Role of the intrinsic transglutaminase in the Ca2+-mediated crosslinking of erythrocyte proteins.

Transamidase (i.e., "transglutaminase") activity of human erythrocytes, lysed by a single freezing and thawing to 37 degrees, was measured by a method of incorporating [14C]putrescine into N,N'-dimethylcasein. In the absence of added calcium ions, virtually no enzyme activity could be detected. An increase in concentration of the cation to about 0.5 mM, however, turned on the enzyme to appreciable levels of activity. Simultaneously, Ca2+ produced formation of high molecular weight, nondisulfied bonded protein polymers either directly in the lysate or in fresh cells when the cation was added together with the A23187 ionophore. The polymers could be readily identified in the isolated cell ghosts by means of disc gel electrophoresis. If the Ca2+-promoted formation of polymers was allowed to take place in the presence of 14C-putrescine, then this tracer became incorporated into the polymeric material. The incorporation indicated that polymerization occurred through gamma-glutamyl-epsilon-lysine bridtes. It is suggested that the intrinsic transamidase mediates protein crosslinking of the erythrocyte membrane whenever there is an increase in intracellular Ca2+ concentration. The presence of suitable transglutaminase substrates, e.g. histamine, inhibited crosslinking when the cells were incubated with Ca2+ and ionophore.