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
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
. This form of amylase is also called ptyalin. It will break large, insoluble starch molecules into soluble starches (amylodextrin
) 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
. In gastric juice adjusted to pH 3.3, ptyalin was totally inactivated in 20 minutes at 370
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 - 5.6–6.9
- Human body temperature - 37 degrees Celsius
- Presence of certain anions and activators:
- Chlorine and bromine - most effective
- Iodine - less effective
- Sulfate and phosphate - least effective
Genetic variation in human ptyalin (salivary amylase)
The salivary amylase gene has undergone duplication during evolution, and DNA hybridization studies indicate that 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. Perry and coworkers reported that 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 4 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 that increased copy number of the salivary amylase gene may have enhanced survival coincident to a shift to a starchy diet during human evolution.
Pancreatic α-amylase randomly cleaves the α(1-4) glycosidic linkages
to yield dextrin
. 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.
Salivary alpha-amylase has been used as a biomarker for stress that does not require a blood draw.
Increased plasma levels in humans are found in:
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.