Introduction to Human Nutrition

136 Introduction to Human Nutrition

Only a proportion of carotene undergoes oxidation
in the intestinal mucosa, and a signifi cant amount of
carotene enters the circulation in chylomicrons. Caro-
tene in the chylomicron remnants is cleared by the
liver; some is cleaved by hepatic carotene dioxygenase,
again giving rise to retinaldehyde and retinyl esters;
the remainder is secreted in very low-density lipopro-
teins (VLDLs), and may be taken up and cleaved by
carotene dioxygenase in other tissues.
Central oxidative cleavage of β-carotene, as shown
in Figure 8.2, should yield two molecules of retinal-
dehyde, which can be reduced to retinol. However, as
noted above, the biological activity of β-carotene, on

a molar basis, is considerably lower than that of

retinol, not twofold higher as might be expected. In

addition to poor absorption of carotene, three factors

may account for this.

● The intestinal activity of carotene dioxygenase is

relatively low, so that a relatively large proportion of

ingested β-carotene may be absorbed unchanged.

● Other carotenoids in the diet may inhibit carotene

dioxygenase and reduce the formation of retinol.

● The principal site of carotene dioxygenase attack is

the central bond of β-carotene, but asymmetric

cleavage also occurs, leading to the formation of 8′-,

CH 3

H 3 C CH 3 CH^3 CH^3

CH 3 CH 3

H 3 C

H 3 C CH 3

β-Carotene

CH 3

H 3 C CH 3 CH^3 CH^3

CH 3 CH 3

H 3 C

OO H^3 C CH^3

H

C

CH 3

H 3 C CH 3 CH^3 CH^3

O

2 × Retinaldehyde

COO-

CH 3

H 3 C CH 3 CH^3 CH^3

all-trans-Retinoic acid

CH 2 OH

CH 3

H 3 C CH 3 CH^3 CH^3

Retinol

O 2

Carotene dioxygenase

NAD(P)H

NAD(P)+

Retinol

dehydrogenase

Aldehyde

oxidase

H 2 O + O 2

H 2 O 2

Figure 8.2 The oxidative cleavage of carotene to yield retinol and retinoic acid. Carotene dioxygenase (EC 1.13.11.21), retinol dehydrogenase
(EC 1.1.1.105), retinaldehyde oxidase (EC 1.2.3.11).