Dietary thiamin level influences levels of its diphosphate form and thiamin-dependent enzymic activities of rat liver

This study was prompted by our incomplete understanding of the mechanism re sponsible for the clinical benefits of pharmacological doses of thiamin in some patients with maple syrup urine disease (MSUD) and the question of whe ther thiamin diphosphate (TDP), a potent inhibitor of the activity of the p rotein kinase that phosphorylates and inactivates the isolated branched-cha in alpha-ketoacid dehydrogenase (BCKDH) complex, affects the activity state of the complex, Rats were fed a chemically-defined diet containing graded levels of thiamin (0, 0.275, 0.55, 5.5, and 55 mg thiamin/kg diet). Maximal weight gain was attained over a 3-wk period only in rats fed diets with 5. 5 and 55 mg thiamin/kg, Feeding rats the thiamin-free diet for just 2 d cau sed loss of nearly half of the TDP from liver mitochondria, Three more days caused over 70% loss, an additional 3 wk, over 90%. Starvation for 2 d had no effect, suggesting a mechanism for conservation of TDP in this nutritio nal state. Mitochondrial TDP was higher in rats fed pharmacological amounts of thiamin (55 mg thiamin/kg diet) than in rats fed adequate thiamin for m aximal growth. Varying dietary thiamin had marked but opposite effects on t he activities of alpha-ketoglutarate dehydrogenase (alpha-KGDH) and BCKDH. Thiamin deficiency decreased alpha-KGDH activity, increased BCKDH activity, and increased the proportion of BCKDH in the active, dephosphorylated, sta te. Excess dietary thiamin had the opposite effects. TDP appears to be more tightly associated with alpha-KGDH than BCKDH in thiamin-deficient rats, p erhaps denoting retention of alpha-KGDH activity at the expense of BCKDH ac tivity. Thus, thiamin deficiency and excess cause large changes in mitochon drial TDP levels that have a major influence on the activities of the keto acid dehydrogenase complexes.