Retinol Vitamin A Family

Early work on this vitamin was confusing because similar outcomes were seen with ingestion of "yellow" vegetables and colorless fish liver oils. It finally was shown that carotene (the yellow pigment) extracts from vegetables were converted to colorless retinal. Because the retinoids are discussed in considerably more detail elsewhere, this chapter presents only a cursory overview of their biochemical functions. .

3.1.1 Chemistry. The commercial form cf vitamin A is all-trans retinol, usually formulated as the acetate or palmitate ester. The active forms are the two oxidation products (Fig. 8.1, (1) retinal, which is a structural component of the visual pigment rhodopsin, and (2) retinoic acid, which is required for cell differentiation. There are specific nuclear receptors for retinoic acid. Although the vitamin is marketed in the all-trans form, retinal and retinoic acid are present, in vivo, in cis forms. There are also commercial forms related to the retinoic acid structure that have cis stereochemistry.

Carotenes are promoted commercially for their antioxidant activity rather than as a source of the retinol group. Nevertheless, the carotenes are the source of the vitamin in yellow vegetables. There are many carotenes, of which three are shown in Fig. 8.2. Only j3-car-otene is symmetrical (note the dashed line)

Vitamin Family Dirivitive

Figure 8.1. Retinol chemistry.

Retinoic acid (cell differentiation)

Figure 8.1. Retinol chemistry.

and theoretically will produce two equivalents of retinal after the enzyme-catalyzed oxidative cleavage. In reality, more recent studies indite that vitamin A activity is six times the itamin A activity derived from /3-carotene. .e problem is one of capacity in theintestinal tucosa cell to cleave the carotenes. Further, iere appears to be regulation of the cleavage of j3-carotene. As the stores in the liver reach capacity, there is less conversion of /3-carotene being oxidized to retinal. This is one of the reasons that j3-carotene nutritional supplements enter the body intact. Further, the bioavailability of j3-carotene is significantly lower than that of retinol (10).

Both retinol esters, whether from animal tissues or a vitamin supplement, and j3-car-otene must be incorporated into mixed micelles along with other lipid material. The retinol esters are hydrolyzed by intestinal esterases. Both retinol and j3-carotene are then absorbed into the mucosa cell, where is oxidatively cleaved to retinal and then reduced to retinol. The retinol, independent of the source, follows the same steps seen with 2-monoglyceride from triglyceride digestion. The retinol is reesteri-fied, usually with palmitic acid, and attached to the chylomicrons along with the other dietary lipids. The chylomicrons first enter the lymph and then move to the circulatory system. The triglycerides are removed from the and deposited in the adipose and skeletal muscle cells, leaving chylomicron remnants, which are transported to the liver where the esterified vitamin is stored (11). Transportation from the liver to tissues where required is done on specific reti-nol-binding proteins. Humans consuming a balanced diet store several months of retinol esters in their livers.

HqC,

HqC,

Transported Chylomicrons Vitamin

Figure 8.2. Carotene chemistry.

^Carotene

Figure 8.2. Carotene chemistry.

oxidation

retinal mucosa cell \ reductase

Retinol esters

(food or vitamin supplements)

retinal & retinoic acid transported on Retinal Binding Protein (RBP)

(2) oxidation storage intestinal esterases reesterify

retino1 mucosa cell ' retino1 P31™^

TGs chylomicron remnent chylomicron

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

Continue reading here: Biochemical Functions and Deficiency

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