Biology of Disease

prevent microorganisms from entering the body (Chapter 4). Retinaldehyde is
necessary for vision and functions as the prosthetic group (visual pigments) of
opsin proteins in light-sensitive retinal cells. Vitamin A is a weak antioxidant
that can protect against free radical damage. There is some evidence that A
carotene reduces the incidence of cardiovascular disease and some forms of
cancer.

The usual dietary form of vitamin D is cholecalciferol (Figure 10.18), although
this is not strictly a vitamin since it can be formed by ultraviolet irradiation
of the skin from 7-dehydrocholesterol. Foods are sometimes fortified with
the synthetic ergocalciferol, which has the same biological activity as
cholecalciferol. Enzyme-catalyzed hydroxylations yield the active metabolites
1 @, 25 dihydroxycholecalciferol (Chapter 8) and calcitriol respectively. Vitamin
D mainly functions in the homeostasis of calcium as described in Chapter 8.

Vitamin E is generally used to describe the tocopherols and tocotrienols that
comprise a number of vitamers of differing biological potencies, of which
the most active is @ tocopherol (Figure 10.19 (A)and(B)). Like vitamins A
and D, vitamin E also has a role in regulating gene expression, although a
receptor has yet to be found, and also in signal transduction. A major function
generally ascribed to vitamin E is to protect cellular membranes against
free radicals (described in Chapters 12and 18 ) and prevent the oxidation of
plasma lipoproteins, especially low density lipoproteins (Chapter 14). The
corresponding reduction of the vitamin produces a relatively unreactive and
therefore less damaging tocopheroxyl radical. This is also relatively long lived
and so persists sufficiently long to be reoxidized back to the active form by
vitamin C or glutathione peroxidases. However, the utility of this mechanism
has been challenged and it has been suggested that the antioxidant role is more
restricted and arises from an inhibition of NADPH oxidase and so reduces the
production of radicals, such as superoxide.

DIET AND NUTRITION

CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY ').

Figure 10.18 The structure of vitamin D

(cholecalciferol).

CH 3

CH 3

CH 3

H 3 C

H 3 C

H 3 C

H 3 C

CH 3

HO O

A)

Figure 10.19 (A) The structure and

(B) a computer generated molecular

model of vitamin E (@ tocopherol).

Oxygen atoms are shown in red,

carbon in black and hydrogen in gray.

CH

HC

CH 2

HO

CH 3

CH 3

H 3 C CH^3