Comparison of ethyleneglycoltetraacetic acid and its magnesium salt as reagents for studying alternative complement pathway function.

The divalent cation chelators, ethyleneglycoltetraacetic acid (EGTA) and its magnesium salt, MgEGTA, were compared in studies of alternative complement pathway function. EGTA (0.01 M) inhibited both the rate and the amount of complement activation by zymosan whether compared to nonchelated serum or to serum chelated with MgEGTA (0.01 M). The rate of alternative pathway activation by zymosan was slightly slower in MgEGTA-chelated serum than in nonchelated serum, but the overall amount of complement consumed by a given amount of zymosan was not decreased. MgEGTA chelation spontaneously activated the alternative pathway, as reflected by lysis of erythrocytes from a patient with paroxysmal nocturnal hemoglobinuria. No evidence could be found that MgEGTA either spontaneously activated C2 or facilitated zymosan activation of C2. Suggested guidelines for the use of these chelators are advanced.