Retinol ester

Transport to the liver Retinol esters present in the diet are hydrolyzed in the intestinal mucosa, releasing retinol and free fatty acids (Figure 28.19). Retinol derived from esters and from the cleavage and reduction of carotenes is reesterified to long-chain fatty acids in the intestinal mucosa and secreted as a component of chylomicrons into the lymphatic system (see Figure 28.19). Retinol esters contained in chylomicrons are taken up by, and stored in, the liver. 

Vitamin A is an essential nutrient for humans and other vertebrates. Dietary sources of vitamin A are provided either by retinol esters, which are present in foods of animal origin and are hydrolyzed in the intestine to form retinol, or by plant carotenoids. More than 600 carotenoids have been identified in nature, of which 50 to 60 possess provitamin A properties and 10 have nutritional relevance (De Flora et al., 1999). 

Fredrikzon, B., Hernell, O., Blackberg, L., Olivecrona, T. 1978. Bile salt-stimulated lipase in human milk evidence of activity in vivo and of a role in the digestion of milk retinol esters. Pediatr. Res. 12, 1048-1052. 

Ito cells (T. Ito, 1951) are also known as fat-storing cells, hepatic stellate cells or lipocytes. These long-lived cells, 5-10 im in size with long thin strands, lie in Disse s space (s. figs. 2.8, 2.9) and contain numerous cytoplasmic fat droplets as well as an abundance of vitamin A (= retinol ester). The retinol esters of the chylomicrons are absorbed by the hepatocytes and hydrolyzed into retinol. The latter is either passed to the blood by means of RBP or transported to Ito cells and stored. In the fat droplets of Ito cells, about 75% of the liver retinoids are present in the form of retinol esters. These fat droplets are characteristic of Ito cells they represent vacuolized... 

Retinol esters present in the diet are hydrolyzed in the intestinal mucosa, releasing retinal and free fatty acids. 

Figure 8.3. Uptake and metabolism of retinol esters and )3-carotene.

Fig. 13.7 Isomerization of 11-trans-retinol ester to the 11-c/s-retinol catalyzed by the isomero-hydrolase.

A few minutes after cis-trans isomerization the 11-cw-retinal is recovered by an isomerase and recombined with a rhodopsin protein. The isomerization all-tram 11-cw is a dark reaction and occurs with activated retinol esters (Fig. 5.2.7). A nucleophilic group of an enzyme is reversibly added in a Michael reaction to Cll and the 5 kcal needed for the unfavorable isomerization comes from the cleavage of the activated ester. The resulting diene with the nucleophile added to the terminal allylic position is then hydrated and the nucleophile eliminated again. The trans-cis isomerization has thus been achieved in a controllable nucleophilic addition-elimination cycle, which is typical for biological reactions. The stereochemistry of the system is not disturbed. The other chemically plausible isomerization, namely via homolysis of the double bond and biradical formation, is not used in biological systems because it may lead to uncontrolled polymerization and side reactions with the protein (Rando, 1990). 

Figure 5.2.7 Enzymatic and reversible addition of a nucleophile to Cll of an activated retinol ester allows the formation of the energy-rich cis isomer in the dark (Rando, 1990).

The term Vitamin A" is here collectively used to encompass (all E)-retinol and (all E)-retinal, also their 3-dehydro-derivatives and double bond isomers, and retinol esters. 

Propionic acid, retinol ester. See Retinyl propionate... 

CAS 7069-42-3 EINECS/ELINCS 230-363-2 Synonyms Propionic acid, retinol ester Retinol, propanoate Vitamin A propionate... 

The original work on which subsequent study and synthesis of retinoids are based was carried out by Karrer and Morf (1933) and Heilbron et al, (1932, 1948). In 1931 Karrer et aL were able to determine the structure of retinol (1) using a highly purified vitamin A extract that they had obtained from shark liver oil (von Euler and Karrer, 1931). Using such retinol preparations, the first oily retinol esters [for example retinyl acetate (9)] were prepared (Karrer et al., 1931 Heilbron et al., 1932). 

M Balz, E Schulte, H-P Thier. Simultaneous determination of retinol esters and tocochromanols in foods using nitro-column HPLC. Fat Sci Technol 97 445-448, 1995. 

It is absorbed directly in the form of retinol esters from liver, butter and eggs and the esters are converted to free retinol by esterases in the body. Plant foodstuffs such as carrots and spinach contain vitamin A in the form of carotene (provitamin A) and this is converted in the body to retinol via the action of beta carotene diotggenase which produces two molecules of retinal from each molecule of beta carotene. It exists in a number of different forms (vitamers Fig. 26.1). 

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