Catalyzes the N-methylation of nicotinamide using the universal methyl donor S-adenosyl-L-methionine to form N1-methylnicotinamide and S-adenosyl-L-homocysteine, a predominant nicotinamide/vitamin B3 clearance pathway (PubMed:8182091. PubMed:21823666, PubMed:23455543). Plays a central role in regulating cellular methylation potential, by consuming S-adenosyl-L-methionine and limiting its availability for other methyltransferases. Actively mediates genome-wide epigenetic and transcriptional changes through hypomethylation of repressive chromatin marks, such as H3K27me3 (PubMed:26571212, PubMed:23455543, PubMed:31043742). In a developmental context, contributes to low levels of the repressive histone marks that characterize pluripotent embryonic stem cell pre-implantation state (PubMed:26571212). Acts as a metabolic regulator primarily on white adipose tissue energy expenditure as well as hepatic gluconeogenesis and cholesterol biosynthesis. In white adipocytes, regulates polyamine flux by consuming S-adenosyl-L-methionine which provides for propylamine group in polyamine biosynthesis, whereas by consuming nicotinamide controls NAD(+) levels through the salvage pathway (By similarity). Via its product N1-methylnicotinamide regulates protein acetylation in hepatocytes, by repressing the ubiquitination and increasing the stability of SIRT1 deacetylase (By similarity). Can also N-methylate other pyridines structurally related to nicotinamide and play a role in xenobiotic detoxification (PubMed:30044909). nicotinamide + S-adenosyl-L-methionine = 1-methylnicotinamide + S-adenosyl-L-homocysteine Inactivated by deimination on Arg-132. Cofactor metabolism. Amino-acid degradation. Monomer. Predominantly expressed in the liver. A lower expression is seen in the kidney, lung, skeletal muscle, placenta and heart. Not detected in the brain or pancreas. Deiminated by PADI1 and PADI2. Prominently expressed in the stroma of high-grade serous carcinomas (PubMed:31043742). In tumorigenesis, regulates the epigenetic reprograming of cancer cells associated with increased cell migration and metastasis (PubMed:23455543, PubMed:31043742). Belongs to the class I-like SAM-binding methyltransferase superfamily. NNMT/PNMT/TEMT family.