Factor XIII or fibrin stabilizing factor is an enzyme (EC 188.8.131.52) of the blood coagulation system that crosslinks fibrin. When thrombin has converted fibrinogen to fibrin, the latter forms a proteinaceous network in which every E-unit is crosslinked to only one D-unit. Factor XIII is activated by thrombin into factor XIIIa; its activation into Factor XIIIa requires calcium as a cofactor.
FXIII is known also as Laki-Lorand factor, after the scientists who first proposed its existence in 1948. A 2005 conference recommended standardization of nomenclature.
Zymogene factor XIII is a 320000 Mr glycoprotein tetramer consisting of twice two subunits (2 A and 2 B), the genes for which are on different chromosomes:
A subunit (6p25-p24). The transglutaminase part; this adds an alkyl group to the nitrogen on a glutamine residue, which binds in turn with a lysine on the other chain. The molecular weight of the A chain is approximately 83000.
B subunit (1q31-q32.1). This has no clear enzymatic activity, and may serve as a carrier for the A subunit. The molecular weight of the B chain is approximately 76500.
Typical concentrations of FXIII in plasma is 10 μg/ml (2A2B heterodimer), while the concentration of free B chain is 22 μg/ml. FXIII has a long half life, ranging from 5-9 days. It is present in plasma, platelets, and monocytes, as well as macrophages and bone marrow precursors of these cell types.
A clot that has not been stabilized by FXIIIa is soluble in 5 mol/L urea, while a stabilized clot is resistant to this phenomenon.
Factor XIII levels are not measured routinely, but may be considered in patients with an unexplained bleeding tendency. As the enzyme is quite specific for monocytes and macrophages, determination of the presence of factor XIII may be used to identify and classify malignant diseases involving these cells.