Biochemical examination of the pancreas

In terms of laboratory diagnostics, the examination of the pancreas can be divided into:


 * examination of endocrine functions of the pancreas;
 * examination of the exocrine part of the pancreas:
 * evidence of acinar cell damage;
 * examination of pancreatic juice secretion.

Endocrine function of the pancreas
The internal secretory part of the pancreas produces mainly insulin, glucagon, and somatostatin. Disorders of the production of these hormones are discussed in the relevant chapters, especially in relation to diabetes mellitus.

Exocrine function of the pancreas
Examination of the exocrine function of the pancreas is not widespread. It is used mainly in the diagnosis of chronic pancreatitis. It is based on direct tests which measure the concentration or activity of pancreatic enzymes (chymotrypsin, elastase) in stool, and indirect tests, based on the administration of suitable substrates of pancreatic enzymes and the detection of fission products made by their digestion.

Biochemical examination of the pancreas
Extensive damage of pancreatic tissue occurs mainly in acute pancreatitis. It is a life-threatening sudden abdominal event in which the digestive enzymes, as a part of the pancreatic juice, are activated, leading to the digestion of the pancreatic tissue. Acute pancreatitis is most often triggered by overpressure in the common pancreatic and bile ducts (in cholelithiasis) and alcoholism.

The breakdown of pancreatic acinar cells leads to the spillage of their components into the blood. High catalytic concentration of pancreatic enzymes can be proven in the serum, especially α-amylase and pancreatic lipase.

α-amylase


α-amylase (AMS, α-1,4-glucan-4-glucan hydrolase, EC 3.2.1.1) hydrolyses α-1-4-glycoside bond; The pH optimum of α-amylase is between 7.0-7.2. It occurs in the body in two forms - according to its organ origin as salivary and pancreatic isoenzymes. Both isoforms of AMS differ from each other by the sugar component and can be distinguished electrophoretically or by precipitation using a special lectin or antibodies. α-amylase is produced by the acinar cells of the pancreas and accumulates in its zymogenic granules. It enters the intestinal lumen as a part of pancreatic secretion (pancreatic juice) along with other digestive enzymes. Under physiological conditions, the enzyme molecule is not absorbed by the intestinal surface and the serum level is low. It corresponds to the activity of the enzyme released into the circulation directly from the glandular cells (lymphatic drainage). The molecular weight of α-amylase is 55,000. Α-Amylase is eliminated from the circulation in the kidneys by glomerular filtration.

The macro form of the enzyme (macroamylase) is formed by the bondage of the enzyme to certain blood serum proteins, especially immunoglobulins, circulating immunocomplexes, or other glycoproteins. The macro form of the enzyme has a significantly higher molecular weight (from 150,000 to 2,000,000) and is therefore not eliminated by glomerular filtration in kidneys. For clinical diagnosis serum and urinary α-amylase levels are determined and the amylase/creatinine clearance index is calculated.

The protein concentration can be determined by immunological techniques or enzyme catalytic concentration using specific substrates. The presence of inhibitors in serum and the formation of enzyme macro forms should be considered when determining both mass concentration and catalytic concentrations of the enzyme. The commonly used determination of α-amylase activity is based on the fission of a chromogenic substrate. Older processes that used derivatives of the natural substrate (starch) were difficult to standardize and are no longer used. Current synthetic substrates are derived from maltose. The most commonly used is chromogen 4-nitrophenyl phosphate. The determination of α-amylase isoenzymes is made possible by the inhibition of one of the two isoenzymes by a specific monoclonal antibody.

In the most common arrangement, the salivary isoenzyme amylase is inhibited first by the antibody in the sample. The reaction is started by adding a substrate protected by ethylidene – glucose heptamer which is bound to one end and a chromogen is at the other end of ethylidene. Amylase from the sample hydrolyzes the oligoglucoside chain. The coloured nitrophenol is then released by the reaction of another component of the reaction mixture, glucosidase. Because α-glucosidase can only cleave end-chain glucose and does not recognize ethylidene-protected glucose, a substrate that has not been hydrolyzed by amylase is protected from the effect of glucosidase.


 * Reference values
 * S-AMS total serum amylase 0.30–1.67 μkat/l
 * U-AMS total urinary amylase <7.67 μkat/l
 * S-pAMS pancreatic amylase in serum 0.22–0.88 μkat/l
 * U-pAMS pancreatic urinary amylase <5.83 μkat/l
 * macroamylase undetectable

Interpretation of findings From a practical point of view, an increase in serum α-amylase activity is an important finding. It can be caused by:


 * increased amylase release from damaged pancreatic or salivary gland cells or
 * by reduced glomerular filtration, where this small protein is filtrated into the primitive urine to a lesser extent than usual.

Hyperamylasemia in damaged pancreas or salivary glands is accompanied by an increase in urinary amylase activity when normal renal function is present; however, it is necessary to take into account that it appears in the urine with a delay of several hours. In this case, it remains to distinguish whether amylase comes from the pancreas or salivary glands. If a decision cannot be made on the basis of the clinical picture, the determination of isoenzymes will give an answer.

Decreased glomerular filtration of amylase is most often due to renal insufficiency. In this case, hyperamylasemia will be accompanied by low urinary concentration and amylase activity. Another, much rarer cause of decreased renal amylase clearance, is macroamylasemia.

Lipase
A newer and more specific marker of pancreatic damage is lipase.

Lipase (triacylglycerolacyl hydrolase, EC 3.1.1.3) is a glycoprotein with 420,449 amino acids and a molecular weight of 46,000-56,000 for pancreatic lipase and 32,000-39,000 for serum lipase.

It is a hydrolytic enzyme that breaks down triacylglycerols with fatty acids chains longer than 12 carbons. In the presence of bile acids, it breaks down fat into monoacylglycerols and diacylglycerols. Fatty acids in the sn-1 and sn-3 positions are cleaved preferably. Like α-amylase, lipase is produced by glandular cells of the pancreas and secreted into the intestinal lumen as a part of the pancreatic juice. The concentration gradient between pancreatic tissue and serum lipase is about 20,000:1.

Determination of lipase activity involves various processes:


 * enzymatic cleavage of the natural substrate;
 * enzymatic breakdown of chromogenic and fluorogenic substrates;
 * immunological methods (ELISA, latex agglutination).

Nephelometric and turbidimetric procedures based on the cleavage of the natural substrate of triacylglycerol are used most commonly. Most lipase enzyme assay kits also contain co-lipase. The turbidimetric determination of lipase activity is based on the clarification of the oil emulsion by the action of lipolytic activity. However, this process can also be affected by other components of the serum, such as the so-called clarification factor pseudolipase. Most often those are circulating immunocomplexes type IgM. For the differential determination of serum pancreatic lipase, in addition to using pseudolipase and a standard turbidimetric procedure, a procedure based on inactivation of pseudolipase with β-mercaptoethanol was developed (it leads to dissociation of IgM complexes). Newer chromogenic assays are based on an enzymatic cascade of lipase that cleaves 1,2-diacylglycerol, glycerol kinase, glycerol-3-phosphate oxidase, and peroxidase with a chromogenic product. A completely new type of technique for the determination of pancreatic lipase is based on changing the conductivity of the solution by releasing fatty acids from the substrate - triolein; it is detected by an acoustic sensor and the measured value is the frequency response.


 * Normal values
 * up to 1 μkat/l

An increase in pancreatic lipase concentration is a more specific sign of acute pancreatitis than α-amylase. Its serum levels remain elevated for about two weeks after the acute event. It does not increase significantly in renal diseases.