α-Amylase is the major form of amylase found in humans and other mammals.
Amylase in human physiology
Although found in many tissues, amylase is most prominent in pancreatic juice and saliva which each have their own isoform of human α-amylase. They behave differently on isoelectric focusing, and can also be separated in testing by using specific monoclonal antibodies. In humans, all amylase isoforms link to chromosome 1p21 (see AMY1A).
Salivary amylase (ptyalin)
- Amylase is found in saliva and breaks starch down into maltose and dextrin. This form of amylase is also called ptyalin. It will break large, insoluble starch molecules into soluble starches (amylodextrin, erythrodextrin, achrodextrin) producing successively smaller starches and ultimately maltose. Ptyalin acts on linear α(1,4) glycosidic linkages, but compound hydrolysis requires an enzyme which acts on branched products. Salivary amylase is inactivated in the stomach by gastric acid.
Optimum conditions for ptyalin
- Optimum pH - 5.6–6.9
- Human body temperature - 37 degrees Celsius
- Presence of certain anions and activators:
- Pancreatic α-amylase randomly cleaves the α(1-4) glycosidic linkages of amylose to yield dextrin, maltose or maltotriose. It adopts a double displacement mechanism with retention of anomeric configuration.
Amylase in human pathology
The test for amylase is easier to perform than that for lipase, making it the primary test used to detect and monitor pancreatitis. Labs will usually measure either pancreatic amylase, or total amylase. If only pancreatic amylase is measured, an increase will not be noted with mumps or other salivary gland trauma.
Unfortunately, because of the small amount present, timing is critical when sampling blood for this measurement. Blood should preferably be taken soon after a bout of pancreatitis pain, otherwise it is excreted rapidly by the kidneys.
Increased plasma levels in humans are found in:
- Salivary trauma (including anaesthetic intubation).
- Mumps — due to inflammation of the salivary glands.
- Pancreatitis — because of damage to the cells that produce amylase.
- Renal failure — due to reduced excretion.
Total amylase readings of over 10X the upper limit of normal (ULN) are suggestive of pancreatitis. 5-10x times the ULN may indicate ileus or duodenal disease or renal failure, and lower elevations are commonly found in salivary gland disease.