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Properdin factor D. II. Activation to D by properdin.

D T Fearon, K F Austen, S Ruddy

The Journal of Experimental Medicine

|August 1, 1974

Summary

Properdin directly activates precursor factor D to factor D in the complement system. This finding reveals a key mechanism for generating factor D in the properdin pathway.

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Area of Science:

Biochemistry
Immunology
Complement System

Background:

The properdin pathway is a crucial component of the innate immune system.
Factor D is a serine protease essential for the activation of the complement cascade.
The precise mechanism of Factor D activation within the properdin pathway has remained incompletely understood.

Purpose of the Study:

To isolate and characterize the protein responsible for converting precursor Factor D to its active form.
To elucidate the direct interaction between properdin and Factor D.

Main Methods:

Sequential ion exchange and gel filtration chromatography were employed for protein purification.
Radial immunodiffusion was used to quantify properdin protein levels.
Enzymatic activity was assessed by measuring the formation of the EAC43B(D) intermediate.
Protein homogeneity was confirmed using acid and alkaline polyacrylamide disc gel electrophoresis.
Hemolytic titrations were performed to determine factor stoichiometry.

Main Results:

The protein responsible for converting precursor Factor D to active Factor D was isolated and identified as properdin.
Purified properdin demonstrated homogeneity and retained its capacity to activate Factor D.
Direct interaction between properdin and Factor D was shown to generate active Factor D.
The activation process was confirmed through stoichiometric analysis.

Conclusions:

Properdin directly activates precursor Factor D to Factor D.
This interaction represents a novel mechanism for generating active Factor D within the properdin pathway.
The findings contribute to a deeper understanding of complement system regulation and function.

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