structure of thiamine
Thiamine consists of a pyrimidine ring attached to a
thiazole ring by a methylene bridge.
Active Form of Thiamine
Thiamine pyrophosphate (TPP) is an active coenzyme
form of vitamin thiamine
Sources
- present in all natural foods but particularly good
dietary sources are unrefined cereals, meat, nuts,
green vegetables, eggs, etc. - White bread and polished rice are very poor sources
of the vitamin thiamine.
Functions of thiamine
- carbohydrate
metabolism - Thiamine pyrophosphate (TPP) is a coenzyme
involved in enzymatic reactions mainly for
oxidative decarboxylation and transketolase
reactions as follows:
1. TPP is a coenzyme for pyruvate dehydrogenase
complex which catalyzes the conversion of pyruvate into acetyl CoA by oxidative decarboxylation. Acetyl-CoA is a precursor for the synthesis of the neurotransmitter acetylcholine
and also for the synthesis of myelin. Thus, thiamine
is required for the normal functioning of the
nervous system.
2. TPP is a coenzyme for α-ketoglutarate dehydrogenase which catalyzes the conversion of α -ketoglutarate to succinyl-CoA in TCA cycle
3. TPP is a coenzyme for the enzyme transketolase,
in the pentose phosphate pathway of glucose
oxidation.
Nutritional Requirements
daily
intake of 1.0 to 1.5 mg of thiamine for adults which
is increased with increased muscular activity, dietary
carbohydrates and in pregnancy and lactation.
Deficiency Manifestations
The deficiency of vitamin B1 results in a condition
called beriberi. Deficiency of thiamine occurs in
population who consume exclusively polished rice
as staple food. Polishing of rice removes thiamine.
types of beriberi
- Dry beriberi
- Wet beriberi
- Infantile beriberi
- cerebral beriberi (Wernicke-Korsakoff Syndrome)
Dry beriberi (neuritic beriberi)
- It develops when the diet chronically contains
slightly less than the thiamine requirements. - This form of beriberi is characterized primarily by
peripheral neuritis, severe muscular weakness and
fatigue. Other symptoms of dry beriberi include dry
skin, mental confusion and poor appetite.
Wet beriberi (cardiac beriberi)
- It develops when the deficiency is more severe in
which cardiovascular system is affected in addition
to neurological symptoms. - Wet beriberi is characterized primarily by edema of
extremities, heart enlargement and cardiac insufficiency. Other symptoms include tachycardia or
bradycardia and palpitation. - Both forms of beriberi may overlap to a varying
degree and patients of beriberi may die due to heart
failure, if not treated.
Infantile beriberi
- Infantile beriberi is observed in breast fed infants
born to mother suffering from thiamine deficiency.
The breast milk of these mothers is deficient in
thiamine. - It is characterized by cardiac dilation (enlargement
of heart), tachycardia, convulsions, edema and GI
disturbances such as vomiting, abdominal colic, etc.
In acute condition, the infant may die due to cardiac
failure.
Wernicke-Korsakoff Syndrome
- It is also known as cerebral beriberi and mostly seen
in alcoholics. - In chronic alcoholics, the nutritional deficiencies
result from either poor intake of food or malabsorption of nutrients from intestine. - Wernicke-Korsakoff syndrome is characterized by
anorexia, nausea, vomiting, nystagmus, depression,
ataxia, loss of memory, mental confusion, peripheral
paralysis, muscular weakness, etc.
Antimetabolites of thiamine
Thiamine can be destroyed if the diet contains thiaminase. Thiaminase is present in raw fish and
seafood.
Thiamine Assay
Whole blood or Erythrocyte transketolase (requiring TPP
as a coenzyme) activity is used as a measure of thiamine
deficiency.
