NAD

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High NAD/NADH ratio in cytoplasm, low in mitochondria

In rat liver it was found that NAD/NADH ratio differs significantly between cytoplasm and mitochondria:

3. The mean NAD+/NADH ratio of rat-liver cytoplasm was calculated as 725 (pH7·0) in well-fed rats, 528 in starved rats and 208 in alloxan-diabetic rats.
4. The NAD+/NADH ratio for the mitochondrial matrix and cristae gave virtually identical values in the same metabolic state. This indicates that β-hydroxybutyrate dehydrogenase and glutamate dehydrogenase share a common pool of dinucleotide.
5. The mean NAD+/NADH ratio within the liver mitochondria of well-fed rats was about 8. It fell to about 5 in starvation and rose to about 10 in alloxan-diabetes.
6. The NAD+/NADH ratios of cytoplasm and mitochondria are thus greatly different and do not necessarily move in parallel when the metabolic state of the liver changes. (R7)

Same paper shows that starving reduces the ratio by 27% in cytosol and by 37% in mitochondria.

Transporters

SLC5A8 - monocarboxylates carrier (SMCT1)

Transports monocarboxylates in sodium and chloride-dependent manner. Monocarboxylates: short-chain fatty acids including L-lactate, D-lactate, pyruvate, acetate, propionate, valerate and butyrate, mocarboxylate drugs (nicotinate, benzoate, salicylate and 5-aminosalicylate) and ketone bodies (beta-D-hydroxybutyrate, acetoacetate and alpha-ketoisocaproate).

Na+:substrate stoichiometry of between 4:1 and 2:1.

Catalyzes passive carrier mediated diffusion of iodide. Mediates iodide transport from the thyrocyte into the colloid lumen through the apical membrane.

May be responsible for the absorption of D-lactate and monocarboxylate drugs from the intestinal tract.

May play a critical role in the entry of L-lactate and ketone bodies into neurons by a process driven by an electrochemical Na+ gradient and hence contribute to the maintenance of the energy status and function of neurons.

Short- and medium- chain fatty acids inhibit uptake of Nicotinate

(14C)Nicotinate uptake was several-fold higher in SMCT1-expressing oocytes than in water-injected oocytes. The uptake was inhibited by short-chain/medium-chain fatty acids and various structural analogs of nicotinate.

Exposure of SMCT1-expressing oocytes to nicotinate induced Na+-dependent inward currents. Measurements of nicotinate flux and associated charge transfer into oocytes suggest a Na+:nicotinate stoichiometry of 2:1.

Monocarboxylate drugs benzoate, salicylate, and 5-aminosalicylate are also transported by human SMCTI. (R5)

TNF-a downregulates SMCT1, L. plantarum upregulates it

Rat intestinal epithelial cell (IEC)-6 or human intestinal Caco-2 cells were treated with TNF-α in the presence or absence of Lactobacilli culture supernatants (CS).

TNF-α treatments for 24 h dose-dependently inhibited SMCT1-mediated, Na(+)-dependent butyrate uptake and SMCT1 mRNA expression in IEC-6 cells and SMCT1 promoter activity in Caco-2 cells.

CS of L. plantarum (LP) stimulated Na(+)-dependent butyrate uptake (2.5-fold, P < 0.05), SMCT1 mRNA expression, and promoter activity.

Furthermore, preincubating the cells with LP-CS followed by coincubation with TNF-α significantly attenuated the inhibitory effects of TNF-α on SMCT1 function, expression, and promoter activity. In vivo, oral administration of live LP enhanced SMCT1 mRNA expression in the colonic and ileal tissues of C57BL/6 mice after 24 h.

Efficacy of LP or their secreted soluble factors to stimulate SMCT1 expression and function and to counteract the inhibitory effects of TNF-α on butyrate absorption could have potential therapeutic value. (R2)

Gene silencing by hypermethylation

The SLC5A8 gene has been reported to function as a tumor suppressor gene that contributes to carcinogenesis and tumor progression. The expression of SLC5A8 is silenced in colon neoplasia by hypermethylation of CpG-rich islands located in exon 1.

Gene silencing of SLC5A8 by hypermethylation was associated with poor prognosis in cases of node-negative stage I and II lung AD. (R3)

Insulin and SGK1 reduce function of SMCT1

We studied the effect of insulin on the function of human SMCT1 expressed in Xenopus oocytes. The addition of insulin reduced α-keto-isocaproate (KIC)-dependent 22Na+ uptake by 29%.

Consistent with this result, the coinjection of SMCT1 with SGK1 cRNA decreased the KIC-dependent 22Na+ uptake by 34%. (R4)

Ibuprofen inhibits SLC5A8

In mammalian cells, hSMCT1- mediated nicotinate uptake was inhibited by ibuprofen and other structurally related NSAIDs. The inhibition was Na+ dependent. With ibuprofen, the concentration necessary for 50% inhibition was 64 +/- 16 microM. In oocytes, the transport function of hSMCT1 was associated with inward currents in the presence of propionate.

Under identical conditions, ibuprofen and other structurally related NSAIDs failed to induce inward currents.

However, these compounds blocked propionate-induced currents. With ibuprofen, the blockade was dose dependent, Na+ dependent, and competitive. However, there was no uptake of [3H]-ibuprofen into oocytes expressing hSMCT1, although the uptake of [14C]-nicotinate was demonstrable under identical conditions. (R6)

SLC22A13

Anion antiporter that mediates the transport of orotate and nicotinate in exchange for organic or inorganic anions (PubMed:35144162).
Translocates orotate across the apical membrane of proximal tubule epithelial cells. Possibly involved in orotate renal reabsorption and nicotinate intestinal reabsorption.

SLC25A51 - mitochondrial NAD carrier

With the realization that SLC25A51 (or MCART1) represents the major mitochondrial NAD+ carrier in mammals, a long-standing mystery in NAD+ biology has been resolved. (R1)

Absorption of NAD+

Bioavailability studies indicated that ingested NAD+ was primarily hydrolyzed in the small intestine by brush border cells. As a first step, NAD+ is cleaved to NMN and 5′-AMP by a pyrophosphatase found either in intestinal secretions or in the brush border. Next, NMN is rapidly hydrolyzed to NR, which in turn is more slowly converted into NAM.

NAM can also be formed directly by the cleavage of NAD+, obtaining ADP-ribose derivates as a side product.

The intestinal production of NAM from NAD+ or NR required the presence of intestinal cells, indicating that the enzymes for this process are membrane-bound or intracellular. The direct perfusion with NAM, however, did not give rise to any of these species, indicating that NAM is the final degradation product and directly absorbed.

In contrast, perfusion of the intestine with NA revealed a substantial cellular accumulation of labeled intermediates of the NAD+ biosynthetic pathway, including NAM, which suggest the presence of active NA metabolism in intestinal cells. In line with this, blood concentrations of NA are relatively low (∼100 nM) yet, when pharmacologically primed, can increase and be rapidly converted to NAM by the liver.

Strikingly, NAM levels in fasted human plasma are also too low to support NAD+ biosynthesis in cells (between 0.3 and 4 μM).

All of these results suggest that these NAD+ precursors are metabolized very quickly in mammalian blood and tissues. (R8)

Note: The paper R8 cites quite old papers from 1983 and even 1972.