In isolated rat enterocytes, both normal (normoenergized) and rotenone-deenergized, riboflavin intracellular metabolic processes, operating in association with membrane specific transport mechanism, were investigated. The time course contents of unlabeled (endogenous) and labeled (exogenous) riboflavin compounds (riboflavin, RF; flavin mononucleotide, FMN; and flavin-adenine dinucleotide, FAD) were determined by a HPLC analytical procedure before and after incubation with tritium labeled riboflavin. In normoenergized enterocytes total labeled riboflavin content, which is the sum of riboflavin membrane transported and intracellular metabolized, steadily increased to a plateau up to 20 min incubation; FMN content reached a plateau after 3-20 min, while FAD and free RF increased constantly. The phosphorylated forms prevailed over the free RF form, being about 60% of total flavins. In deenergized enterocytes total riboflavin content was significantly lower than in normoenergized enterocytes and reached a plateau after only 3 min incubation; FMN and FAD contents were significantly lower than in normoenergized enterocytes and free RF represented the prevailing form (about 70% of total RF content). In both normoenergized and deenergized enterocytes the contents of unlabeled total RF, FMN and FAD significantly decreased after 20 min incubation, whereas free RF significantly increased only in normoenergized enterocytes.
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