Kallikrein

Kallikreins are a subgroup of serine proteases, enzymes capable of cleaving peptide bonds in proteins. In humans, plasma kallikrein (KLKB1) has no known homologue, while tissue kallikrein-related peptidases (KLKs) encode a family of fifteen closely related serine proteases. These genes are localised to chromosome 19q13, forming the largest contiguous cluster of proteases within the human genome. Kallikreins are responsible for the co-ordination of various physiological functions including blood pressure, semen liquefaction and skin desquamation.

Occurrence
E. Werle reported in 1934 finding a substance in the pancreas of men and various animals in such great amounts that the pancreas could be taken for its site of origin. He named it kallikrein, by derivation from the Greek word for pancreas. Since then similar enzymes have been found in the biological fluids of humans and other mammals, as well as in some snake venoms.

Plasma kallikrein
The KLKB1 gene encoding plasma kallikrein is located on chromosome 4q34-35. It is synthesised as an inactive precursor, prekallikrein, which must undergo proteolytic processing to become activated. This is facilitated by factor XII, PRCP or other stimuli.

Plasma kallikrein liberates kinins (bradykinin and kallidin) from the kininogens, peptides responsible for the regulation of blood pressure and activation of inflammation. It is also capable of generating plasmin from plasminogen:



Tissue kallikreins
Distinct from plasma kallikrein, tissue kallikreins (KLKs) are expressed throughout the human body and perform various physiological roles. As some kallikreins are able to catalyse the activation of other kallikreins, several cascades involving these proteases have been implicated in the regulation of homeostatic functions.

Function
Similar to KLKB1, three tissue kallikreins KLK1, KLK2 and KLK12 also participate in regulation of blood pressure via the activation of bradykinin. KLK2, KLK3, KLK4, KLK5 and KLK14 are expressed in the prostate and are thought to be responsible for regulating semen liquefaction through hydrolysis of seminogelin. Desquamation of the skin is likely controlled by KLK5, KLK7 and KLK14, which are expressed in the outermost layer of the epidermis and cleave cellular adhesion proteins. Additionally, KLK6 and KLK8 are associated with neuronal plasticity in the central nervous system.

Genes
There are 15 known tissue kallikreins: KLK1, KLK2, KLK3, KLK4, KLK5, KLK6, KLK7, KLK8, KLK9, KLK10, KLK11, KLK12, KLK13, KLK14, KLK15

Clinical significance
Kallikrein-related peptidases are targets of active investigation by drug researchers as possible biomarkers for cancer.

Prostate-specific antigen (PSA; hk3, human kallikrein gene 3) and human glandular kallikrein (hK2) are used as tumor markers for prostate cancer.