NicotinamideN-Oxidation by CYP2E1 in Human Liver Microsomes

Abstract

Excess nicotinamide, a form of vitamin B 3 , is metabolized through two enzymatic systems and eventually excreted from the body. The first system starts with the methylation of nicotinamide by nicotinamide N-methyltransferase, which can subsequently be oxidized by aldehyde oxidase. The second enzymatic system oxidizes nicotinamide to nicotinamide N-oxide. It is located in the endoplasmic reticulum of hepatocytes but the precise enzyme is unknown. We have used human liver microsomes in combination with selective cytochrome P450 inhibitors, specific substrates, and antibodies to identify CYP2E1 as the main activity producing nicotinamide N-oxide. Our results suggest the potential use of nicotinamide N-oxide as a biomarker of CYP2E1 activity from urine or blood samples.

NAD+ metabolism: pathophysiologic mechanisms and therapeutic potential

“…Excess NAM that is not recycled is metabolized through two enzymatic systems and eventually excreted from the body. 55 The first system methylates the NAM into MNAM by NNMT, which utilizes the SAM as methyl donor. 56 The MNAM together with their oxidized compounds, 4py and 2py, are eventually eliminated in the urine.…”

“…While an acute pharmacological dose of NAM can be converted by CYP2E1 to nicotinamide N-oxide, which is then excreted to the urine. 55,58,59 Therefore, NNMT and CYP2E1 divert NAM from recycling to NAD + , restraining NAM accumulation and inhibition of NAD + -dependent signaling. 60 The Km of the human NNMT enzyme for NAM (approximately 430 μM) is much higher than the affinity of NAMPT for NAM (<1 μM), suggesting an unsaturated NNMT under normal conditions.…”

Nicotinamide N -Methyltransferase: More Than a Vitamin B3 Clearance Enzyme

“…The product of NNMT, MNAM, can be further oxidized by aldehyde oxidase (Aox) into two related compounds, N1-methyl-2-pyridone-5-carboxamide (2py) and N1-methyl-4-pyridone-3-carboxamide (4py), and all three metabolites are eventually excreted in the urine [2] (Figure 1). A secondary NAM clearance pathway starts with the direct oxidation of NAM to NAM N-oxide by cyp2E1, followed by elimination in the urine [3]. Under most conditions, methylation is quantitatively by far the predominant NAM clearance pathway with the exception of an acute pharmacological dose of NAM, which is mainly converted to NAM N-oxide [4,5].…”

Melatonin Alleviates the Toxicity of High Nicotinamide Concentrations in Oocytes: Potential Interaction with Nicotinamide Methylation Signaling

“…The vital role of NAD + , generated via NAM-mediated metabolism, in regulating metabolic homeostasis and activating the key enzymes responsible for cellular survival and longevity highlights the therapeutic potential of NAM [21]. However, under excessive NAM accumulation, the methylation pathway modulated by NAM-Nmethyltransferase (NNMT) to generate methylated NAM (N-methyl-nicotinamide, metNAM) [22] and the direct oxidation of NAM to form NAM-N-oxide through the effect of cytochrome P450 2E1 (CYP2E1) [23] are two metabolic pathways that can be activated. Unlike the normal conditions, NNMT is induced under the increase in the dietary NAM intake, which accordingly catalyzes the NAM hypermethylation pathway where different underlying metabolites are produced [24].…”