Kinetic scheme for Ca2+-arsenazo III interactions

Temperature-jump relaxation kinetic studies show that the complex formation between Ca2+ and the metallochromic dye arsenazo III (Ar) is associated with a rapid mode (less than or equal to 10 mus-range) involving both Ca2+ and Na+ of the Na-salt of Ar and a slower mode (approximately 10 ms range) which can be attributed to structural rearrangements in the 1:2 complex CaAr2. The kinetic data suggest the scheme: Ca+2Ar = CaAr+Ar = CaAr2 = CaAr2. The relatively slow rate-limiting step sets a limit for the use of arsenazo III to study the kinetics of Ca2+ processes in cell biology.