Akiyama T, Selhub J, Rosenberg I H
J Nutr. 1982 Feb;112(2):263-8. doi: 10.1093/jn/112.2.263.
Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), are two major coenzyme forms of dietary riboflavin. Yet little attention has been given to the release of the vitamin from its coenzyme forms during the absorptive process. Homogenates from rat intestine catalyze the hydrolases of these flavin coenzymes. To determine the location of FMN and FAD hydrolases, homogenates of intestinal mucosa were fractioned. FMN and FAD phosphatases were localized in brush border membranes. FAD pyrophosphatase activity was maximal at pH between 6.5 and 8.5 while FMN phosphatase has a pH optimum of 7.5 - 8.0. FAD pyrophosphatase is more stable to heat. The two enzymes separate on ion exchange chromatography of an isobutanol extract of intestinal brush border membrane fraction. Inhibition of 14C-riboflavin uptake by FMN and FAd in everted rings of rat intestine is directly related to the amount of conversion of these coenzymes to free riboflavin by intestinal enzymes. When FMN and FAD conversion to riboflavin is inhibited by EDTA, competition with 14C-riboflavin for transport was correspondingly decreased. These studies are best explained by a sequential process in which hydrolysis of FMN and FAD by enzymes of the intestinal brush border is followed by absorption of free riboflavin.
黄素单核苷酸（FMN）和黄素腺嘌呤二核苷酸（FAD）是膳食核黄素的两种主要辅酶形式。然而，在吸收过程中，维生素从其辅酶形式的释放很少受到关注。大鼠肠道匀浆可催化这些黄素辅酶的水解酶反应。为了确定FMN和FAD水解酶的位置，对肠黏膜匀浆进行了分级分离。FMN和FAD磷酸酶定位于刷状缘膜。FAD焦磷酸酶活性在pH 6.5至8.5之间最大，而FMN磷酸酶的最适pH为7.5 - 8.0。FAD焦磷酸酶对热更稳定。这两种酶在肠刷状缘膜部分的异丁醇提取物的离子交换色谱上分离。FMN和FAD对大鼠肠外翻环中14C-核黄素摄取的抑制作用与这些辅酶被肠酶转化为游离核黄素的量直接相关。当FMN和FAD向核黄素的转化被EDTA抑制时，与14C-核黄素的转运竞争相应降低。这些研究最好用一个连续的过程来解释，即肠刷状缘的酶先水解FMN和FAD，然后游离核黄素被吸收。
