# Defining NAD(P)(H) catabolism
## 4 Ribosylated Catabolites of NAD(P)(H)  
While less often measured than Me-PYs, carboxamide pyridone ribosides (PYRs) have been detected in biological samples since the 1970s and were more formally characterized and isolated from human urine in the 1980s [111; 112]. Once again, three isomeric forms exist based on the site of oxidation (Figure 5; Table 1). In human urine, the 1-ribosylpyridin-4-one-3-carboxamide (4-PYR; Figure 1) is found to be the most abundant circulating PYR [113]. The next most abundant isomer is 1-ribosylpyridin-6-one-3-carboxamide (6-PYR; Figure 5), also sometimes referred to 1-ribosylpyridin-2-one-5-carboxamide in the literature [114]. The last isomer is 1-ribosylpyridin-2-one-3-carboxamide (2-PYR), found to be much less abundant. Only a few of the PYR phosphorylated derivatives (Figure 5) have been described and characterized in the literature to date (Table 1). These derivatives include the nucleotide series, for which PYR can be mono, di, or triphosphorylated (PYR-NP), or conjugated to an adenosine diphosphate unit, as a pyridone adenine dinucleotide (ox-NAD; Figure 5) [111; 114; 115; 116; 117; 118]. Although PYR derivatives in bio-specimens have been quantified for more than 40 years, their origin remains mostly speculative. A biochemical relationship between the methylated (N-Me-PYs) and ribosylated (PYRs) forms of the pyridone species has been proposed but has yet to be identified. Instead, we posit that such a relationship does not exist and that PYRs were generated from the ribosylated forms of nicotinamide by over-oxidation of the pyridinium ring. The 2 and the 6 isomer ribosylated catholites of NR, NMN, NAD, and NADP can be generated by Fenton chemistry [70]. However, this chemistry does not account for the formation of the 4 isomer. [PERSON] observed that an over-oxidized form of ox-NADP (Figure 5) could be generated by the flavin oxidoreductase, adrenodoxin (FDXR [119; 120; 121], while we observed that in the presence of oxygen, NQO2 could oxidize NRH but not NR to 4-PYR [62; 122]. This observation indicates that electron transfer to oxygen via FADH\({}_{2}\) is necessary and sufficient to enable the reaction between superoxide and pyridinium ring to generate the 4-hydroxylated pyridinium that then iso  
Figure 5: Hyperoxidized ribosylated catholites of NAD(P). R = H, PYR; R = PO(OH)\({}_{2}\), PYR-MP; R = PO(OH)OPO(OH)\({}_{2}\), PYR-DP; R = PO(OH)OPO(OH)OPO(OH)\({}_{2}\), PYR-TP; R = ADP; ox-NAD; R = APDP; ox-NADP. merizes to the pyridone. This mechanism is supported by the fact that NQO2 can generate superoxide [123], that can react with the pyridium ring of NR\({}^{+}\). One can consider that NQO1, a component of complex I, might facilitate NAD\({}^{+}\) over-oxidation (4-ox-NAD [124], Figure 6) under conditions that favor superoxide formation rather than electron transport during oxidative phosphorylation. The over-oxidation of NAD\({}^{+}\) to 4-ox-NAD offers the platform for the formation of 4-PYR-MP via pyrophosphatase activity, and 4-PYR via phosphatases, offering a mechanism for the formation of circulating 4-PYR. Aside from the potential of 4-PYR as a possible indicator of tumor burden in malignancy [125], its abundance has been closely associated with the aging process and nephrotic dysfunction [126]. 4-PYR was also identified as one of the markers of good prognosis for survival in AIDS patients, and an independent predictor for AIDS progression [127]. Unlike the 2-PYR or the 6-PYR, this isomer of the pyridone series possesses a quinonoid structure that can act as an electrophile and interact with nucleophiles like cysteine and glutathione, in addition to DNA in a manner like a quinone. In the year 1979, [PERSON] and his team isolated and characterized 4-PYR from the urine samples of chronic myelogenic leukemia patients which was reported as the first pyridine-containing nucleoside derived from patient urine samples [112]. 4-PYR is commonly found in the plasma of healthy individuals in the nano-molar range (0.013 \(\pm\) 0.006 uM).