Positively charged amino groups on the surface of normal and cancer cells

Abstract

On the surface of Ehrlich ascites tumour (EAT) cells, human blood platelets, and lymphocytes, various ionizable groups of high pK (amino and sulphydryl) are present but their proper characterization had proved extremely difficult without resorting to severe chemical treatment. The present work describes the development of a suitably mild chemical treatment of intact live cells using citraconic anhydride (CA) and 2,3-dimethylmaleic anhydride (DMA) at pH 6 · 4 to 6 · 9 at room temperature (ca. 18°C) to block the cell surface amino groups reversibly. From electrokinetic data on chemically-modified cells, it was possible to calculate the numbers of positively charged amino groups on the cell surface without undue complications from the presence of the cell surface sulphydryl groups. The reaction of the sulphydryl groups with the reagents under the experimental conditions is irreversible. When the positively charged amino groups are blocked, the anodic mobility of the treated cells increases by about 20% because of the elimination of the positive charges and also because the adduct introduced into the cell periphery, carrying an ionizable carboxyl group, becomes operative in the electrophoretic plane of shear. If the treated cells are washed with saline of pH 5 or 6, the adduct is split off and the mobility of the cells returns to the control value thus showing that the blocking of the amino groups is reversible and that the calculations are not complicated by any involvement of the cell surface sulphydryl groups. Treatment of the tumour cells with N-ethylmaleimide, which is not specific for thiols, increases the mobility nearly to the same extent as with the two anhydrides, CA and DMA, suggesting that perhaps all the three reagents (CA, DMA and NEM) block the same sites. From the electrokinetic data the number of positively charged amino groups and the actual number of electron charges on the surface of the three cell types (Ehrlich ascites tumour cells, human blood platelets and lymphocytes) have been calculated and discussed in relation to their significance for the adhesion and aggregation phenomena.