Riboflavin 5'-pyrophosphate: a contaminant of commercial FAD, a coenzyme for FAD-dependent oxidases, and an inhibitor of FAD synthetase.

Commercially available preparations of flavin adenine dinucleotide (FAD) have been found to be 94% pure, the remaining 6% being composed of four or five minor contaminants which can be separated from FAD by reverse-phase high-performance liquid chromatography. FAD purified in this manner has been shown to be 100% pure. One of the contaminants has been identified as riboflavin 5'-pyrophosphate (RPP) by spectroscopic and chemical methods of analysis. This compound has been shown to exhibit biological activity as a weak cofactor for two FAD-requiring enzymes. With the apoprotein of porcine D-amino-acid oxidase, values determined for RPP were 8.4 microM for Km and 0.10 for Vmax compared to 0.47 microM and 0.28 (36 U/mg), respectively, for FAD. With fungal glucose apooxidase, values determined for RPP were 474 nM for Km and 0.02 for Vmax and 45 nM and 0.09 (105 U/mg), respectively, for FAD. RPP can also inhibit FAD biosynthesis. For bovine liver FAD synthetase, a Ki value for RPP against FMN was determined to be 9 microM where Km for FMN was 5.5 microM. These studies illustrate the value of riboflavin 5'-pyrophosphate as a flavin analog for use in the study of structure/function relationships within certain flavin-dependent enzymes.