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chapter 37

Mineral Metabolism

FIGURE 37-1

Conversion of provitamin D (ergosterol or 7-dehydrocholesterol) to vitamin D in the skin and its transport in the blood

compiexed with vitamin D-binding protein (also known as transcalciferin, Gc-protein, and group-specific component).

Increased oxalate absorption can be reduced by calcium

adm inistered w ith m eals as a w ater-soluble salt.

Vitamin D Metabolism and Function

A lthough rickets w as first described in the m id -1600s,

it w as not until the 1920s that deficiency o f vitam in D

w as recognized as its cause. D espite its designation as

a vitam in, dietary vitam in D is needed only if a per-

son receives inadequate exposure to sunlight. Norm ally,

vitam in D

3

is synthesized in the skin by irradiation o f

7-dehydrocholesterol (Figure 37-1). Sufficient exposure

to ultraviolet radiation can cure rickets.

The principal com pound form ed in the skin is

chole-

calciferol

(vitam in D 3). The critical step requiring irra-

diation is the breaking o f the 9 ,1 0 bond in the sterol B

ring to form a secosterol. A secosterol occurs w hen one

o f the rings o f the steroid skeleton cyclopentanoperhy-

drophenanthrene has undergone carbon-carbon breakage.

R ing opening is accom plished by light o f w avelength from

290 to 320 nm (ultraviolet-B radiation) with a m axim al

effect at 297 nm. 7-D ehydrocholesterol is present at high

concentration in the stratum spinosum and stratum basale

o f the epiderm is. Thermal isom erization o f previtamin D

3

to vitam in D

3

occurs slow ly, over 2 -3 days. The vitam in

D

3

form ed diffuses gradually through the basal layers o f

the skin into the circulation. E xcessive exposure to solar ra-

diation causes conversion o f previtam in D

3

to tachysterol

3

and Iumisterol3, w hich are b iologically inactive. M elanin

can reduce form ation o f previtam in D

3

by absorbing part

o f the solar radiation. This effect m ay partly explain the

greater susceptibility o f dark-skinned children to rickets.

T he concentration o f 7-dehydrocholesterol in skin is re-

ported to decrease with increasing age; if so, inadequate

synthesis o f vitam in D

3

m ay contribute to senile osteo-

porosis. The R D A for vitam in D is 4 0 0 IU.

Ergocalciferol

(vitam in D 2) is form ed by irradiation

o f ergosterol, a plant sterol com m on in the diet. It dif-

fers from ch olecalciferol in the sid e chain attached to the

D ring (Figure 37-1). Irradiation o f ergosterol is an im -

portant com m ercial m ethod for the synthesis o f vitam in

D 2, w hich is used for enriching c o w ’s m ilk. The practice