Abstract
Quinolinate phosphoribosyltransferase (QPRTase, EC 2.4.2.19) was purified and crystallized from Alcaligenes eutrophus subsp. quinolinicus IAM 12305 and hog kidney. The physicochemical properties such as molecular weight, sedimentation coefficient, stokes' radius etc, of the both QPRTases were almost the same. But the enzymatic properties of the two QPRTases were much different, e.g., specific activity and optimum pH. Alcaligenes QPRTase was inhibited by a product, nicotinic acid mononucleotide. On the other hand, hog kidney QPRTase was inhibited by a substrate, 5-phosphoribosyl-1-pyrophosphate at an alkaline and a physiological pHs but not an acidic pH. The type for this inhibition by 5-phosphoribosyl-1-pyrophosphate at an alkaline pH, was competitive for quinolinic acid. Therefore, 5-phosphoribosyl-1-pyrophosphate may play as a regulator in the biosynthesis of NAD from tryptophan.
