Catalyzes the deamidation of nicotinamide, an early step in the NAD(+) salvage pathway. Prevents the accumulation of intracellular nicotinamide, a known inhibitor of poly(ADP-ribose) polymerases (PARP enzymes). H2O + nicotinamide = NH4(+) + nicotinate kcat is 0.93 sec(-1) with nicotinamide as substrate. Optimum pH is 6.5-7.0. Cofactor biosynthesis; nicotinate biosynthesis; nicotinate from nicotinamide: step 1/1. Expressed in roots and stems, and at lower levels in flowers, siliques and leaves. No visible phenotype under normal growth conditions, but mutant plants have decreased endogenous levels of NAD and NADP and display abnormal hypersensitivity to exogenous treatment with abscisic acid (ABA) and sodium chloride (NaCl). Belongs to the isochorismatase family.