Hydrolysis of FMN and FAD by alkaline phosphatase of the intestinal brush-border membrane.

Both FMN and FAD were found to be hydrolysed with saturation kinetics by purified alkaline phosphatase (aPase E.C. 3.1.3.1) as well as by a brush-border membrane preparation (BBMp) from rat jejunum. With aPase the KM-value was 11.0 mmole/l when FMN was applied and 4.4 mmole/l when FAD was used. The apparent KM-values with the BBMp were calculated to be 22.9 mmole/l for FMN and 5.7 mmole/l for FAD as substrates. The BBMp contained FMN- and FAD-hydrolysing activity besides that due to the aPase. Regarding the high phosphatase activities associated with the brush-border membrane, it seems unlikely that FMN and FAD penetrate this membrane without being split. The transmural intestinal transport of 14C-riboflavin was tested in vitro in the presence of non-labelled FMN and FAD. The transport rate of the labelled riboflavin was found to be reduced by the coenzymes. It could be concluded that 14C-riboflavin competed with the non-labelled riboflavin released by the phosphatases for the binding sites of a hypothetical transport carrier.