. "The enzyme, nicotinate nucleotide pyrophosphorylase, is specific for quinolinate. Its activity is strictly dependent on Mg2+ ions being present. A phosphoribosyl group is transferred to quinolinate to form nicotinate D-ribonucleotide. This reaction represents another rate-limiting step of the pathway from tryptophan to NAD+."^^ . . . . . . "A phospho-ribosyl group is added to quinolinate"^^ . "2.4.2.19"^^ . . . . . . . . . .