Vitamin D 2 & D 3 - Hormones?

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What is vitamin D and why is it important:

Chemistry:

There are two chemical forms of vitamin D, namely vitamin D2 (sometimes referred

to as ergocalciferol) and vitamin D3 (sometimes referred to a cholecalciferol).

The natural form of vitamin D for animals and man is vitamin D3 that is produced

in their bodies from cholesterol and 7- dehydrocholesterol. An alternative

vitamin D2 is derived from the yeast sterol ergosterol by chemical procedures.

The molecular structure of vitamin D is closely allied to that of the classical

steroid hormones, e.g. cortisol, estradiol, progesterone, aldosterone, and

testosterone (1). All steroid hormones and vitamin D3 are chemically related to

the well known sterol cholesterol. Cholesterol in animals and man is a precursor

substance for all steroid hormones and as well vitamin D3.

Technically the molecule called vitamin D3 is not really a vitamin because it

can be produced by exposure of the skin (higher animals and humans) to

ultraviolet light or sunlight. The skin of many animals and man has a high

concentration of the sterol cholesterol which is converted by enzymes in the

skin to the sterol 7-dehydrocholesterol. Exposure of skin (including human skin)

to sunlight for regular intervals results in the photochemical conversion of

7-dehydrocholesterol into vitamin D3. This sunlight- generated vitamin D3 is a

precursor of the steroid hormone 1,25(OH)2D3. Under these circumstances vitamin

D3 is not a vitamin because it has been produced by the body (with the

assistance of sunlight). However, if the animal or man lives in the absence of

sunlight (e.g., Alaska in the winter) or exclusively indoors, then there is

indeed an absolute regular requirement for the fat soluble vitamin D, that must

be met through proper dietary intake.

Therefore for nutritional and public health reasons, vitamin D3 continues to be

classified even today in 2000 officially as a vitamin. Thus many vitamin

capsules and food sources including cows milk are supplemented with vitamin D3

to improve their nutritional value. In the 1940’s this milk supplementation

process reduced the incidence rate of juvenile rickets by 85% in the United

States.

Importance:

Vitamin D3 is essential for life in higher animals. Classically vitamin D3 has

been shown to be one of the most important biological regulators of calcium

metabolism through stimulating the absorption of calcium from food across the

intestine and participating in the incorporation of the absorbed calcium in to

the skeleton (2). These important biological effects are only achieved as a

consequence of the metabolism of vitamin D into a family of daughter

metabolites, including 1,25(OH)2-vitamin D3 [1,25(OH)2D3]. 1,25(OH)2D3, is

considered to be a steroid hormone because the general mechanism by which it

produces the biological responses attributed to vitamin D is similar to those of

steroid hormones (3;4).

It has become increasingly apparent since the 1980s that 1,25(OH)2D3 also plays

an important multidisciplinary role in tissues not primarily related to mineral

metabolism, e.g. the hematopoietic or blood system, effects on cell

differentiation and proliferation including important interactions with

keratinocytes and cancer cells, and participation in the processes of

parathyroid hormone and insulin secretion (3).

Prepared by:

Professor W. Norman

Department of Biochemistry & Biomedical Sciences

University of California

Riverside CA 92521

E-MAIL: Norman@...

FYI,

Lottie Duthu