Lung contains an inhibitor for nicotinatemononucleotide pyrophosphorylase (carboxylating) of NAD biosynthesis

Rat, cow and foal lung extracts contained an inhibitor for the liver NAD biosynthetic-pathway enzyme, nicotinatemononucleotide pyrophosphoryase (carboxylating) [EC 2.4.2.19]. The inhibitor was not dialyzable, was labile at 100°C, was retained by a 30, 000 dalton pore size Amicon membrane and, when partially purified by precipitation at 40-100% ammonium sulfate, inhibited the enzyme stoichiometrically. Lung reportedly does not contain nicotinatemononucleotide pyrophosphorylase or make NAD de novo. However, the inhibitor would mask detection of the enzyme in lung extracts. We detected a low nicotinatemononucleotide pyrophosphorylase-like activity (0.003 ± 0.001 nanomoles CO₂ produced from quinolinic acid per mg of extract protein) in rat lung but none in foal or cow lung.