Thiamin plays an important role in glucose metabolism. TPP is a coenzyme in the transketolase reaction that is part of the direct oxidative pathway (pentose phosphate cycle) of glucose. The pentose pathway occurs in the cell cytoplasm of red blood cells, liver, brain, adrenal cortex and kidney, but not in skeletal muscle. The pentose phosphate cycle is the only mechanism known for synthesis of ribose, which is required for nucleotide (RNA, DNA) formation. The pentose pathway also produces nicotinamide adenine dinucleotide phosphate (NADPH) that provides reducing equivalents for synthesis of fatty acids and other products.
A more recently discovered role of thiamin is as coenzyme for the oxidative decarboxylation of iso-keto acids derived from the degradation of the branched-chain amino acids (leucine, isoleucine, valine). A dehydrogenase enzyme complex similar to pyruvate dehydrogenase has been reported (Gubler, 1991).
Little is known of thiamin function in nervous tissue. However, evidence has accumulated for a specific role of thiamin in nerve transmission that is independent of its coenzyme function. Possible mechanisms of thiamin action in neural tissue include (Muralt, 1962; Cooper et al., 1963): (1) thiamin is involved in the synthesis of acetylcholine, a neurotransmitter; (2) thiamin participates in the passive transport of sodium (Na+) through nerve cell membranes, particularly in the membrane of ganglionic cells and (3) thiamin’s role in glucose metabolism, via pentose phosphate pathways and pyruvate dehydrogenase complex, directly and indirectly influences the efficiency of energy metabolism and synthesis of fatty acids in the nervous system.
Additionally there may a direct role of thiamin phosphates in generation of nerve impulses (Grubber,1991)