All-trans retinal

All-trans retinal

All-trans retinal (also known as all-trans retinene, or simply retinal) is a form of vitamin A that plays a crucial role in the visual cycle. It is a light-absorbing substance that is essential for the conversion of light into nerve signals in the retina of the eye.

Structure and Properties

All-trans retinal is a polyene chromophore, and is the light-absorbing part of the visual pigment. It is a yellow, oil-soluble substance with a cyclic end group. The molecule is composed of a long hydrocarbon chain with alternating single and double bonds, which allows it to absorb light in the visible spectrum.

Role in Vision

In the photoreceptor cells of the retina, all-trans retinal is combined with the protein opsin to form rhodopsin, the visual pigment used in low-light conditions. When light strikes rhodopsin, the all-trans retinal is isomerized to 11-cis retinal, which triggers a series of chemical reactions that create an electrical signal. This signal is then transmitted to the brain, where it is interpreted as vision.

Metabolism

All-trans retinal is derived from vitamin A, which is obtained from the diet. The body converts dietary vitamin A into all-trans retinal, which is then stored in the liver until it is needed. When light enters the eye, the all-trans retinal is released from its binding protein and isomerized to 11-cis retinal. After it has been used in vision, the 11-cis retinal is converted back into all-trans retinal, and the cycle begins again.

Health Implications

Deficiency in all-trans retinal can lead to night blindness, as the body is unable to produce enough rhodopsin for low-light vision. In severe cases, it can also lead to xerophthalmia, a condition that can cause blindness. Conversely, an excess of all-trans retinal can be toxic and can lead to hypervitaminosis A, a condition characterized by hair loss, skin problems, and liver damage.

See Also

• Vitamin A
• Retinol
• Retinoic acid
• Visual cycle
• Rhodopsin