Carotene

Epsilon-carotene-(R,R)

β-Carotene represented by a 3-dimensional stick diagram

Carotene is responsible for the orange colour of carrots and the colours of many other fruits and vegetables and even some animals.

Lesser Flamingos in the Ngorongoro Crater, Tanzania. The pink colour of wild flamingos is due to Astaxanthin (a carotenoid) they absorb from their diet of Brine shrimp. If fed a carotene-free diet they become white.

Carotene (also known as carotin), derived from the Latin carota (referring to the carrot), encompasses a group of unsaturated hydrocarbon substances characterized by the formula C40Hx. These compounds, which exhibit vibrant hues perceptible to the human eye, play a crucial role in the photosynthetic processes of plants. Interestingly, while plants can synthesize these compounds, animals cannot.

Molecular Structure

Chemically, carotenes are polyunsaturated hydrocarbons made up of 40 carbon atoms, a varying number of hydrogen atoms, and no other elements. They may be terminated by hydrocarbon rings at one or both ends. All carotenes display colors to the human eye, a result of their extensive systems of conjugated double bonds. In essence, carotenes are tetraterpenes, derived biochemically from four 10-carbon terpene units, which are themselves formed from eight 5-carbon isoprene units.

These carotenoids are found in two primary forms: alpha-carotene (α-carotene) and beta-carotene (β-carotene). Other variants such as gamma-, delta-, epsilon-, and zeta-carotene (γ, δ, ε, and ζ-carotene) also exist. Notably, since they're hydrocarbons and devoid of oxygen, carotenes are fat-soluble and insoluble in water. This distinguishes them from other carotenoids like xanthophylls, which do contain oxygen.

Dietary Sources

A variety of foods are rich in carotenes. They include:

Sweet potatoes Carrots Wolfberries or goji Cantaloupe melon Mangoes Apricots Persimmon ... [shortened for brevity] Notably, the absorption of carotenes from these foods is enhanced when consumed with fats due to their fat-solubility. Cooking these foods until their cell walls break and release their color also facilitates better absorption. Each 6 μg of dietary β-carotene provides an equivalent of 1 μg of retinol, or 1 RE (Retinol Equivalent). This is tantamount to 3⅓ IU of vitamin A.

Role in Photosynthesis

Carotenes contribute indispensably to photosynthesis by transmitting the light energy they capture to chlorophyll. They also play a protective role in plant tissues, absorbing energy from singlet oxygen – a high-energy form of the O2 molecule – that emerges during photosynthesis.

Carotene and Vitamin A

β-Carotene stands out as it is made up of two retinyl groups. In the human small intestine's mucosa, it is cleaved by β-carotene 15,15'-monooxygenase to produce retinal, a form of vitamin A. Given that β-Carotene can be stored in body fat and the liver, and later be converted to retinal as required, it functions as a form of vitamin A for humans and certain mammals. Other carotenes, including α-carotene and γ-carotene, possess some vitamin A activity due to their single retinyl group (β-ionone ring). However, not all carotenoids confer vitamin A activity. For instance, lycopene doesn't have any β-ring, thus lacking vitamin A activity, though it may possess antioxidant capabilities.

Variability Among Animal Species

Animal species exhibit significant variations in their ability to convert retinyl (beta-ionone) containing carotenoids to retinals. While carnivores are generally inefficient converters of dietary ionone-containing carotenoids, pure carnivores like ferrets entirely lack the β-carotene 15,15'-monooxygenase enzyme, making them unable to convert any carotenoids to retinals. In contrast, cats can convert a minuscule amount of β-carotene to retinol, but this is woefully inadequate for their daily retinol requirements.

See also

• Vitamin A
• Chlorophyll
• Photosynthesis
• Lycopene
• Beta-carotene

Carotene Resources
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