infarction.
Most of the CK released after a myocardial infarction is, in fact, the MM isoenzyme (CK-MM), which is found in both skeletal and myocardial muscle and has a longer half-life than the MB fraction. After about 24 h, the finding of a high MM and undetectable MB does not exclude myocardial damage as a cause of high total CK activities . A raised plasma total CK activity, due entirely to CK-MM, may follow recent intramuscular injection, exercise, surgery or various skeletal muscular disorders Plasma enzyme activities are raised in about 95 per cent of cases of myocardial infarction . The degree of rise is a crude indicator of the size of the myocardial infarct, but is of limited prognostic value .
Troponins
Troponins are muscle-regulatory proteins present in skeletal and cardiac muscle. Three troponins have been reported, namely troponin C (TnC), troponin I (TnI) and troponin T (TnT)There are no structural differences between cardiac and skeletal muscle TnC. The cardiac and skeletal forms of TnI and TnT are structurally different and can be distinguished by immunological assays.Troponin I and TnT appear in the plasma 4–8 h or earlier with high-sensitivity troponin assays after symptoms of acute myocardial infarction, and are best measured 12 h after the start of chest pain. They are therefore not early markers of acute myocardial infarction, but they do stay elevated for about 7–10 days in plasma, which makes them useful in the late presentation of chest pain.Troponin T may be elevated in patients with chronic kidney disease and thus may not be so cardiospecific. An increased TnI or TnT concentration is a sensitive marker of occult myocardial damage even in nonischaemic conditions. Plasma troponin concentrations are increased in subarachnoid haemorrhage (due to vasoactive peptide release affecting the myocardium), hypertension, tachyarrhythmias, cardiac surgery, sepsis, congestive cardiac failure, pulmonary embolism, hypothyroidism. So that the universal definition of an acute myocardial infarction requires not only a change in troponin but also clinical or ECG evidence.
Myoglobin : myoglobin is a low molecular- weight haem-containing protein found in both skeletal and cardiac muscle. Because of its low molecular weight, it is rapidly released from themyocardium upon damage, and a typical rise occurs within 2–4 h after the onset of acute myocardial infarction. This is useful for the early diagnosis of acute myocardial infarction, as this rise is generally earlier than that of the other currently used cardiac markers.Unfortunately, myoglobin is not cardiac specific, being also found in skeletal muscle, and thus is less useful in the diagnosis of acute myocardial infarction unless used in conjunction with other markers.