Phar 722Pharmacy Practice III

Vitamins-

Biotin

Spring 2006

Biotin Study Guide

• The applicable study guide items in the Vitamin Introduction

• History

• Structure of the vitamin and cofactor forms

• Function of the cofactor including specific types of reactions catalyzed

• Deficiency condition

History• 1916

– It was recognized that rats fed a diet high in raw egg white exhibited muscle incoordination, dermatitis and loss of hair.

• Cooked egg white was not toxic.

– The toxicities from the raw egg white could be prevented by feeding liver or yeast to the animals.

– Deficiencies can be induced by a diet restricted to raw egg white.

• 1942– The biotin structure was published.

Biotin Chemistry

• Biotin consists of two 5-membered rings cis-fused to each other.

• The d-isomer is the only active form.

S

NHN

O

H H

(CH2)4 OH

O

H

S

NHHN

O

H H

(CH2)4 OH

O

Enolic form Keto or ureido form

H H

Biotin Uptake and Metabolism• Little is known about biotin uptake. • There does not seem to be a cofactor form

different from the basic structure. • The carboxyl chain forms an amide linkage

with the ε-amino nitrogen of lysine which binds the vitamin to the enzyme's active site.

• Biotin is required for the addition of carbon dioxide in many, but not all, carboxylation reactions.

• There is some debate about the structure of the cofactor intermediate that transfers the carbon dioxide. See the next slide.

S

NHN

O

H

H H

(CH2)4 NH

O

(CH2)4CH

C

NH

OLys residue

Coenzyme form of Biotin

S

NHN

O

H H

(CH2)4 Enz

O

-O C O

O

Bicarbonate

HO P

O

O-

O C

O

O H

H

Phosphoric-carbonic acid anhydride

ATP

ADP

Pi

C

O

O

H

S

NHN

O

H H

(CH2)4 Enz

O

H

C

-O

O

Addition of CO2 to biotin for subsequent carboxylation reactions.

Biotin Function-1• Methylmalonyl CoA from propionyl CoA

CH3 CH2 CSCoA

O CO2

Propionyl CoA Methyl Malonyl CoA

-O C CH

O CH3

C

O

SCoA

cobalamin

-O C CH2

O

CH2 C SCoA

O

Succinyl CoA

Val, Ile, Met, Thr

Biotin Function-2• Oxalacetate from pyruvate (gluconeogenesis;

anapleurotic reactions)

CH3 C

O

C

O

O-

CO2

-O C CH2

O

C

O

C

O

O-

Pyruvate Oxalacetate

Biotin Function-3• Formation of carbamyl phosphate (urea cycle)

H2N C

O

O PO3

Carbamyl Phosphate

Biotin Function-4• Malonyl CoA from acetyl CoA (fatty acid synthesis)

CH3 C

O

SCoA

CO2

CH2 C

O

SCoA

Acetyl CoA Malonyl CoA

C-O

O

Biotin Function-5• Metabolism of leucine

Biotin Deficiency• One of the ways to induce a deficiency in humans is

to feed the volunteer a raw egg white diet. – Deficiency symptoms include hair loss, a rash around the

nose and mouth, and conjunctivitis.

• A good source of biotin is egg yolk. – Therefore, eating whole raw eggs will not cause a deficiency

of this vitamin (although person could be at increased risk for salmonella poisoning).

• What is the problem with raw egg white?– Egg white contains a basic protein known as avidin which

forms salt linkages with the acidic biotin that might be in the intestinal tract (possibly produced by intestinal bacteria?).

• This complex is not absorbed, but remains in the intestine eventually to be excreted.

– Cooking the egg white ties up the avidin in the rest of the egg albumin preventing its binding biotin.

Hypervitaminosis Biotin

• None has been reported in humans.

• There is no UL.

Dosage Forms

• The synthetic racemic mixture is used commercially. – As with racemic pantothenic acid and pantothenol,

only half of the racemic biotin is active.

• Biotin is considered one of the more expensive vitamins and can add to the cost of the final product.

• Solubility 0.3 -0.4 mg/ml (1 gm/2,500-3,300 ml)

DRIs• AI

– Infants 5 - 6 μg/day– Children (1 - 13 years) 8 - 20 μg/day– Adolescents (14 - 18 years) 25 μg/day– Adults 30 μg/day– Pregnancy 30 μg/day– Lactation 35 μg/day

• EAR– None reported

• RDA– None reported

• UL– None reported

Sources• Liver

• Kidney

• Yeast

• Animal and plant tissue in general;

• Possibly the intestinal bacteria