Table 1.

Light-Independent Pathways for Rod Photoreceptor Activation

Receptor	Free Opsin	Opsin-ATR Noncovalent Complex	Opsin-11CR Noncovalent Complex	Meta-II (Thermally Activated)
Number/roda	∼104 to 105b	–c	∼10 to 102 s−1d	∼10−2 to 10−1 s−1e
Relative activity	10−6f	∼0.03 to 0.14g	–h	1
Predicted response to extra 11CR	decrease	decrease	transiently increasei	unchanged
Predicted response to extra ATR	decrease	increase	decrease	unchanged

^aOne rod cell contains ∼10⁸ to 10⁹ molecules of Rho, depending on the species (26).

^bCalculated from Eqs. 1 to 4.

^cThe value is dependent on the concentration of ATR in dark-adapted retina, which was estimated to be less than 5% of 11CR (39).

^dCalculated based on the k_off (2.2 × 10⁻⁷ s⁻¹ at 37°C) and the total number of Rho (∼10⁸ to 10⁹). At the equilibrium state, the rate of 11CR binding is equal to the dissociation rate. The activity of this noncovalent complex is transient and only detected by electrophysiological recording.

^eEstimated based on the discrete dark noise, using a k_off of 5.2 × 10⁻¹¹ s⁻¹ in monkey retina (5).

^fEstimated by measuring phosphodiesterase (31) and transducin (32) activation.

^gMeasured by transducin activity assay (35, 36).

^hAlthough the opsin-11CR noncovalent complex cannot be purified and characterized because of its transient nature, the electrophysiological data showed substantial signaling by the complex (37).

ⁱUpon addition of 11CR, the number of the opsin-11CR noncovalent complexes will transiently increase and then relax to the original level.