trans-alpha, beta-Diformamido-beta-(5'-phosphoribosylamino)acrylamide: a possible new intermediate in de novo purine...

The nucleotide trans-alpha, beta-diformamido-beta-(5'-phosphoribosylamino)acrylamide (DAR) has been chemically synthesized and is converted to inosine 5'-phosphate (IMP) by enzyme activities found in chicken, rat, and human liver. The increase in optical density at 250 nm when DAR is converted to IMP is used as the basis of the assay. The Km values for DAR at pH 7.4 were 2.8 and 4.2 microM with the chicken and rat liver enzymes, respectively. The integrated Michaelis--Menten equation was used to determine the kinetic parameters of the chicken liver enzyme from pH 5.6 to 10.1. The pH--activity profiles show ionizations with pKa values of 6.1, 7.1, and 8.8. The possibilities that DAR is a substrate analogue or a new intermediate in the pathway of purine biosynthesis de novo are discussed.