Introduction to Human Nutrition

168 Introduction to Human Nutrition

of bacterial contamination of the lakes where the

algae were harvested.

Absorption and metabolism of vitamin B 12

Absorption

Very small amounts of vitamin B 12 can be absorbed

by passive diffusion across the intestinal mucosa, but

under normal conditions this is insignifi cant; the

major route of vitamin B 12 absorption is by attach-

ment to a specifi c binding protein in the intestinal

lumen.

This binding protein is intrinsic factor, so called

because in the early studies of pernicious anemia it

was found that two curative factors were involved: an

extrinsic or dietary factor, which is now known to be

vitamin B 12 , and an intrinsic or endogenously pro-

duced factor. Intrinsic factor is a small glycoprotein

secreted by the parietal cells of the gastric mucosa,

which also secrete hydrochloric acid.

Gastric acid and pepsin play a role in vitamin B 12

nutrition, serving to release the vitamin from protein

binding, so making it available. Atrophic gastritis is a

relatively common problem of advancing age; in the

early stages there is failure of acid secretion but more

or less normal secretion of intrinsic factor. This can

result in vitamin B 12 depletion due to failure to release

the vitamin from dietary proteins, although the

absorption of free vitamin B 12 (as in supplements or

fortifi ed foods) is unaffected. In the stomach, vitamin

B 12 binds to cobalophilin, a binding protein secreted

in the saliva.

In the duodenum cobalophilin is hydrolyzed,

releasing vitamin B 12 to bind to intrinsic factor. Pan-

creatic insuffi ciency can therefore be a factor in the

development of vitamin B 12 defi ciency, since failure to

hydrolyze cobalophilin will result in the excretion of

cobalophilin-bound vitamin B 12 rather than transfer

to intrinsic factor. Intrinsic factor binds the various

vitamin B 12 vitamers, but not other corrinoids.

N

N

N

N

Co

H 3 C

H 3 C

H 2 C C NH 2

O

H 2

C

H 2

C C NH 2

O

CH 3

CH 3

H 2 C

H 2

C C NH 2

O

CH 3

CH 3

CH 2

CH 2

C O

C

H 2

H 2 N C

O

H 3 C

H 3 C

H 2
H 2 N C C

O

H 2 C

H 2 N C CH 2

O

NH

CH 2

C

H

H 3 C O P O

O

O

O

OH

HO CH 2

N

N CH^3

CH 3

Figure 8.13 Vitamin B 12. Four coordination sites on

the central cobalt atom are occupied by nitrogen

atoms of the ring, and one by the nitrogen of

the dimethylbenzimidazole side-chain. The sixth

coordination site may be occupied by cyanide

(cyanocobalamin), a hydroxyl ion (hydroxocobala-

min), water (aquocobalamin), or a methyl group

(methylcobalamin).