Uptake of Nicotinic Acid and Nicotinamide by Rat Erythrocytes

Abstract

Abstract The uptake of nicotinic acid and nicotinamide by rat erythrocytes consists of two processes, diffusion and enzyme-catalyzed conversion to nucleotides which do not readily diffuse from the cell. These two processes provide for a very rapid removal of these compounds, especially nicotinic acid, from the external medium. The two processes have been investigated separately with the use of 14C-labeled substrates and fluoride to prevent the formation of nucleotides. Inorganic phosphate stimulates the formation of nucleotides 2- to 3-fold at an optimum concentration of 20 mm. Glycolytic inhibitors other than fluoride, including arsenate and iodoacetate, also arrest the formation of nucleotides. Replacement of glucose with 2-deoxyglucose likewise prevents nucleotide formation. The passage of nicotinic acid and its amide into and out of fluoride-inhibited erythrocytes differed greatly. The passage of the acid ceased when the temperature was lowered to 0° and exhibited a Q10 of about 2 for uptake between 10° and 30°, suggesting that it was transported by facilitated diffusion. On the other hand, free diffusion can account for the rapid uptake and removal of nicotinamide from the cells. Equilibration occurred within 1 min at temperatures between 0° and 37°. Quinolinic acid was taken up only very slightly, and there was no evidence of nicotinic acid mononucleotide formation.