The influence of chloride and other univalent inorganic anions on the visible-absorption spectrum of aspartate aminotransferase.

1. The influence of Cl(-), Br(-), NO(3) (-) and F(-) ions on the visible-absorption spectrum of deionized aspartate aminotransferase was investigated. 2. Except for F(-), these anions caused an increase of the extinction at 430mmu with a concomitant decrease of that at 362mmu. 3. The affinity constants for Cl(-) and NO(3) (-) ions were calculated by a procedure based on the assumption that the anion stabilizes the protonated form of the enzyme chromophore (lambda(max.) 430mmu). 4. The true pK of the chromophore of the enzyme was found to be 5.25.