Α-Amylase

Endohydrolysis of (1→4)-α-D-glucosidic linkages in polysaccharides containing three or more (1→4)-α-linked D-glucose units

It is the major form of amylase found in humans and other mammals. It is also present in seeds containing starch as a food reserve, and is secreted by many fungi. It is a member of glycoside hydrolase family 13.

In human biology

Although found in many tissues, amylase is most prominent in pancreatic juice and saliva, each of which has its 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 into maltose and dextrin. This form of amylase is also called "ptyalin" , which was named by chemist Jöns Jacob Berzelius. The name derives from the Greek word πτυω (I spit), because the substance was obtained from saliva. It will break large, insoluble starch molecules into soluble starches (amylodextrin, erythrodextrin, and achrodextrin) producing successively smaller starches and ultimately maltose. Ptyalin acts on linear α(1,4) glycosidic linkages, but compound hydrolysis requires an enzyme that acts on branched products. Salivary amylase is inactivated in the stomach by gastric acid. In gastric juice adjusted to pH 3.3, ptyalin was totally inactivated in 20 minutes at 37 °C. In contrast, 50% of amylase activity remained after 150 minutes of exposure to gastric juice at pH 4.3. Both starch, the substrate for ptyalin, and the product (short chains of glucose) are able to partially protect it against inactivation by gastric acid. Ptyalin added to buffer at pH 3.0 underwent complete inactivation in 120 minutes; however, addition of starch at a 0.1% level resulted in 10% of the activity remaining, and similar addition of starch to a 1.0% level resulted in about 40% of the activity remaining at 120 minutes.

Optimum conditions for ptyalin

Optimum pH – 7.0; 5.6-6.9

Human body temperature - 37 degrees Celsius

Presence of certain anions and activators:

Chloride and bromide – most effective

Iodide – less effective

Sulfate and phosphate – least effective

Genetic variation in human salivary amylase

The salivary amylase gene has undergone duplication during evolution, and DNA hybridization studies indicate many individuals have multiple tandem repeats of the gene. The number of gene copies correlates with the levels of salivary amylase, as measured by protein blot assays using antibodies to human amylase. Gene copy number is associated with apparent evolutionary exposure to high-starch diets. For example, a Japanese individual had 14 copies of the amylase gene (one allele with 10 copies, and a second allele with four copies). The Japanese diet has traditionally contained large amounts of rice starch. In contrast, a Biaka individual carried six copies (three copies on each allele). The Biaka are rainforest hunter-gatherers who have traditionally consumed a low-starch diet. Perry and colleagues speculated the increased copy number of the salivary amylase gene may have enhanced survival coincident to a shift to a starchy diet during human evolution.

Pancreatic amylase

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. Two very similar forms of the enzyme are produced in humans, AMY2A and AMY2B, which are found in roughly equal quantities in pancreatic juice (produced by acinar cells). In humans, the salivary amylase is thought to have evolved from a copy of one of the pancreatic amylase genes.

In pathology

The test for amylase is easier to perform than that for lipase, making it the primary test used to detect and monitor pancreatitis. Medical laboratories 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.

However, because of the small amount present, timing is critical when sampling blood for this measurement. Blood should be taken soon after a bout of pancreatitis pain, otherwise it is excreted rapidly by the kidneys.

Salivary α-amylase has been used as a biomarker for stress and as a surrogate marker of sympathetic nervous system (SNS) activity that does not require a blood draw.

Interpretation

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
• Kidney failure – due to reduced excretion

Total amylase readings of over 10 times the upper limit of normal (ULN) are suggestive of pancreatitis. Five to 10 times the ULN may indicate ileus or duodenal disease or kidney failure, and lower elevations are commonly found in salivary gland disease.

Genes

• salivary – AMY1A, AMY1B, AMY1C
• pancreatic – AMY2A, AMY2B