Melatonin attenuates metabolic disorders due to streptozotocin-induced diabetes in rats

Enhanced oxidative stress and impairments in nitric oxide synthesis and bioavailability are of considerable importance in the pathogenesis of diabetic vascular diseases. The aim of the present work was to evaluate the metabolic effects of pharmacological doses of the melatonin, a known antioxidant, on streptozotocin-induced diabetic damage in rats. We investigated the indolamine's influence on the cellular redox-balance, nitric oxide (NO) level, and the activities of antioxidative defence enzymes, as well as the activities of enzymes involved in phase II detoxication and NADPH-generating pentose phosphate pathway. Blood glucose, glycated hemoglobin, bilirubin, as well as plasma alanine aminotransferase activities increased and body weight was reduced in rats with streptozotocin-induced (60 mg/kg, i.p.) diabetes (25 days). The NO level was markedly increased in diabetic plasma (by 50%) and aortic tissue (by 30%). The hyperglycemia resulted in reduced activities of glutathione peroxidase (by 25%), catalase (by 20%), glucose-6-phosphate dehydrogenase (by 55%) and transketolase (by 40%) in liver tissue of diabetic animals. Melatonin treatment (10 mg/kg, 18 days) did not influence the level of hyperglycemia or glycated hemoglobin and it had little effect on the activities of antioxidative enzymes. However, melatonin markedly reversed the activities of glucose-6-phosphate dehydrogenase and transketolase in liver tissue of diabetic rats. The most pronounced effect of the melatonin administration was the prevention of an increase in nitric oxide levels in blood plasma and aortic tissue during diabetes. In in vitro experiments, nitrosomelatonin formation in the presence of nitrosodonors was observed. This implies that melatonin might operate as an NO scavenger and carrier. Thus, melatonin treatment may have some beneficial effects in controlling diabetic vascular complications.

The TKTL1 gene influences total transketolase activity and cell proliferation in human colon cancer LoVo cells

A plasmid carrying DNA to be transcribed into a small interfering RNA against transketolase-like-1 mRNA was constructed and transfected into a human colon cancer cell line. The mRNA expression of transketolase gene family in the human colon cell line was determined by real-time polymerase chain reaction. The effect of anti-transketolase-like-1 small interfering RNA on cell proliferation and cell cycle in the human colon cancer cell line cells was detected by flow cytometry and 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyl tetrazolium bromide. The transketolase-like-1 gene was significantly downregulated in human colon cancer cell line cells transfected with small interfering RNA transketolase-like-1 constructs compared with the cells transfected with control vector and the cells without transfection. In addition, the anti-transketolase-like-1 small interfering RNA construct significantly decreased the level of transketolase in the transfected human colon cancer cell line cells, arrested them in G0/G1 phase and substantially inhibited cell proliferation. No significant difference was found in the other two genes (transketolase and transketolase-like-2 genes) between the transfected human colon cancer cell line cells and the controls (P>0.05). Our data demonstrated that the transketolase-like-1 gene plays an important role in total transketolase activity and in the cell proliferation of human colon cancer. Transketolase-like-1 may serve as a target for novel anticancer therapies.

Which stage of the process of apotransketolase interaction with thiamine diphosphate is affected by the regulatory activity of the donor substrate?

The interaction of thiamine diphosphate (ThDP) with transketolase (TK) involves at least two stages: [formula: see text] During the first stage, an inactive intermediate complex (TK...ThDP) is formed, which is then transformed into a catalytically active holoenzyme (TK* - ThDP). The second stage is related to conformational changes of the protein. In the preceding publication (Esakova, O. A., Meshalkina, L. E., Golbik, R., Hübner, G., and Kochetov, G. A. Eur. J. Biochem. 2004, 271, 4189 - 4194) we reported that the affinity of ThDP for TK considerably increases in the presence of the donor substrate, which may be a mechanism whereby the activity of the enzyme is regulated under the conditions of the coenzyme deficiency. Here, we demonstrate that the substrate affects the stage of the reverse conformational transition, characterized by the constant k(-1): in the presence of the substrate, its value is decreased several fold, whereas K(d) and k(+1) remain unchanged.

Modification of thiamine pyrophosphate dependent enzyme activity by oxythiamine inSaccharomyces cerevisiaecells

Oxythiamine is an antivitamin derivative of thiamine that after phosphorylation to oxythiamine pyro phos phate can bind to the active centres of thiamine-dependent enzymes. In the present study, the effect of oxythiamine on the viability of Saccharomyces cerevisiae and the activity of thiamine pyrophosphate dependent enzymes in yeast cells has been investigated. We observed a decrease in pyruvate decarboxylase specific activity on both a control and an oxythiamine medium after the first 6 h of culture. The cytosolic enzymes transketolase and pyruvate decarboxylase decreased their specific activity in the presence of oxythiamine but only during the beginning of the cultivation. However, after 12 h of cultivation, oxythiamine-treated cells showed higher specific activity of cytosolic enzymes. More over, it was established by SDS–PAGE that the high specific activity of pyruvate decarboxylase was followed by an increase in the amount of the enzyme protein. In contrast, the mitochondrial enzymes, pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase complexes, were inhibited by oxythiamine during the entire experiment. Our results suggest that the observed strong decrease in growth rate and viability of yeast on medium with oxythiamine may be due to stronger in hibition of mitochondrial pyruvate dehydrogenase than of cytosolic enzymes.Key words: pyruvate dehydrogenase, 2-oxoglutarate dehydrogenase, transketolase, pyruvate decarboxylase, activity, oxythiamine, inhibition.

Effects of Chloramphenicol on Photosynthesis, Protein Profiles and Transketolase Activity under Extremely High CO2 Concentration in an Extremely-high-CO2-tolerant Green Microalga, Chlorococcum littorale

An extremely-high-CO2-tolerant alga, Chlorococcum littorale, showed high quantum efficiency of PSII (PhiII) in the light at 40% CO2, as well as at 5% CO2. However, PhiII decreased greatly when chloramphenicol (CAP) was added at 40% CO2, while no such decrease was observed at 5% CO2. Cycloheximide showed no effect on PhiII at either 5% or 40% CO2. The amount of a 76 kDa polypeptide (p76) on SDS-PAGE decreased markedly in the presence of CAP at 40% CO2 but not at 5% CO2. A partial amino acid sequence of p76 was 71-100% identical (10-14 identical residues out of 14 amino acids determined) to those of transketolases (TKLs) reported in higher plants and a cyanobacterium. In agreement with these observations, the TKL activity in C. littorale was decreased by CAP at 40% CO2, but not at 5% CO2. The transient decrease in TKL activity caused by CAP under 40% CO2 was well correlated with that in PhiII. These results indicate that the addition of CAP directly or indirectly influences the stability of TKL in C. littorale at 40% CO2, but not at 5% CO2, and that photosynthetic activity was reduced by a decrease in TKL activity.

[The effect of cocarboxylase treatment on erythrocyte transketolase and blood thiamine in patients with end stage renal disease undergoing maintenance hemodialysis]

The depressed ETKA in ESRD patients is supposed to be caused and/or aggravated by several factors among which the diminished content of thiamine in blood and/or disturbances of thiamine utilization seem to play the major role. This role stems from the fact that thiamine acts as the cofactor of transketolase. In order to check the therapeutic significance of this relationship we introduced the thiamine pyrophosphoric acid ester chloride (Cocarboxylasum-CC) administration in 25 patients (mHD + CC). Immediately after each HD performance CC was given i.v. during 12 weeks in a doses of 5 mg/kg b.w., 3 times a week. The blood for ETKA value, free and total thiamine in plasma and erythrocytes, as well as, the total protein and albumins/globulins index investigation was drawn before, after 6 and 12 weeks of CC administration, and 3 months after cessation of this therapy. In 10 patients, on maintenance HD nontreated by CC (mHD), the blood was drawn at the same time intervals. Normal values we obtained from 15 healthy volunteers. For ETKA evaluation photocolorimetric method was used, thiamine content in blood was estimated by fluorimetric method. At the beginning of the study the mean value of ETKA, in two examined groups, was found statistically decreased (p < 0.01) when compared with normals. Mean values of thiamine in plasma and erythrocytes were lower but did not differ significantly from those in normals. After 6 weeks of CC administration ETKA value increased, but only after 12 weeks it increased significantly (p < 0.01), reaching normal value. On the other hand, striking increase in plasma thiamine and erythrocyte thiamine levels was observed after 6 weeks of CC administration already (p < 0.01). Three months after cessation of CC administration significant decrease in ETKA value and thiamine level in blood was observed (p < 0.01). ETKA returned to lower value than in normals even in the presence of still high thiamine levels in blood. In mHD patients nontreated by CC the ETKA value and thiamine levels in blood did not change significantly during all periods of study. The nutritional status assessed by total protein and albumins/globulins index did not change in both groups through the study. We conclude, the administration of high doses of CC to ESRD patients on maintenance hemodialysis HD was successful in terms of increasing ETKA value and thiamine levels in blood without any side effects. Thus, supplementation with large doses of CC deserves further study because it promises to be another adjunct in the treatment of potential thiamine deficiency and metabolic disturbances in the course of dialysotherapy.

Oxythiamine and dehydroepiandrosterone induce a G1 phase cycle arrest in Ehrlich's tumor cells through inhibition of the pentose cycle

Transketolase (TK) reactions play a crucial role in tumor cell nucleic acid ribose synthesis utilizing glucose carbons, yet, current cancer treatments do not target this central pathway. Experimentally, a dramatic decrease in tumor cell proliferation after the administration of the TK inhibitor oxythiamine (OT) was observed in several in vitro and in vivo tumor models. Here, we demonstrate that pentose cycle (PC) inhibitors, OT and dehydroepiandrosterone (DHEA), efficiently regulate the cell cycle and tumor proliferation processes. Increasing doses of OT or DHEA were administered by daily intraperitoneal injections to Ehrlich's ascites tumor hosting mice for 4 days. The tumor cell number and their cycle phase distribution profile were determined by DNA flow histograms. Tumors showed a dose dependent increase in their G0-G1 cell populations after both OT and DHEA treatment and a simultaneous decrease in cells advancing to the S and G2-M cell cycle phases. This effect of PC inhibitors was significant, OT was more effective than DHEA, both drugs acted synergistically in combination and no signs of direct cell or host toxicity were observed. Direct inhibition of PC reactions causes a G1 cell cycle arrest similar to that of 2-deoxyglucose treatment. However, no interference with cell energy production and cell toxicity is observed. PC inhibitors, specifically ones targeting TK, introduce a new target site for the development of future cancer therapies to inhibit glucose utilizing pathways selectively for nucleic acid production.

Effects of magnesium, high energy phosphates, piracetam and thiamin on erythrocyte transketolase

Erythrocyte transketolase activity coefficient (ETK-AC) and affinity for coenzyme (Km TPP) were assessed in 50 patients with transketolase abnormalities such as fibromyalgia or senile dementia of Alzheimer's type, before and after magnesium (Mg), thiamin+pyridoxine (B1,B6), high energy phosphates (HEP) (phosphocreatinine of adenosine triphosphate), and piracetam. Compared to 12 untreated patients, ETK-AC was significantly decreased with B1,B6 (P < 0.05, n = 10); Km-TPP was significantly decreased with HEP (P < 0.05, n = 20) and piracetam (P < 0.01, n = 5). In nine other patients treated with HEP + B1,B6 + magnesium, ETK-AC and Km TPP were both significantly decreased.

[Effect of manganese on transketolase activity and total pentose content in rabbits]

Different doses of manganese are shown to exert a positive effect on the transketolase activity in the blood serum and in certain tissues as well as on the amount of total pentoses in the blood serum of rabbits. This confirms intensification of the pentose-phosphate exchange of carbohydrates under the effect of manganese.