The binding of arsenazo III to cell components.

The Ca2+ indicator, arsenazo III, binds to subcellular fractions of rabbit skeletal muscle with sufficient affinity that in living muscle containing 1--2 mM arsenazo III, the estimated free arsenazo III concentration is only 50--200 microM; 80--90% of the bound arsenazo III is associated with soluble proteins. The binding of arsenazo III to soluble proteins decreases the optical response of the dye to Ca2+; this is due to a decrease in the affinity of the protein-bound dye for Ca2+. Approximately half of the bound arsenazo III is released from the particulate fraction and soluble proteins upon addition of 5 mM Ca2+, suggesting that the Ca-arsenazo complex has lower affinity for the protein binding sites than the free dye. The Ca2+ binding to the soluble protein fraction of rabbit skeletal muscle is attributable largely to its parvalbumin content.