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Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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702<br />

Chapter | 23 Vitamins<br />

H CH<br />

H 3 C CH 3 H<br />

3 cis-configuration<br />

C C 8 C 10 C H<br />

1 7 9 11<br />

H 2 C 2 6 C C C 12 C<br />

3 5<br />

13<br />

H 2 C 4 C H H C H<br />

C CH 3 H 3 C C<br />

14<br />

H 2<br />

11-cis-retinal<br />

C<br />

15<br />

H O<br />

Capillary<br />

all-transretinol<br />

Retinal Pigment Epithelium<br />

11-cis-retinal<br />

11-cis-retinol<br />

all-trans-retinyl ester<br />

all-trans-retinol<br />

Interphotoreceptor<br />

Matrix<br />

Photoreceptor (Rod) Cell<br />

11-cis-retinal<br />

Rhodopsin<br />

Opsin<br />

Opsin<br />

+<br />

all-trans-retinal<br />

all-trans-retinol<br />

LIGHT<br />

Neuronal<br />

signaling<br />

VISION<br />

Visible light<br />

P-Opsin<br />

II-CIS Retinal<br />

ATP Opsin<br />

Rhodopsin<br />

h<br />

H CH<br />

H 3 C CH 3 H CH 3 3<br />

C C 8 C 10 C 12 C 14<br />

1 7 9 11 13<br />

H 2 C 2 6 C C C C C<br />

3 5<br />

H 2 C 4 C H H H H<br />

C CH 3<br />

H 2 trans-configuration<br />

all-trans-retinal<br />

H<br />

C<br />

15<br />

O<br />

all-trans Retinal<br />

GTP + Transducin-GDP<br />

Membrane<br />

hyperpolarization<br />

Pi<br />

GTP<br />

Metarhodopsin II<br />

+<br />

Transducin-GTP + GDP<br />

+<br />

Phosphodiesterase<br />

cGMP<br />

Na uptake<br />

+<br />

GMP<br />

FIGURE 23-7 Vitamin A and vision. Retinal, as a component <strong>of</strong> rhodopsin, facilitates the transfer <strong>of</strong> energy from photons <strong>of</strong> light to electrochemical<br />

signals. The series <strong>of</strong> events includes the cis - trans isomerization, which results in a highly strained form <strong>of</strong> rhodopsin that is converted to metarhodopsin<br />

with subsequent deprotonation. The deprotonated metarhodopsin interacts with transducin, one <strong>of</strong> the proteins in the transmembrane G-protein family.<br />

This interaction stimulates cGMP phosphodiesterase activity, which results in a decrease in cGMP and signal amplification. The local changes in cGMP<br />

concentration result in turn in changes in cation flux (Na and Ca ions) across rod cell membranes to initiate firing <strong>of</strong> cells <strong>of</strong> the optic nerve.<br />

Retinyl<br />

Ester<br />

Retinol<br />

Retinyl esters<br />

mRNA<br />

Retinoic Acid<br />

Retinol<br />

Retinal<br />

RAR X 9cRAR<br />

PPAR X 9cRAR<br />

VDR X 9cRAR<br />

Retinoic Acid<br />

9-cis or 13-cis<br />

Retinoic Acid<br />

FIGURE 23-8 Vitamin A and cell regulation. Within cells all- trans retinol associates with cytosolic retinol-specific binding proteins, and the resulting<br />

complex become vehicles for subsequent processing. For example, all- trans retinol may be oxidized and isomerized to all- trans, 9- cis, or 13- cis retinoic<br />

acid, which subsequently binds to retinoic acid-specific binding proteins that act as transcription factors in protein regulation and cellular differentiation.<br />

The nature <strong>of</strong> the metabolic control has many facets including the regulation <strong>of</strong> specific receptor concentrations and their combination to form specific<br />

signaling complexes. The figure shows duplexes for the retinoic acid receptor (RAR) with (1) an isomeric form, (2) complexes with peroxisomal activation<br />

receptors, and (3) receptors under the control <strong>of</strong> vitamin D derivatives.

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