Vitamin C inhibits the synthesis of a molecule essential for bacteria survival
That vitamin C, an anti-oxidant agent, boosts and strengthens immunity is well known. Its ability to speed-up recovery from tuberculosis and impede the TB causing bacteria from causing disease, and even kill the bacteria in culture at high concentration are also known. Now, a study by a team of researchers at the Indian Institute of Science (IISc), Bengaluru has found the molecular mechanism by which vitamin C impedes and even kills Mycobacterium smegmatis, a non-pathogenic bacterium that belongs to the same genus as the TB-causing mycobacteria. The results were published in the journal FEMS Microbiology Letters.
During times of stress or hostile conditions, such as increased temperature and presence of antibiotics, bacteria tend to come together and form a biofilm to protect themselves. The stress response pathway is crucial for bacteria to survive during hostile conditions. So blocking this pathway is a sure way of killing the bacteria.
In mycobacterium, the (p)ppGpp (Guanosine pentaphospahte or Guanosine tetraphosphate) is a key molecule in the stress response pathway. The (p)ppGpp is synthesised by Rel protein, which in turn is made by the Rel gene.
The team led by Dipankar Chatterji from the Molecular Biophysics Unit at IISc looked at the effects of vitamin C on the stress response pathway. “We chose vitamin C because its structure is similar to (p)ppGpp,” says Prof. Chatterji. “So we hypothesised that vitamin C should be competing to bind to the Rel enzyme and inhibiting (p)ppGpp synthesis.”
To test their hypothesis, the researchers conducted experiments using M. smegmatis. M. smegmatis is used as a model organism for TB-causing Mycobacterium tuberculosis.
Role of vitamin C
In vitro studies showed “significant” inhibition of (p)ppGpp synthesis in the presence of vitamin C. The inhibition level was seen to increase as the vitamin C concentration increased. The more the vitamin C concentration, the greater the possibility of vitamin C binding to the Rel enzyme, thus inhibiting (p)ppGpp synthesis. At about 10 mM concentration, the synthesis of (p)ppGpp molecule was completely inhibited.
The binding of vitamin C to the Rel enzyme is weak and this explains why high concentration of vitamin C is needed to inhibit (p)ppGpp synthesis.
“Using Mycobacterial cells we found that 1 mM of vitamin C produced 50% inhibition in (p)ppGpp synthesis. Vitamin C is able to get inside cells and inhibit (p)ppGpp synthesis,” says Kirtimaan Syal from IISc, the first author of the paper.
When 2 mM of vitamin C was added, “significant” defect in biofilm formation was seen. There was more than 50% reduction in viability of cells in a matter of four days when M. smegmatis was treated with 2mM of vitamin C. The viability of cells reduced even further with time, raising the possibility of therapeutic implications.
In the early literature, vitamin C deficiency was associated with pneumonia. After its identification, a number of studies investigated the effects of vitamin C on diverse infections. A total of 148 animal studies indicated that vitamin C may alleviate or prevent infections caused by bacteria, viruses, and protozoa. The most extensively studied human infection is the common cold. Vitamin C administration does not decrease the average incidence of colds in the general population, yet it halved the number of colds in physically active people. Regularly administered vitamin C has shortened the duration of colds, indicating a biological effect. However, the role of vitamin C in common cold treatment is unclear. Two controlled trials found a statistically significant dose–response, for the duration of common cold symptoms, with up to 6–8 g/day of vitamin C. Thus, the negative findings of some therapeutic common cold studies might be explained by the low doses of 3–4 g/day of vitamin C. Three controlled trials found that vitamin C prevented pneumonia. Two controlled trials found a treatment benefit of vitamin C for pneumonia patients. One controlled trial reported treatment benefits for tetanus patients. The effects of vitamin C against infections should be investigated further.
Vitamin C Has Effects on the Immune System
Vitamin C levels in white blood cells are tens of times higher than in plasma, which may indicate functional roles of the vitamin in these immune system cells. Vitamin C has been shown to affect the functions of phagocytes, production of interferon, replication of viruses, and maturation of T-lymphocytes, etc. in laboratory studies [1,23,42,43,44]. Some of the effects of vitamin C on the immune system may be non-specific and in some cases other antioxidants had similar effects.
The Diverse Biochemical, Physiological, and Psychological Effects of Vitamin C
Biochemistry textbooks usually mention the role of vitamin C in collagen hydroxylation. However, the survival time of vitamin C deficient guinea pigs was extended by carnitine  and by glutathione , which indicates that scurvy is not solely explained by defects in collagen hydroxylation, and it is not clear whether hydroxylation is important at all in explaining scurvy . Vitamin C participates in the enzymatic synthesis of dopamine, carnitine, a number of neuroendocrine peptides, etc. [6,47,48,49,50]. Vitamin C is also a powerful antioxidant, as mentioned above.
Experimentally induced vitamin C deficiency leads to depression and fatigue [11,51]. Recently, vitamin C was reported to improve the mood of acutely hospitalized patients [52,53]. Such effects cannot be explained by collagen metabolism, and vitamin C effects on the immune system are not plausible explanations either. Instead, the effects of vitamin C on the neuroendocrine system or carnitine metabolism might explain such effects. Thus, if vitamin C has beneficial effects on patients with infections, that does not unambiguously indicate that these effects are mediated by the immune system per se.
The Effects of Antioxidants against Infections May Be Heterogeneous
It is quite a common assumption that the effects of vitamins are uniform. Thus, if there is benefit, it is often assumed that the same benefit applies to all people. However, it seems much more likely that the effects of vitamins, including vitamin C, vary between people depending on biology and their lifestyle. Thus, it is possible that there are benefits (or harms) restricted to special conditions or to particular people. In the case of vitamin E, there is very strong evidence for the heterogeneity in its effects on pneumonia [54,55] and on the common cold . Although the factors modifying the effects of vitamin E cannot be extrapolated to vitamin C, it seems probable that there is comparable heterogeneity in the effects of vitamin C.