Metabolic properties of normal and mutant mannan-binding proteins in mouse plasma

Human mannan-binding protein (MBP) is a serum lectin involved in innate immunity. MBP activates the complement pathway through its interaction with mannose-rich carbohydrates on various microorganisms and a common opsonic defect has been shown to be associated with a low serum concentration of MBP. This low serum concentration is closely associated with a single base mutation in codon 52, 54 or 57 of the human MBP gene, which results in a change of Arg52 to Cys, Gly54 to Asp, or Gly57 to Gln, respectively, in the collagen-like region of the molecule and prevents the formation of higher oligomers. However, the mechanism underlying the low serum concentration in such patients is completely unknown. The levels of protein synthesis and secretion of the normal and mutant MBPs seem to be similar according to our previous in vitro results. In this study, we examined the plasma clearance of the normal and mutant human (Gly54Asp) MBPs in mice, and found that the half-life of the mutant MBP is about half that of the normal MBP, explaining in part the difference in the plasma levels between the two types of MBP.