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Reactions Between Lipoids and Antibodies

I. The Isoelectric Point and Composition of the Aggregates Obtained on Adding Beef-Heart Lipoid to Syphilitic Serum

From the Department of Bacteriology, School of Medicine, University of Pennsylvania, Philadelphia

Abstract

No analytically demonstrable protein is adsorbed from normal serum by the lipoid crystals used as antigen in the diagnostic tests for syphilis. However, their cataphoretic isoelectric point after being placed in normal serum and washed free of excess protein is found to be changed from an original value of pH 1.9 to approximately pH 3.5. This increase is probably to be ascribed to the retention of minute quantities of adsorbed protein on the surface of the particles, too small to be detected analytically by the methods used.

The isoelectric point of the particles after similar sensitization with syphilitic serum rises as high as pH 4.85, the exact value obtained depending on the reagin content of the particular serum used. Even this is probably not the maximum value obtainable, as no serum has yet been found with a sufficiently high reagin titer to make the isoelectric point of the lipoid-reagin aggregates reach a definite maximum. The striking change in cataphoretic mobility caused by syphilitic serum is probably due to the deposition of the specific reactive protein, reagin, on the surface of the lipoid particles.

The isoelectric point of reagin itself in the antigen-reagin complex is probably greater than pH 4.9, and thus falls into the same zone as the specific antibodies to bacteria, proteins and red blood cells, further evidence for the thesis that reagin, despite its apparent non-specificity, is a true antibody. The determination of its exact isoelectric point when in solution awaits its dissociation in quantity from the lipoid-reagin complex studied in the present paper.

Quantitative analysis of the lipoid-reagin aggregates for nitrogen shows protein to constitute up to 6 per cent of the compound. The theoretical value calculated on the assumption that the reagin is deposited as a unimolecular layer of closely packed spherical molecules seven µµ in diameter, is on the order of 10 per cent. It is therefore suggested as a working hypothesis that the reagin protein of syphilitic serum is deposited on the surface of the antigen particles as a unimolecular layer which does not completely cover the individual particles.

The actual concentration of reagin protein in syphilitic serum necessary to give a positive diagnostic test is estimated to be on the order of 0.3 mgm. per 100 cc. The average syphilitic serum, containing 5 to 50 such units, thus contains 1.5 to 15 mgm. per cent of the reactive factor.

It is estimated, with the reservations entailed by the necessary assumptions, that approximately one-twentieth of the surface of the lipoid particles here studied must be covered with reagin to cause visible aggregation.