Water Soluble Vitamins Midterm #2 Flashcards
Generalities: Metabolism and Storage
only B12 and folate are stored in the body, in general water soluble vitamins are excreted rapidly and not stored. Depletion is more of a problem than toxicity.
Generalities: Toxicity
Only niacin and pyridoxine can become toxic at high concentrations. In general there are very few toxicities.
Generalities: Measuring body levels
water soluble vitamins are coenzymes for various common biochemical reactions and their status can be determined by measuring the appropriate enzyme activities. Typically enzyme activity is measured in the absence and the presence of exogenously added coenzyme and then determined if the patient needs more of the vitamin.
Vitamin B1
Thiamin

Vitamin B1 (thiamin): Structure
pyrimidine and thiamin ring. Thiazole is the “business end”. phosphorylation occurs at the alcohol group off the thiamin ring. Addition of one ATP forms TMP (thiamine monophosphoate) and addition of a second ATP forms TPP (Thiamine pyrophosphate).

Vitamin B1 (thiamine): Function
Metabolism of carbohydrates in glycolysis and TCA cycle. Cofactor for pyruvate dehydrogenase (glycolysis) and alpha-ketoglutarate dehydrogenase (TCA).

Vitamin B1 (Thiamine): Reactions
Oxidative carboxylation of alpha-keto acids
and transfer of alpha-ketols
Vitamin B1 (Thiamine): oxidative decarboxylation of alpha-keto acids enzymes
Pyruvate dehydrogenase complex
alpha-ketoglutarate dehydrogenase complex
Vitamin B1 (thiamine): pyruvate dehydrogenase reaction
Oxidative decarboxylation of alpha-keto reaction.
form acetyl-CoA intermediate for the TCA cycle and NADH

Vitamin B1 (Thiamine): alpha-ketoglutarate dehydrogenase reaction
Oxidative decarboxylation of alpha-keto acids
form succinyl CoA and NADH in TCA cycle.

Vitamin B1 (Thiamine): transfer of alpha-ketos reaction
used in the pentose phosphate pathway (pentose shunt in glycolysis).
Provides 5 carbon pentose sugars for DNA and RNA synthesis and also produces NADPH.
Runs both directions.

Vitamin B1 (Thiamine): Mechanism
All occurs on the thiamin ring. The 2-C loses a hydrogen atom to form a negative charge on the carbon and a positive charge on the nitrogen. The negative charged carbon can attack C=O to form a tetrahedral intermediate. Alpha-hydroxy-ethyl TPP is the intermediate with a two carbon structure attached and is used to move 2 carbon groups around.

Vitamin B1 (Thiamin): needs
Thiamin needs are proportional to caloric intake.
Vitamin B1 (Thiamin): measuring deficiency
Measure deficiency using erythrocyte transketolase assay
Vitamin B1 (thiamin): deficiency in alcoholics
About 40%-50% of chronic alcoholics are deficient. Low intake and alcohol affects the conversion of thiamin to TPP. Also, increased urine flow may cause thiamin washout. This includes fetal alcohol syndrome, which can be helped with supplementing the mother with thiamin.
Vitamin B1 (thiamin): Signs of deficiency
Early signs: anorexia, nausea, vomiting, fatigue, weight loss, nystagmus, tachycardia.
Late signs: beriberi (polyneuritis). Wet beriberi includes cardiac issues including cardiac edema, and enlargement of heart.
Wernicke-Korsakoff syndrome
Vitamin B1 (thiamin deficiency)
neurological disorder resulting in impaired mental functioning usually requiring institutionalization. Symptoms include confusion, memory loss, confabulation, psychotic behavior, and maybe irreversible in part. Seen in alcoholics.
Vitamin B1 (thiamin): factors causing deficiency
Increased carbohydrate intake, i.e. TPN and alcoholics
Decreased absorption, i.e. ulcerative colitis and alcoholics
Decreased intake, i.e. poor diet, geriatrics, breast fed infant from B1 deficient mother
alcoholism
Vitamin B1 (thiamin): cellular uptake
Thiamin specific receptor in intestinal cells (hTHTR) which only recognizes thiamin, not TPP. Common to ingest thiamin in plants and TPP in meats, this has to be broken down to thiamin for uptake. Have been polymorphisms in the gene encoding hTHTR which may cause thiamin-responsive megaloblastic anemia.
Vitamin B1 (Thiamin): Source
Present in most tissues as TPP and plants as thiamin.
Rich sources include lean meat, especially pork, cereal grains, eggs, yeast, nuts.
Thiaminase in some raw fish and raw shellfish have an enzyme that can hydrolyze thiamin
In the milling or processing of rice and flour, thiamin is lost. Whole wheat or rice contains 10X the amount of thiamin as white. In the US most white flour, rice, pastas are “enriched” to restore thiamin levels. “Enriched” products also have added riboflavin, niacin, iron and folic acid.
Vitamin B1 (thiamin): Stability
labile at pH > 4 and when heated.
Vitamin B1 (thiamin): diagnosis of deficiency
increased pyruvate and lactate in plasma (because can’t form pyruvate) or the transkelotase activity in RBC. You compare the activity of the enzyme in the presence and absence of thiamine. If addition of thiamine increases activity that person is said to be deficient.
Vitamin B1 (Thiamin): Uses
- deficiency states - alcoholics
- thiamin responsive inborn errors of metabolism
- mosquito repellant - questionable efficacy
- Acute alcoholism - have to give IV or IM because don’t absorb well orally
- Alzheimer’s disease - huge dose, questionable efficacy
Vitamin B1 (Thiamine): Responisve inborn errors of metabolism
- Wernicke-Korsakoff: transketolase defect
- Maple Syrup urine disease: failure to decarboxylate branched chain amino acids defect
- Thiamin responsive megaloblastic anemia
- Hyperalanemia: unknown defect
- Hyperpyruvate acidurea: pyruvate dehydrogenase defect























