The alternative pathway of complement represents an important humoral component of natural
defense against microbial attack. The interaction of the proteins C3, factor B, and factor D
results in the formation of the alternative C3- and C5-convertases, i.e. C3bBb and
C3bBbC3b(n). These multicomponent enzymes assemble on the surface of alternative
pathway of complement activators and are stabilized by properdin (P). The participation of the
alternative pathway of complement has been implicated in the pathogenesis of a wide variety
of human diseases.
Factor D, a 24 kD serine protease of the alternative complement pathway, is synthesized as a
precursor single-chain molecule. Factor D is unique among serine proteases because it
requires neither enzymatic cleavage for expression of proteolytic activity nor activation by a
serpin for its control. Factor D is highly specific and cleaves factor B bound to C3b, generating
the C3bBb enzyme. Factor D is the rate-linking C3 convertase enzyme of the alternative
pathway. Furthermore, factor D plays a role in fatty tissue distinct from its role as a complement
protein.
Normal values in human EDTA plasma are 1.05 (± 0.27) μg/ml. In healthy individuals factor D
is rapidly eliminated via the kidney and neither modified extrarenal catabolism nor changes in
synthesis contribute to elevated factor D levels observed in patients with renal failure. The
levels in patients with chronic renal failure (CRF) increase up to 4.35 (± 3.03) μg/ml and in endstage
renal disease (ERDS) up to 12.1 (± 3.53) μg/ml.
Measurable quantities of factor D were detected in urine of 85% of healthy individuals (0.62 ±
0.33 ng/ml). The urinary concentrations of factor D in patients with tubular proteinuria are
elevated to 230 (± 313) ng/ml.
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