Alkaline phosphatase (EC 3.1.3.1) (ALP) [1] is a zinc and magnesium-containing metalloenzyme which hydrolyzes phosphate esters, optimally at high pH. It is found in nearly all living organisms, with the exception of some plants. In Escherichia coli, ALP (gene phoA) is found in the periplasmic space. In yeast it (gene PHO8) is found in lysosome-like vacuoles and in mammals, it is a glycoprotein attached to the membrane by a GPI-anchor.

In mammals, four different isozymes are currently known [2]. Three of them are tissue-specific: the placental, placental-like (germ cell) and intestinal isozymes. The fourth form is tissue non-specific and was previously known as the liver/bone/kidney isozyme.

Streptomyces' species involved in the synthesis of streptomycin (SM), an antibiotic, express a phosphatase (EC 3.1.3.39) (gene strK) which is highly related to ALP. It specifically cleaves both streptomycin-6-phosphate and, more slowly, streptomycin-3"-phosphate.

A serine is involved in the catalytic activity of ALP. The region around the active site serine is relatively well conserved and can be used as a signature pattern.