22.05.2022 Views

DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

22

1281

23 25

1

19

HO

3 7

provitamin D 3

(7-dehydrocholesterol)

HO

provitamin D 2

(ergosterol)

UV light

CHAPTER 44

HO

HO

19

CH 2

CH 2

vitamin D 3

(cholecalciferol)

25 OH

calcifediol

(25-OH-D 3 )

25 OH

liver

25-hydroxylation

kidney

1--hydroxylation

HO

HO

CH 2

CH 2

vitamin D 2

(ergocalciferol)

OH

vitamin D 2

(25-OH-ergocalciferol)

OH

AGENTS AFFECTING MINERAL ION HOMEOSTASIS AND BONE TURNOVER

1

calicitriol

HO OH 1, 25-(OH) 2 vitamin D 3

HO OH

CH 2

Figure 44–4. Photobiology and metabolic pathways of vitamin D production and metabolism.

1, 25-(OH) 2 -ergocalciferol

1, 25-(OH) 2 vitamin D 2

disappears from plasma with a t 1/2

of 20–30 hours but is stored in fat

depots for prolonged periods.

Metabolic Activation. Whether derived from diet or endogenously

synthesized, vitamin D requires modification to become biologically

active. The primary active metabolite of the vitamin is calcitriol

[1α,25-dihydroxyvitamin D, 1,25(OH) 2

D], the product of two successive

hydroxylations of vitamin D (Figure 44–4).

25-Hydroxylation of Vitamin D. The initial step in vitamin D activation

occurs in the liver, where cholecalciferol and ergocalciferol are

hydroxylated in the 25-position to generate 25-OH-cholecalciferol

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!