The role of metarhodopsin in the generation of spontaneous quantum bumps in ultraviolet receptors of Limulus median eye. Evidence for reverse reactions into an active state.

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Abstract
The origin of spontaneous quantum bumps was examined in the UV photoreceptors of Limulus median eye. These cells have a rhodopsin with a .lambda.max at 360 nm and a stable photoproduct, metarhodopsin, with a .lambda.max at 470 nm. The steady-state rate of spontaneous quantum bumps was found to be higher when the metarhodopsin concentraiton was high than when the rhodopsin concentration was high. This result implicates metarhodopsin in the generation of spontaneous quantum bumps. This result is consistent with the idea that the reaction which inactivates metarhodopsin (terminates the ability of metarhodopsin to initiate the reactions leading to a quantum bump) is reversible and that such reversions can be a significant source of spontaneous quantum bumps. Given that the rate of spontaneous quantum bumps is .apprx. 1/s under conditions where the number of inactive metarhodopsin molecules is .apprx. 109, it follows that the molecular switch that inactivates metarhodopsin reverses with a probability of < 10-9. A model is presented of how a molecular switch with this reliability might be constructed.