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Mechanism of Cleavage of Rabbit IgG in Two Stages by Soluble Papain and Reducing Agent¹

From the Department of Microbiology, University of Illinois College of Medicine, Chicago, Illinois, and the Department of Microbiology, University of Texas, Austin, Texas

Abstract

The mechanism of an apparent two-stage cleavage of rabbit IgG by soluble papain and L-cysteine was investigated. In the absence of reducing agent papain had no detectable effect on the sedimentation velocity or precipitating activity of antibody. After treatment with iodoacetamide and gel filtration, in an effort to inactivate and remove papain, addition of reducing agent resulted in cleavage into univalent 3.5 S fragments. This was shown to be attributable to partial resistance of papain to alkylation, particularly in the presence of IgG, and to adherence of an appreciable quantity of the enzyme to IgG during gel filtration. The results are consistent with a mechanism in which papain acts through cleavage of the heavy chains, and in which the reducing agent activates the enzyme but does not necessarily act directly on the IgG molecule.

Footnotes

This investigation was supported by grants from the National Institute of Allergy and Infectious Diseases (AI-06281 and AI-07184), and from the National Science Foundation (GB-5424).

² United States Public Health Service International Postdoctoral Research Fellow (FO 5-TW-1101); on leave from the Department of General Pathology, University of Naples, Naples, Italy.

³ United States Public Health Service Research Career Award, K6-2947.

⁴ United States Public Health Service Career Development Award, 1 K3 GM 21 252.