4.1. The Oxidative Branch of PPPLooking at the oxidative irreversible part of PPP, apparently devoted to NADPH replenishment (Figure 1), the failure of glucose-6-phosphate dehydrogenase (G6PDH) activity, the enzyme which catalyzes the first and limiting step of the pathway, may determine dramatic pathological situations [44]. The deficiency of this enzyme derives from a genetic disorder, especially pronounced in specific world regions, characterized by several point mutations with a wide phenotypic heterogeneity [45,46].Such a deficiency leads to a number of severe pathophysiologic states, such as neonatal jaundice [47], chronic non-spherocytic haemolytic anaemia [48], predisposition to haemolytic crisis induced by drugs or infections [49,50,51]. The pathology defined as “favism”, in which compounds present in fava beans, such as divicine and isouramil are inducers of haemolysis, is emblematic [52]. The haemolysis linked to G6PDH deficiency may also be triggered in diabetes, in myocardial infarction, or even during extreme physical exercise [53,54,55]. Cardio circulatory dysfunctions also have been related to deficiency of G6PDH activity. Also in this case, the effect was ascribed to a decrease of NADPH levels, which leads to the loss of modulation of the redox status of endothelial cells, which in turn induces ROS accumulation [56,57,58]. Alterations at level of 6-phosphogluco-δ lactone dehydrogenase (6PGLDH), which catalyses the second irreversible step of PPP, are rarer. Deficiency of this enzyme have been reported to be associated to a reduction of red blood cell resistance [59,60]. However, possibly since it is not a limiting step for G6P flux into PPP, the loss of approximately 50% to 80% in 6PGLDH activity does not determine severe consequences as it occurs for the G6PDH deficiency.When 6-phosphogluco-δ lactone lactonase (6PGL) deficiency occurs on top of G6PDH deficiency, the situation may become more critical. In fact, in polymorphic variants of G6PDH in which no chronic hemolysis takes place, the concomitant lack of 6PGL may induce hemolysis [61]. Also, overexpression of G6PDH is associated with pathological states. This is the case, for instance, of the metabolic support offered by the over activity of the enzyme to cancer cell growth [62,63]. 4.2. The Non-Oxidative, Reversible Branch of PPPThe most evident role of the reversible branch of PPP is to modulate recovery of pentose sugars to feed, through glucose-6P resynthesis, the oxidative PPP branch when reducing power, rather than R5P-linked nucleotide synthesis, is required. This pathway consists of four enzymes: two isomerases and two transferases. The isomerases refer to a ribulose-5-phosphate isomerase (Ru5PI), converting ribulose-5P (Ru5P) to R5P and a Ru5P epimerase (Ru5PE) converting Ru5P to xylulose-5P (Xu5P). The transferases refer to a transketolase (TKT) and a transaldolase (TAL). TKT is a thiaminepyrophosphate-dependent enzyme able to transfer a two-carbon group (a glycolaldehyde residue) from an α-ketosugar to an aldose, as is the case of the reactions between Xu5P and R5P, generating glyceraldehyde-3P (Ga3P) and sedoeptulose-7P (Se7P), and between Xu5P and erythrose-4P (Er4P), generating Ga3P and F6P. Finally, TAL catalyzes the transfer of a three carbon group (dihydroxyacetone residue) from an α-ketosugar (i.e., F6P and Se7P) to an aldose (i.e., Ga3P and Er4P) (Figure 2).All of the reactions of this branch of PPP are reversible. However, nothing is more wrong than thinking of this pathway as a simple sealed container of processes at equilibrium. In fact, an articulated signaling network affecting the level of the involved enzymes occurs, making this pathway a relevant metabolic checkpoint for the cell health, in which different intermediates are functional links between the formation of NADPH and R5P and the different metabolic needs (Figure 2). Both TKT and TAL, which are differently represented in different tissues [64], catalyze relevant regulatory steps able to fulfil the main goals of PPP (i.e., to guarantee reducing power for anabolic demand and antioxidation protection and to allow nucleotides synthesis). The interplay of the enzymes involved in the two branches of PPP appears peculiar for different cells systems, leading, when altered, to cell type-specific pathologies [62,65]. Indeed, both the deficiency and the overexpression of TKT and TAL appeared to be linked to pathological states characterized by cell proliferation and cell death.Transketolase-like 1, one of the mutated transketolase transcript isoforms of TKT, was found overexpressed in a variety of tumors. Gastric, uterine, colorectal, endometrial, renal, thyroid and liver cancer [66,67,68,69,70,71,72] are characterized by an elevated level of the enzyme, which was suggested as a marker of poor prognosis of the tumor and as potential antitumor target. TAL and G6PDH, are over expressed in different hepatomas [62]. The overexpression of these enzymes, which catalyze limiting steps of the two branches of the PPP, should fulfil the increased need of nucleotide synthesis for tumor cells. This would be accomplished through the channeling of glucose towards R5P synthesis from both the oxidative and the reversible arms of the pathway. The former would proceed through NADP+ reduction and the latter would favour, through the TKT-dependent generation of Er4P, the utilization of the glycolytic intermediates F6P and Ga3P [64]. Indeed, the interconnection between G6PDH and TAL appears to occur both on a structural and a functional basis, being the two enzymes in human neutrophils involved in a supramolecular structure, which appears to control the overall flux of the PPP [73]. This event led to hypothesize that the two enzymes are part of a functional strategic device devoted to equilibrate the two arms of the PPP. In such a competitive game, however, as predicted by a theoretical analysis of factors affecting the fluxes of G6P metabolism in red blood cells [74], TAL overexpression appears to act as a dominating factor in determining an overall decrease in NADPH production and thus a loss of antioxidant ability [64]. On the other hand, in Jurkat human leukemic T cells, the overexpression of TAL is accompanied by a deficiency of both G6PDH and 6PGDH. In this case, no competition between the two PPP branches occurs and, as predictable, a concomitant significant loss of NADPH, reduced levels of GSH and high rate of ROS formation were observed. These cells are prone to apoptotic events induced by H2O2 and NO, by signaling factors as tumor necrosis factor-α, anti-FAS monoclonal antibodies or simply by serum deprivation [75].As it occurs for overexpression of TAL, the activation of the enzyme through phosphorylation resulted in a depletion of NADPH and induction of oxidative stress in fibroblasts from patients affected by Xeroderma pigmentosum or transformed by SV40. In these cell systems, the apparent involvement of TAL in the homeostasis of cell reduced state is concomitant with a low activity of catalase, another antioxidant enzyme whose activity could be restored by NADPH supplementation [76].Not only hyperactivity, but also a reduced activity of TKT and TAL has a significant impact on cell function. A reduced activity of TKT was shown in neurodegenerative diseases, diabetes mellitus and cancer [77,78] and associated in diabetes to an increase in circulating ribose and glycation phenomena. Indeed, while the complete deficiency of TK and G6PDH is lethal, TAL deficiency may be tolerated by some cell types and tissues, even though the human organism cannot survive in the absence of this enzyme. TAL deficiency is associated to a variety of different pathologies, as multiple sclerosis and rheumatoid arthritis as well as different tumors [79]. It is remarkable how the modulation of NADPH and ROS levels, exerted by TAL activity, affects the mitochondrial trans-membrane potential [79]. In this regard, both TKT and, more incisively, TAL deficiency were reported to activate in vivo the mitochondrial unfolded protein response (UPRmt) in Caenorhabditis elegans. In particular, it was shown how TAL deficiency determines, through complex oxidative stress and a starvation-like responses, mitochondrial morphology changes, impairment of mitochondrial respiration and decreased fats level, and increases the longevity of the animal. To conclude, the impact of TAL activity in modulating NADPH levels and controlling the ROS production mainly associated to mitochondrial oxidative processes, appears the result of the ability of the enzyme to favour R5P accumulation. In the case of TAL overexpression, R5P accumulation would occur through the recruitment of glycolytic precursors. In the case of TAL deficiency, the impairment of the recycle of R5P into G6P, limits the efficiency of the oxidative branch of PPI. In this respect, the accumulation of neurotoxic molecules, as erythritol, arabitol and ribitol, concomitant with a marked decline of NADPH levels, led to propose the involvement in the process of aldose reductase [80], a NADPH-dependent enzyme for which four and five carbon atom aldoses, including ribose, are indeed better substrates than glucose [81].We can complete the survey on the reversible branch of PPP by looking at the remaining two enzymes, namely Ru5PI and Ru5PE.These enzymes, through their action on Ru5P, connect the two branches of the pathway. In fact, the two reaction products, Xu5P and R5P, can be addressed towards G6P resynthesis, thus allowing the complete utilization of the glucose molecule and then the maximization of NADPH generation. On the other hand, R5P is the indubitable precursor for nucleotide synthesis either de novo, through further activation of R5P to PRPP, or salvage, through its isomerization to R1P by phosphopentomutase [82,83]. As mentioned above, also in this case, the flux of intermediates of the reversible branch of PPP is not functionally isolated. In fact, the epimerization of Ru5P to Xu5P affects lipogenesis, being Xu5P (together with glucose and fructose-1,6-bisphosphate) the inducer of lipogenic genes through the activation of a protein serine/threonine phosphatase acting on the ChREBP (Carbohydrate-responsive element binding protein) transcription factor [84,85,86]. Also, the efficiency of the isomerization reaction between Ru5P and R5P may enter in this metabolic control, which may dangerously evolve to cancer when significantly stressed, as reported, for instance, for Ru5P-isomerase overexpression in human hepatocellular carcinoma [87,88]. In this regard, we may conclude underlining that a body of experimental evidence merge in the conclusion that dysregulation of the PPP promotes carcinogenesis [89].All of these considerations evidence that an intervention at any point on PPP, which alters the level of an intermediate, may affect a complex body of processes whose relevance in being damaging or even beneficial on health has not yet been fully clarified [90]. Thus, ribose intake, forcing the generation of R5P by a high substrate concentration, besides pushing toward nucleotide synthesis, would influence PPP with not yet clearly assessed consequences. If this practice may be in principle understandable or even acceptable in severe pathological situations as described above, in which “possible” damage is overcome by “ascertained” damage linked to the pathological status, it is not anymore understandable when it is applied to healthy, even “tired” subjects. On the basis of the above considerations, it should be clear when and why the ribose supply might be beneficial. “When”, in the case the generation of pentose phosphates through the G6PDH/6PGDH couple is knockdown by a failure of the oxidative PPP and/or in the case of a failure of the reversible PPP branch in recruiting glycolysis intermediate for R5P synthesis. “Why”, in the attempt to counteract an emergency. It should be obvious, however, that the treatment should occur under a strict medical control and for a proper limited time. This is due to the fact that the metabolic and functional “price” of such a therapeutic approach may be high (see below).