Alkaline phosphatase, tissue-nonspecific
isozyme is an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
that in humans is encoded by the ''ALPL''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
.
Function
There are at least four distinct but related
alkaline phosphatase
The enzyme alkaline phosphatase (EC 3.1.3.1, alkaline phosphomonoesterase; phosphomonoesterase; glycerophosphatase; alkaline phosphohydrolase; alkaline phenyl phosphatase; orthophosphoric-monoester phosphohydrolase (alkaline optimum), systematic ...
s: intestinal, placental, placental-like, and liver/bone/kidney (tissue-nonspecific). The first three are located together on
chromosome 2, whereas the tissue-nonspecific form is located on
chromosome 1. The product of this gene is a membrane-bound glycosylated enzyme that is not expressed in any particular tissue and is, therefore, referred to as the tissue-nonspecific form of the enzyme. The exact physiological function of the alkaline phosphatases is not known. A proposed function of this form of the enzyme is matrix mineralization. However, mice that lack a functional form of this enzyme show normal skeletal development.
Clinical significance
This enzyme has been linked directly to a disorder known as
hypophosphatasia
Hypophosphatasia (; also called deficiency of alkaline phosphatase, phosphoethanolaminuria, or Rathbun's syndrome; sometimes abbreviated HPP) is a rare, and sometimes fatal, inherited metabolic bone disease. Clinical symptoms are heterogeneous, ...
, a disorder that is characterized by low serum ALP and undermineralised bone (osteomalacia). The character of this disorder can vary, however, depending on the specific mutation, since this determines age of onset and severity of symptoms.
The severity of symptoms ranges from premature loss of deciduous teeth with no bone abnormalities to stillbirth
depending upon which amino acid
is changed in the ALPL gene. Mutations in the ALPL gene lead to varying low activity of the enzyme tissue-nonspecific alkaline phosphatase (TNSALP) resulting in hypophosphatasia (HPP).
There are different clinical forms of HPP which can be inherited by an autosomal recessive trait or autosomal dominant trait,
the former causing more severe forms of the disease. Alkaline phosphatase allows for mineralization of calcium and phosphorus by bones and teeth.
ALPL gene mutation leads to insufficient TNSALP enzyme and allows for an accumulation of chemicals such as inorganic pyrophosphate
to indirectly cause elevated calcium levels in the body and lack of bone calcification.
The mutation E174K, where a glycine is converted to an alanine amino acid at the 571st position of its respective polypeptide chain, is a result of an ancestral mutation that occurred in Caucasians and shows a mild form of HPP.
References
Further reading
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External links
GeneReviews/NCBI/NIH/UW entry on Hypophosphatasia*
{{Esterases
Enzymes