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Quantitative Study of the Combination of Lima Bean Lectin with Human Erythrocytes¹

From the Boston University School of Medicine, Boston, Massachusetts

Abstract

The combination of lima bean anti-A lectin with human erythrocytes was studied quantitatively by use of lectin labeled by coupling with diazotized p-iodoaniline containing I¹³¹. The anti-A consistently took up two to four times as much I¹³¹ as did the nonspecific proteins present. It was found that the labeled lima bean proteins combined with erythrocytes in greater and greater amounts as the ratio of lectin to cells was increased. The relation between amount of lectin per cell and ratio of lectin to cells added was found to be substantially linear if both were plotted logarithmically, so that the amount of lectin per cell did not reach a maximum in the range studied. In addition to the anti-A which combined with group A red cells and agglutinated them, there was present in the lectin preparations large amounts of a protein that combined with all human erythrocytes without agglutinating them. Homozygous A₁A₁ cells seemed to combine with more anti-A than did heterozygous A₁O cells. A₂ cells may combine with somewhat less, but on the whole the results suggest that the main difference between A₁ and A₂ is qualitative rather than quantitative. A₃ cells combined with much less anti-A lectin than did A₁ or A₂ cells.

That the extra radioactivity taken up by group A cells was due to the combination of anti-A lectin with them was shown by the fact that N-acetyl-D-galactosamine would inhibit this combination while leaving the amount of nonspecific protein combined unchanged, and also by the fact that after the labeled lectin had reacted with A substance it no longer added more radioactivity to A cells than to O or B cells.

If it is assumed that each molecule of the anti-A lectin that combines with a group A erythrocyte is combined with an A combining site, we can calculate, making plausible assumptions as to the molecular weight of the lectin, how many such A-specific sites there are in a group A cell. The results are astoundingly high, as much as 8 or 10 million. Nevertheless, it is thought that these figures may not be in error, though possibly they may refer to the A-combining sites in the entire stroma rather than merely to those available on the surface.

Footnotes

¹ This work was supported by Research Grants H-1076(C8), RG-4704(C4), and RG-6939(C1) from the National Institutes of Health and Research Grant G-15082 from the National Science Foundation. One of us (HMB) was a National Institutes of Health Foreign Post-doctoral Fellow in 1961.

² Present address: Blood Group Reference Centre, Human Variation Unit, Indian Cancer Research Centre, Bombay 12, India.