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There are two pathways all-trans-retinal can take after detaching from the scotopsin: (1) it can convert back to 11-cis-retinal, or (2) it can convert to all-trans-retinol (form of vitamin A), which then converts to 11-cis-retinol and then 11-cis-retinal. My question is what factors determine which pathway will occur?

The first sounds like it doesn’t involve vitamin A at all and just recycles the retinal. Hence it wouldn’t be affected by any vitamin A deficiency, yet we know vitamin A deficiency affects the stock of retinal in the eye. Yet I don’t see any disadvantages in simply recycling our retinal; why would we go through the process of converting it to vitamin A and then back to 11-cis-retinal?

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Short answer
A complex visual cycle in vertebrates involving vitamin A is useful for storage, re-distribution of retinoids to both rods and cones, to improve photosensitivity and reduce noise in the visual system.

Background
The visual cycle in mammalian rods is depicted in Fig. 1.

Visual Cycl
Fig. 1. Visual rod cycle. Source: Palczewski (2010)

11-cis-retinal isomerizes to all-trans retinal by absorption of a photon. Your notion that this pathway can involve direct re-isomerization back to 11-cis-retinal is only true for invertebrates. They simply absorb another photon to back-isomerize all-trans to 11-cis (Saari, 2012).

In vertebrates, however, this bi-stable configuration is replaced by a complex visual cycle (or retinoid cycle, Fig. 1). The reason is that the bi-stable invertebrate system is less sensitive, as photons used for regeneration cannot be used to initiate phototransduction. Moreover, the invertebrate system is more prone to noise, as photons can both start the phototransduction or recycle all-trans retinal (Saari, 2012).

In vertebrates it is thought that the visual cycle only emerged after appearance of the cone system (Saari, 2012). Notably, the visual cycle depends on the retinal pigment epithelum (RPE), as shown if Fig.2.

visual cycle 2
Fig. 2. Rod and cone visual cycle. Source: Saari (2012)

Fig. 2 shows that vitamin A (all-trans-retinol) is present in the RPE. RPE cells are able to store vitamin A by converting it to fatty acids and store it in oil droplets. Moreover, RPE cells can funnel this stored vitamin A to both rods and cones, another reason why the complex visual cycle and vitamin A conversion makes sense (Palczewski, 2010)

References
- Palczewski Trends Pharmacol Sci (2010); 31(6): 284-95
- Saari, Annu Rev Nutr (2012); 32: 125–45

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  • $\begingroup$ It's entirely wrong statement about sensitivity. Invertebrate photoreceptors are extremely sensitive compare to vertebrates and they can respond to ideally single photon (Randall et al 2015, Hardie 2001 ) $\endgroup$ – Dexter Nov 20 '15 at 12:12
  • $\begingroup$ @Dexter - one photon is necessary to regenerate invertebrate photopigment after absorption of the first photon. Efficiency=sensitivity hence reduced by 50%. Vertebrate rods can too respond to a single photon. $\endgroup$ – AliceD Nov 20 '15 at 12:27
  • $\begingroup$ You might want to look at this. I remember one review comparing sensitivity of both. I am looking into it. And I don't think "efficiency=sensitivity" is true. $\endgroup$ – Dexter Nov 20 '15 at 12:37
  • $\begingroup$ @Dexter the responsiveness of photoreceptors also depends on their stack number, height, concentration of photopigment and what not. The simple fact is that when you use x to convert A to B and x to recycle B to A, it's noisy and inefficient. And using two photons to detect one is per definition less sensitive than detecting one and independently recycling the pigment. $\endgroup$ – AliceD Nov 20 '15 at 12:40
  • $\begingroup$ I think it's better to provide reference for claim than fight over different points. Another logical points I can raise is that Calcium channels are being closed in vertebrates in contrast to invertebrates. And opening of channel (which also has strong calcium feedback) is much faster than closing channels (which also has calcium feedback but on 100x slower time scale). $\endgroup$ – Dexter Nov 20 '15 at 12:43

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