# Cyclic-FMN Is a Detectable, Putative Intermediate of FAD Metabolism

## TL;DR

Scientists synthesized and studied a compound called cyclic-FMN, which may be an intermediate in FAD metabolism and can now be detected in biological samples.

## Contribution

The paper introduces a new method to synthesize and detect cyclic-FMN, a potential intermediate in FAD metabolism.

## Key findings

- Cyclic-FMN can be synthesized from FAD using ZnSO4 and is stable under acidic and basic conditions.
- Cyclic-FMN can be extracted and detected in biological samples using liquid chromatography and mass spectrometry.
- Liver tissue extracts can hydrolyze cyclic-FMN into FMN and riboflavin, though the exact mechanism is unknown.

## Abstract

Free flavin adenine dinucleotide (FAD) is metabolized to flavin mononucleotide (FMN) and adenine monophosphate (AMP) by hydrolases and to 4′,5′-cyclic phosphoriboflavin (cFMN) and AMP by the triose kinase FMN cyclase (TKFC). Yet, the lack of analytical standards for cFMN might have resulted in the incidence of cFMN in biological specimens being underreported. To address this shortcoming, cFMN was synthesized from either FMN or FAD. The optimization of the FAD to cFMN reaction conditions revealed that an equimolar ratio of ZnSO4 and FAD yielded pure cFMN upon the precipitation of AMP-Zn salts. cFMN is stable to aqueous acidic and basic conditions and is readily extracted from biological samples for detection by liquid chromatography coupled with mass spectrometry. Although cFMN is hydrolyzed by liver tissue extracts to FMN and riboflavin, the mechanisms for this conversion remain elusive.