We are very familiar with amylase, no matter in biochemistry, biochemical inspection, or internal medicine, we cannot avoid amylase. Amylase testing is also one of the most commonly used testing items in clinical laboratories.
The discovery of amylase can be traced back to the first development stage of biochemistry. In 1833, Payen, who worked in a sugar factory, separated a soluble substance from malt that could convert starch into sugar and called it amylase. In 1878, Cunai first proposed the concept of “enzyme” based on previous chemical research. Amylase became the first enzyme discovered in the history of enzymology.
What is amylase
Amylase is an exocrine hydrolase that is mainly derived from pancreatic synthesis, mainly in the digestive tract, and can hydrolyze α-1,4 glycosidic bonds. Of course, there is not only amylase that can hydrolyze glycosidic bonds in the body, but also phosphorylase that hydrolyzes α-1,4 glycosidic bonds in the process of glycogen decomposition, which has the bidirectional function of forming and decomposing α-1,4 glycosidic bonds. Branch chain transferase, α-1,6 glycosidase that hydrolyzes α-1,6 glycosidic bonds; amylase that enters the blood rarely exerts catalytic activity.
The source of amylase
There are two main isoenzymes of amylase in the body, one is pancreatic amylase: mainly synthesized by pancreas and testicular cells, the pancreas is the main organ for synthesizing amylase; the other is salivary amylase: mainly from the salivary glands and also found in lungs, ovaries and other tissues.
What can the source distribution of amylase show? (1) The occurrence of diseases in these tissues will cause the increase of serum amylase; (2) The pancreas is the main organ for the synthesis of amylase, so the diseases of the tissues and organs around the pancreas can also affect the pancreas and cause the increase of serum amylase.
The nature of amylase
In enzymology, amylase is a metalloenzyme, or calcium ion metalloenase to be precise. This property of amylase determines: (1) In pathophysiology, calcium ion levels are inextricably linked with amylase and acute pancreatitis; (2) In clinical tests, amylase test samples meet following requirements, that is, the usual sodium citrate, EDTA, and even heparin (although the main anticoagulation mechanism of heparin is not to chelate calcium ions, heparin also has calcium binding sites) and other anticoagulant specimens that bind calcium ions. It is not advisable to detect amylase. In terms of molecular weight, the molecular weight of amylase is about 45kd. The molecular weight of 45kd indicates that amylase is a medium-molecular-weight protein, so amylase can be partially filtered out through the glomerular filtration membrane and appears in the urine. It also indicates that the kidney’s filtration capacity will affect the level of urine amylase and reduce the glomerulus diseases of filtration capacity may cause a slight increase in blood amylase.
Blood amylase and pancreatitis
Amylase is used for the auxiliary diagnosis and differential diagnosis of clinical pancreatitis. To be precise, it is used for the clinical diagnosis of acute pancreatitis and the clinical diagnosis of acute episodes of chronic pancreatitis, especially edematous acute pancreatitis. It has no diagnostic effect on general chronic pancreatitis. Necrotizing pancreatitis can have normal or decreased amylase levels.
Urine amylase and pancreatitis
In acute pancreatitis, urine amylase rises late, starting to rise after 12-14 hours; but it lasts for a long time, lasting 7-14 days. It can be seen that compared with blood amylase, not only amylase can be present in urine, but the duration of amylase in urine is longer, the detection window is longer, and the reference interval is wide.
(1) The molecular weight of amylase is small
As mentioned above, the molecular weight of amylase is only about 45kd, indicating that amylase can be partially filtered out through the glomerular filtration membrane.
(2) Renal tubules partially reabsorb amylase
Whether amylase appears in the final urine also depends on the kidney’s reabsorption and secretion mechanisms. Do these mechanisms have an effect on urine amylase? The renal tubules have partial reabsorption capacity for amylase.
(3) The priority filtering mechanism of the kidney
Mere filtration and partial reabsorption are not enough to explain the long duration of urine amylase and the long detection window. The kidney also has a preferential filtering mechanism for blood amylase, that is, the kidney can preferentially filter amylase, and the higher the blood amylase, the stronger the kidney’s filtering ability. The preferential filtration mechanism allows urine amylase to show higher levels in a relatively regular period of time after the blood amylase begins to decrease.
(4) The urine concentration and dilution mechanism of the kidney
The body interacts with the kidneys through the action of antidiuretic hormone, and uses the different transmembrane transport mechanisms of NaCl and urea in different sections of the renal tubular collecting duct to realize the concentration and dilution of urine. The urine concentration and dilution mechanism undoubtedly caused the urine amylase to present a wider reference interval in the normal population, and thus did not have better diagnostic sensitivity.