59] Quinolinate phosphoribosyltransferase (beef liver)

Abstract

Publisher Summary This chapter describes the assay, purification, and properties of quinolinate phosphoribosyltransferase. It catalyzes the conversion of quinolinic acid to nicotinic acid ribonucleotide and CO 2 in the presence of 5-phosphoribosyl-1-pyrophosphate (PP-ribose-P). The enzyme is assayed routinely by measuring the PP-ribose-P-dependent formation of radioactive CO 2 from the α -carboxyl group of quinolinic acid, which is labeled with 14 C. Radioactive quinolinic acid to be used as the substrate can be prepared either enzymatically from tryptophan whose benzene ring is uniformly labeled with 14 C or chemically from uniformly labeled aniline- 14 C. A unit of the activity is defined as the amount of enzyme required to decarboxylate 1 micromole of quinolinate per minute. The enzyme is absolutely specific for quinolinic acid. The purified enzyme preparation is practically free from nicotinic acid ribonucleotide pyrophosphorylase. Mg 2+ is required for the activity with a sharp optimum concentration of 5 x 10 -4 M. Adenosine triphosphate (ATP) is not required for the enzyme. The enzymatic decarboxylation of quinolinic acid is not enhanced by ATP with any preparation of the enzyme.