Effect of calcium chelators on the Ca2+-dependent luminescence of aequorin

Abstract

The luminescence of aequorin, a useful tool for studying intracellular Ca2+, was recently found to be inhibited by the free EDTA and EGTA [ethylene glycol bis(.beta.-aminoethyl ether)-N,N,N'',N''-tetraacetic acid] that are present in Ca buffers. The effect of the free forms of various chelators in the calibration of [Ca2+] with aequorin was examined. Free EDTA and EGTA in low-ionic-strength solutions strongly inhibited the Ca2+-triggered luminescence of aequorin, causing large errors in the calibration of [Ca2+] (.apprx. 2 pCa units), whereas in solutions containing 150 mM-KCl errors were relatively small (0.2-0.3 pCa units). Citric acid in low-ionic-strength solutions and [(carbamoylmethyl)imino]diacetic acid in high-ionic strength solutions showed no inhibition and did not cause detectable error in the calibration of [Ca2+], indicating that they are better chelators than EDTA and EGTA for use with aequorin.