Enzymic formation of riboflavin 4′,5′-cyclic phosphate from FAD: evidence for a specific low-Km FMN cyclase in rat liver1

Abstract

An enzyme activity splitting FAD to AMP and riboflavin 4ʹ,5ʹ-cyclic phosphate (4ʹ,5ʹ-cFMN), with a Km of 6-8 μM, was partially purified from the cytosolic fraction of rat liver homogenates. 4ʹ,5ʹ-cFMN was characterized by enzyme, HPLC, UV-visible and NMR spectroscopic analyses. The data suggest that a novel enzyme, tentatively named FAD-AMP lyase (cyclizing) or FMN cyclase, is involved. Also, 4ʹ,5ʹ-cFMN was hydrolysed to 5ʹ-FMN by a rat liver cyclic phosphodiesterase. The results indicate a novel enzymic pathway for flavins in mammals, and support the biological relevance of 4ʹ,5ʹ-cFMN, perhaps as a flavocoenzyme or a regulatory signal.