Enzymology of extracellular NAD metabolism

Abstract

Extracellular NAD represents a key signaling molecule in different physiological and pathological conditions. It exerts such function both directly, through the activation of specific purinergic receptors, or indirectly, serving as substrate of ectoenzymes, such as CD73, nucleotide pyrophosphatase/phosphodiesterase 1, CD38 and its paralog CD157, and ecto ADP ribosyltransferases. By hydrolyzing NAD, these enzymes dictate extracellular NAD availability, thus regulating its direct signaling role. In addition, they can generate from NAD smaller signaling molecules, like the immunomodulator adenosine, or they can use NAD to ADP-ribosylate various extracellular proteins and membrane receptors, with significant impact on the control of immunity, inflammatory response, tumorigenesis, and other diseases. Besides, they release from NAD several pyridine metabolites that can be taken up by the cell for the intracellular regeneration of NAD itself. The extracellular environment also hosts nicotinamide phosphoribosyltransferase and nicotinic acid phosphoribosyltransferase, which inside the cell catalyze key reactions in NAD salvaging pathways. The extracellular forms of these enzymes behave as cytokines, with pro-inflammatory functions. This review summarizes the current knowledge on the extracellular NAD metabolome and describes the major biochemical properties of the enzymes involved in extracellular NAD metabolism, focusing on the contribution of their catalytic activities to the biological function. By uncovering the controversies and gaps in their characterization, further research directions are suggested, also to better exploit the great potential of these enzymes as therapeutic targets in various human diseases.

The Extracellular NADome Modulates Immune Responses

“…When in the extracellular space, eNAD functions are linked to the modulation of cell surface P2X and P2Y purinergic receptor families, thereby acting in an apparently enzyme-independent way and eliciting pro-inflammatory immune responses ( Figure 1 ). In addition, within the extracellular space, a complete network of different ectonucleotidases can rapidly hydrolyze eNAD generating intermediates that modulate signaling, cell metabolism, adhesion, migration and activate immunoregulatory circuits ( 14 , 39 , 47 ), as summarized in Figure 1 . eNAD is degraded by different classes of ectoenzymes: the NADases CD38 and CD157 ( 48 – 50 ), the ADP-ribosyltransferases (ARTs) ( 51 ), the Ectonucleotide Pyrophosphatase/Phosphodiesterase 1 (ENPP1) and the ecto-5′-nucleotidase CD73 ( 34 , 52 , 53 ).…”

“…All these mechanisms of nucleotide/nucleoside release to alert or switch off the immune system, respectively, are enhanced during acute and chronic inflammation, including cancer ( 29 , 37 , 38 ). Even though very unlikely, eNAD synthesis has not been conclusively ruled out, also in consideration of the presence of several key NAD biosynthetic enzymes (NBEs) ( 16 , 39 ).…”

Preparation of ZIF@ADH/NAD-MSN/LDH Core Shell Nanocomposites for the Enhancement of Coenzyme Catalyzed Double Enzyme Cascade

“…The loss of enzyme activities may occur as follows: (1) The centrifugal recovery of ZIF@ADH/NAD-MSN/LDH resulted in enzyme leakage and reduced enzyme activity [69]. (2) The acetaldehyde catalyzed by ethanol would affect the stability and catalytic effect of the enzyme in the circulation system. (3) The efficiency drops majorly resulted from the inevitable polymer weight loss in the separation from the reacting suspension [69].…”

“…The coenzyme regeneration is to regenerate the coenzyme from the oxidized state to the reduced state, or vice versa, so as to keep the coenzyme at a certain level of catalyst amount. In recent years, in order to solve the problem of coenzyme regeneration, a series of methods have been proposed, including regeneration systems such as chemistry, photochemistry, enzymology, and electrochemistry [ 1 , 2 , 3 , 4 ].…”

Dihydronicotinamide Riboside Is a Potent NAD+ Precursor Promoting a Pro-Inflammatory Phenotype in Macrophages

“…Several extracellular NADmetabolizing enzymes rapidly degrade extracellular NAD + and its precursors into metabolites such a nicotinamide mononucleotide (NMN), nicotinamide riboside (NR), adenosine diphosphate ribose (ADPR), adenosine, and nicotinamide (NAM) that can be further metabolized, serving as signaling molecules or precursors for NAD synthesis. Once taken up by cells, these NAD metabolites and vitamin B3 derivatives can be used directly or converted intracellularly into other metabolites that will be used to regenerate the intracellular NAD pool (1,2).…”