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The Journal of Immunology, 1954, 73: 146-158.
Copyright © 1954 by The American Association of Immunologists, Inc.
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Studies on the Mechanism of Inactivation of Human Complement by Plasmin and by Antigen-Antibody Aggregates

I. THE REQUIREMENT FOR A FACTOR RESEMBLING C'1 AND THE ROLE OF CA++1,2,

Irwin H. Lepow, Leona Wurz, Oscar D. Ratnoff and Louis Pillemer

From the Institute of Pathology, Western Reserve University, and the Department of Medicine, Western Reserve University School of Medicine, and University Hospitals, Cleveland

Abstract

The inactivation of C'2 and C'4 of human complement by streptokinase-activated plasmin and by antigen-antibody aggregates requires the presence of a factor which differs from plasminogen but which has all of the known properties of C'1. The similarity between this factor and C'1 is shown by the following data:

1. Complete inactivation of the factor in normal serum heated at 56°C for 30 minutes; nearly complete inactivation at 52°C for 30 minutes.
2. Nearly complete inactivation of the factor in Ca++-depleted serum heated at 37°C for 2 hours.
3. Absence of the factor in R1; presence of the factor in R2, R3 and R4.
4. Absence or marked depletion of the factor in serum treated with streptokinase or with immune aggregates.
5. Correlation between C'1 titer and presence of the factor in heated R2 and in preliminary purification of R2.
6. Absence of a correlation between the presence of the factor and the amount of plasminogen or plasmin in the system.

Ca++ potentiates the inactivation of C'2 and C'4 of human complement by streptokinase-activated plasmin and, to a lesser extent, by antigen-antibody aggregates. Ca++ spares the amount of factor (C'1) required for the inactivation of C'2 and C'4 at a given concentration of streptokinase or of antigen-antibody. At least in part, this sparing effect is attributable to the greater stability of the factor (C'1) in the presence of Ca++.

A method is described which is useful for the study of factor requirements for the inactivation of complement by plasmin and by antigen-antibody aggregates. The method employs R1 as substrate and as an incomplete enzyme system. The role of a given factor is determined by its ability to initiate or potentiate the inactivation of C'2 and C'4 in R1 in the presence of streptokinase or of antigen-antibody aggregates.

An interrelationship between the plasmin and antigen-antibody-complement systems is proposed in which it is postulated that the inactivation of C'2 and C'4 of human complement by plasmin and by antigen-antibody aggregates is the result of conversion of an enzyme precursor (C'1) to an active enzyme.

Footnotes

1 Presented, in part, before the American Association of Immunologists, Atlantic City, New Jersey, April 12–16, 1954.

2 This investigation was supported by a grant from Lederle Laboratories Division, American Cyanamid Company, Pearl River, New York, and by research grants from the National Institutes of Health, Public Health Service.






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