10.3 Thiamin (Vitamin B1)

Jill Reid, VCU

10.3 Thiamin (Vitamin B1)

Thiamin (Vitamin B1)

Thiamin in a water-soluble B vitamin consisting of 2 rings that are bridged together as shown below.

Figure 10.31 Structure of thiamin1

Because it was one of the original vitamins, (remember vitamine), it was originally named thiamine. The -e has since been dropped from its spelling. Thiamin is sensitive to heat, so prolonged heating, such as during cooking, causes the cleavage of thiamin between the 2 rings destroying its activity2.

Like most of the B vitamins, thiamin’s primary function is as a cofactor for enzymes. It is not thiamin alone that serves as a cofactor, but instead thiamin diphosphate (thiamin + 2 phosphates), which is more commonly referred to as thiamin pyrophosphate (TPP). The structure of thiamin pyrophosphate is shown below.

Figure 10.32 Structure of thiamin pyrophosphate (aka thiamin diphosphate)3

Common sources of thiamin include whole grains, meat, and fish. The most common sources of thiamin in American diets are cereals and bread. Pork is also a good source of thiamin, while fruits and dairy products generally have low levels6.

In plants, thiamin is found in its free form, but in animals it is mostly thiamin pyrophosphate. These phosphates must be cleaved before thiamin is taken up into the enterocyte4.

Thiamin uptake and absorption is believed to be an efficient process that is passive when thiamin intake is high and active when thiamin intakes are low4. There are two thiamin transporters (THTR), THTR1 and THTR2, that are involved in thiamin uptake and absorption. THTR1 is found on the brush border and basolateral membrane, while THTR2 is only found on the brush border membrane as shown in Figure 10.335.

Figure 10.33 Thiamin uptake and absorption

Like most water-soluble vitamins there is little storage of thiamin. For more information on thiamin, see the Required Web Link below.

Required Web Link:Thiamin Fact Sheet for Health Professionals

Subsections:

10.31 Thiamin Functions
10.32 Thiamin Deficiency & Toxicity

References & Links

http://en.wikipedia.org/wiki/File:Thiamin.svg
Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.
http://en.wikipedia.org/wiki/File:Thiamine_diphosphate.png
Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
Said H, Mohammed Z. (2006) Intestinal absorption of water-soluble vitamins: An update. Curr Opin Gastroenterol 22(2): 140-146.
Thiamin Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Thiamin- HealthProfessional/

Links

Thiamin Fact Sheet for Health Professionals – https://ods.od.nih.gov/factsheets/Thiamin- HealthProfessional/

Thiamin Functions

There are three functions of thiamin1:

Cofactor for decarboxylation reactions (TPP)
Cofactor for the synthesis of pentoses (5-carbon sugars) and NADPH (TPP)
Membrane and nerve conduction (Not as a cofactor)

Decarboxylation Reactions

A decarboxylation reaction is one that results in the loss of carbon dioxide (CO₂) from the molecule as shown below.

Figure 10.311 Decarboxylation reaction2

The transition reaction and one reaction in the citric acid cycle are decarboxylation reactions that use TPP as a cofactor. The conversion of pyruvate to acetyl CoA in the transition reaction is a decarboxylation reaction that requires TPP as a cofactor. CO₂ is produced as a result of this reaction.

A similar TPP decarboxylation reaction occurs in the citric acid cycle converting alpha- ketoglutarate to succinyl-CoA. CO₂ is also given off as a result of this reaction as well.

TPP also functions as a cofactor for the decarboxylation of valine, leucine, and isoleucine (branched-chain amino acids)1.

Synthesis of Pentoses and NADPH

TPP is a cofactor for the enzyme transketolase. Transketolase is a key enzyme in the pentose phosphate pathway. This pathway is important for converting 6-carbon sugars into 5-carbon sugars (pentose) that are needed for synthesis of DNA, RNA, and NADPH. In addition, pentoses such as fructose are converted to forms that can be used for glycolysis and gluconeogenesis3.

Membrane and Nerve Conduction

In addition to its cofactor roles, thiamin, in the form of thiamin triphosphate (TTP, 3 phosphates), is believed to contribute to nervous system function, but its exact role is not yet fully understood1.

References & Links

Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
https://commons.wikimedia.org/wiki/File:Decarboxylation_reaction.png
Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.

Thiamin Deficiency & Toxicity

Thiamin deficiency is rare in developed countries, but still occurs in poorer countries where white (a.k.a. polished) rice is a staple food. During the polishing process, thiamin, and many other nutrients, are removed. Some people also have a mutation in THTR1 (the thiamin transporter mentioned above) that causes them to become thiamin deficient1. Thiamin deficiency is known as beriberi, which, when translated, means “I can’t, I can’t.” The symptoms of beriberi are illustrated in the Required Web Link below.

Required Web LinkBeriberi

There are two major forms of beriberi: dry and wet. Dry beriberi affects the nervous system, with symptoms such as loss of muscle function, numbness, and/or tingling. Wet beriberi affects the cardiovascular system resulting in pitting edema, along with enlargement of the heart1. A picture of a person with beriberi is shown in Figure 10.321.

Figure 10.321 A person suffering from beriberi2

Another group that is at risk for thiamin deficiency is alcoholics. There are three reasons why alcoholics are prone to becoming deficient3:

Alcohol displaces foods that are better sources of thiamin
Liver damage decreases TPP formation
Increased thiamin excretion

The thiamin deficiency found in alcoholics is known as Wernicke-Korsakoff Syndrome. Symptoms of this condition include paralysis or involuntary eye movement, impaired muscle coordination, memory loss and confusion3. The TV show House is fictional, but the writers did use real medical information to script their episodes as you can see below.

Required Web LinkVideo: Dr. House Explains Korsakoff Syndrome

Thiamin toxicity has never been reported as a result of oral intake. Thus, there is little worry about thiamin toxicity4.

References & Links

http://www.nlm.nih.gov/medlineplus/ency/article/000339.htm
http://en.wikipedia.org/wiki/File:Beriberi_USNLM.jpg
Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.