The effect of Ca2+ on the metarhodopsin I-II transition. I. Experiments.

The effect of Ca2+ on kinetics and equilibrium of the Meta I-II transition was studied in rhodopsin-digitonin-solutions using flash-photometry. With increasing Ca2+-concentrations the Meta I-II-equilibrium is shifted to Meta I. The pH-dependence of the Meta I-II equilibrium is suppressed by Ca2+. To obtain the same effect as with bivalent cations about the 10-fold concentration of univalent ions is required. Ca2+-ions have also an effect on the rate of equilibrating Meta I-II: with increasing Ca2+-concentration the rate-constants of the rapid and slow component decrease and become equal to the value at pH8. This observation can be described as an inhibition of the catalytic effect of protons by Ca2+. Similar results are obtained with Mg2+, whereas K+ and Na+ are practically ineffective. In the presence of the Ca2+-blocking agents verapamil (Isoptin) and D-600 the rate of equilibrating Meta I-II is reduced. These and several former observations can be explained by a model in which the Meta I-II transition is coupled with the separation of negative fixed charges, which can be clamped by Ca2+.