Summary
Although the site and manner of normal catabolism of most of the fibrinogen pool is
uncertain, certain pathways have been defined for various fibrinogen derivatives.
Several organs, including the kidneys and reticuloendothelial system (RES) have been
directly implicated as catabolic sites for various fibrinogen derivatives. The catabolic
sites are not the same for different derivatives. These differences in catabolism
are probably in part related to biochemical differences between fibrinogen and its
various derivatives. Fibrinogen itself may be catabolized in endothelial cells, although
little experimental data is available. RES uptake of intact fibrinogen does not occur,
and removal of sialic acid does not result in the rapid hepatic uptake seen with other
desialop rote ins. In contrast, a variety of studies have shown that fibrin is taken
up by the RES by at least 2 mechanisms. The first is phagocytosis of microparticulate
fibrin. The second involves a RES cell membrane binding of soluble fibrin which remains
soluble in the blood, when complexed to fibrinogen or degradation products. Fibrinogen
degradation products alone may in part also be cleared in the RES. Fragments D and
E appear to be catabolized in the kidney, although both the intrarenal site of catabolism
and the means of cellular uptake is unknown. It is clear that normally there is no
urinary excretion of D and E. Another fibrinogen derivative, low molecular weight
clottable fraction 1–8, is derived in vivo from intact fibrinogen. 1–8 is found normally
in the blood and has a shorter t ½ than fibrinogen although much longer than D and
E. While originally thought to be the result of limited plasmin degradation, 1–8 may
be the result of another type of proteolysis. The sites of both 1–8 formation and
degradation are unknown. Catabolism via fibrin, 1–8, or D and E appears to be only
a small percent of normal turnover, albeit of much greater significance in disease.
The relationship of these pathways to the as yet unknown catabolic site for the bulk
of normal fibrinogen remains to be determined.