Subacute ruminal acidosis in high-producing dairy cows: an integrated diet-microbiota-epithelium-host framework for diagnosis and nutritional management.

The redox couples of NAD/NADH and NADP/NADPH and the thiol/disulfide system involving reduced glutathione (GSH) and its oxidized form (GSSG) play critical roles in regulating redox homeostasis. The NAD/NADH ratio is a key determinant of cellular redox status and plays a central role in regulating catabolic metabolism and energy production. A delicate equilibrium is maintained between the concentrations of free NADH and NAD on one hand and the levels of reduced and oxidized substrates for essential cellular dehydrogenases on the other hand. NADPH and NADP are essential for anabolism, antioxidant defense mechanisms, and regulation of the thiol/disulfide balance, thereby contributing to the overall redox status of the organism. The NADH/NAD and NADPH/NADP redox systems are interconnected through mitochondrial nicotinamide nucleotide transhydrogenase, which links these redox couples and contributes to their coordinated regulation. NADPH serves as a reducing agent during anabolic processes, such as the synthesis of fatty acids and amino acids []. The formation of NADH from NAD occurs mainly during glycolysis, the TCA cycle, and fatty acid (FA) oxidation. NADH supplies electrons to the electron transport chain, thereby producing ATP while being oxidized back to NAD. NAD also serves as a substrate for various enzymes, including NAD-dependent deacetylases (Sirtuins or SIRTs), Poly (ADP-ribose) polymerases (PARPs), NAD glycohydrolase, and cyclic ADP ribose (cADPR) synthase. These enzymes are involved in physiological and biochemical processes that include energy metabolism, DNA repair, regulation of gene transcription, epigenetic genome regulation, and cellular aging []. Moreover, NAD can be converted to NADP by NAD kinases. The redox ratio of NAD/NADH is a key regulator of cellular energy metabolism that influences the activity of glycolytic enzymes, the TCA cycle, oxidative phosphorylation (OXPHOS) in mitochondria, and other processes []. The NADPH-dependent thioredoxin (TRX) system operates in a similar manner using NADPH, while NADPH binding helps maintain catalase stability and activity. Low-molecular-weight thiol compounds, of which GSH is the most abundant, are ubiquitous in living organisms, where they serve as redox buffering agents that shield cells from the harmful effects of reactive molecules. The conversion of GSSG to GSH is catalyzed by glutathione reductase (GSR), which reduces GSSG to the potent antioxidant GSH. Glutathione serves as a substrate for other antioxidant enzymes, including glutathione-S-transferase (GST) and glutathione peroxidases (GPx) [,]. GSH can interact directly with oxidants and cysteine residues in proteins via the reversible process of S-glutathione conjugation. GSH also plays a crucial role in the recycling of ascorbate and provides reducing equivalents for the enzymes glutathione peroxidase (GPx) and peroxiredoxin (PRx) [,]. Methionine residues in proteins can also undergo reversible oxidative modification by their conversion to methionine sulfoxide, which can be reduced by methionine sulfoxide reductase enzymes [].