Dietary Thiamin Level Influences Levels of Its Diphosphate Form and Thiamin-Dependent Enzymic Activities of Rat Liver

Abstract

This study was prompted by our incomplete understanding of the mechanism responsible for the clinical benefits of pharmacological doses of thiamin in some patients with maple syrup urine disease (MSUD) and the question of whether thiamin diphosphate (TDP), a potent inhibitor of the activity of the protein kinase that phosphorylates and inactivates the isolated branched-chain 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 caused 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 nutritional state. Mitochondrial TDP was higher in rats fed pharmacological amounts of thiamin (55 mg thiamin/kg diet) than in rats fed adequate thiamin for maximal growth. Varying dietary thiamin had marked but opposite effects on the 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, state. Excess dietary thiamin had the opposite effects. TDP appears to be more tightly associated with alpha-KGDH than BCKDH in thiamin-deficient rats, perhaps denoting retention of alpha-KGDH activity at the expense of BCKDH activity. Thus, thiamin deficiency and excess cause large changes in mitochondrial TDP levels that have a major influence on the activities of the keto acid dehydrogenase complexes.

Wild birds of declining European species are dying from a thiamine deficiency syndrome

“…This is in fair agreement with published values of liver KGDH latency in nonthiamine deficient rats: 6.5% (22), 5.4% (23), and 4.8% (24). We noted that there was a negative linear relationship between liver KGDH activity and latency in the thiamine deficient rats (22) (R 2 ϭ 0.88, P ϭ 0.00062), but no such relationship in nonthiamine deficient rats (22), or in the nonthiamine deficient domestic chicken pulli (Fig. S4 a-c in the SI Appendix).…”

“…1; Table S5 in the SI Appendix; P ϭ 0.012). The liver KGDH latency was, however, Ն25% throughout (Table S5 in the SI Appendix), i.e., strongly indicating thiamine deficiency (21,22) in all investigated regions, including Iceland.…”

Effect of low doses of bezafibrate and fenofibrate on liver 2-oxo-glutarate dehydrogenase complex in low-protein diet fed rats

“…Tissue extracts for the assay of liver 2-OGDH were prepared as described previously (Blair et al, 1999). Briefly, a portion of frozen liver was powdered in liquid nitrogen, weighed and homogenized in an extraction buffer.…”

Inhibition of Branched-Chain α-Ketoacid Dehydrogenase Kinase by Thiamine Pyrophosphate at Different Potassium Ionic Levels

“…2B) K þ levels, respectively. Considering that the TPP level in mitochondria is approximately equal to that in whole liver tissue, 15) the TPP concentration in rat liver mitochondria has been estimated to be about 41 mM. 15,16) The physiological K þ concentration in mitochondria may be around 100 mM.…”

“…This physiological effect of TPP has not been considered by others, 8) even though others have reported a low K i value (4 mM). Furthermore, because TPP functions as a coenzyme to other complexes and has higher affinity for the mitochondrial -ketoglutarate dehydrogenase, 15) it is likely that the TPP concentration around BCKDC is lower than 41 mM, and this suggests that BDK is somewhat less influenced by TPP under physiological K þ concentrations. Further studies are needed to clarify the TPP-dependent BCKDC activation under physiological conditions.…”