Thiamine triphosphate: a ubiquitous molecule in search of a physiological role.

Thiamine triphosphate (ThTP) was discovered over 60 years ago and it was long thought to be a specifically neuroactive compound. Its presence in most cell types, from bacteria to mammals, would suggest a more general role but this remains undefined. In contrast to thiamine diphosphate (ThDP), ThTP is not a coenzyme. In E. coli cells, ThTP is transiently produced in response to amino acid starvation, while in mammalian cells, it is constitutively produced at a low rate. Though it was long thought that ThTP was synthesized by a ThDP:ATP phosphotransferase, more recent studies indicate that it can be synthesized by two different enzymes: (1) adenylate kinase 1 in the cytosol and (2) FoF1-ATP synthase in brain mitochondria. Both mechanisms are conserved from bacteria to mammals. Thus ThTP synthesis does not seem to require a specific enzyme. In contrast, its hydrolysis is catalyzed, at least in mammalian tissues, by a very specific cytosolic thiamine triphosphatase (ThTPase), controlling the steady-state cellular concentration of ThTP. In some tissues where adenylate kinase activity is high and ThTPase is absent, ThTP accumulates, reaching ≥ 70% of total thiamine, with no obvious physiological consequences. In some animal tissues, ThTP was able to phosphorylate proteins, and activate a high-conductance anion channel in vitro. These observations raise the possibility that ThTP is part of a still uncharacterized cellular signaling pathway. On the other hand, its synthesis by a chemiosmotic mechanism in mitochondria and respiring bacteria might suggest a role in cellular energetics.

Thiamin(e): the spark of life.

One of the earliest vitamins to be discovered and synthesized, thiamin was originally spelled with an "e". The terminal "e" was dropped when it was found that it was not an amine. It is still spelled with and without the "e" depending on the text. This chapter provides a brief historical review of the association of thiamin with the ancient scourge of beriberi. It emphasizes that beriberi is the model for high calorie malnutrition because of its occurrence in predominantly white rice consuming cultures. Some of the symptomatology of this ancient scourge is described, emphasizing the difference from that seen in starvation. High calorie malnutrition, due to excessive ingestion of simple carbohydrates, is widely encountered in the U.S.A. today. Thiamin deficiency is commonly associated with this, largely because of its cofactor status in the metabolism of glucose. The biochemistry of the three phosphorylated esters of thiamin and the transporters are discussed and the pathophysiology of thiamin deficiency reviewed. The role of thiamin, and particularly its synthetic derivatives as therapeutic agents, is not fully appreciated in Western civilization and a clinical section describes some of the unusual cases described in the scientific literature and some experienced by the author. The possible role of high calorie malnutrition and related thiamin deficiency in juvenile crime is hypothesized.

[Vitamin B1: metabolism and functions].

The review highlights metabolism and biological functions of vitamin B1 (thiamine). Thiamine transport systems, enzymes of its biosynthesis and degradation in various organisms, as well as molecular basis of thiamine-dependent hereditary patologies are considered. A special emphasis is given to discuss the role of thiamine triphosphate and adenylated thiamine triphosphate, a new thiamine derivative recently discovered in living cells.

Thiamine triphosphate, a new signal required for optimal growth of Escherichia coli during amino acid starvation.

Thiamine triphosphate (ThTP) is present in low amounts in most organisms from bacteria to humans, but its biological role remains unknown. Escherichia coli grown aerobically in LB medium contain no detectable amounts of ThTP, but when they are transferred to M9 minimal medium with a substrate such as glucose or pyruvate, there is a rapid but transient accumulation of relatively high amounts of ThTP (about 20% of total thiamine). If a mixture of amino acids is present in addition to glucose, ThTP accumulation is impaired, suggesting that the latter may occur in response to amino acid starvation. To test the importance of ThTP for bacterial growth, we used an E. coli strain overexpressing a specific human recombinant thiamine triphosphatase as a glutathione S-transferase (GST) fusion protein (GST-ThTPase). Those bacteria were unable to accumulate measurable amounts of ThTP. On minimal medium supplemented with glucose, pyruvate, or acetate, they exhibited an intermediate plateau in cell growth compared with control bacteria expressing GST alone or a GST fusion protein unrelated to thiamine metabolism. These results suggest that the early accumulation of ThTP initiates a reaction cascade involved in the adaptation of bacteria to stringent conditions such as amino acid starvation. This is the first demonstration of a physiological role of this ubiquitous compound in any organism.

A non-cofactor role of thiamine derivatives in excitable cells?

Thiamine diphosphate (TDP) is an important cofactor of pyruvate (PDH) and alpha-ketoglutarate (KGDH) dehydrogenases and transketolase. Thiamine deficiency leads to reversible and irreversible brain lesions due to impaired oxidative metabolism. A specific non-cofactor role for thiamine has also been proposed in excitable cells and thiamine triphosphate (TTP) might be involved in the regulation of ion channels. Thiamine is taken up by neuroblastoma cells through a high affinity transporter. Inside the cells, it is rapidly phosphorylated to TDP. This high turnover TDP pool is the precursor for TTP. Most of the TDP however has a low turnover and is associated with PDH and KGDH in mitochondria. In excised inside-out patches from neuroblastoma cells, TTP, at a concentration of 1 microM, activates chloride channels of large unitary conductance, the so-called maxi-Cl- channels. These channels are inhibited by oxythiamine from the outide. In addition to the role of TTP in the regulation of chloride channels, thiamine itself, or a presently unknown analog, may have trophic effects on neuronal cells.