isoriboflavin

8-Demethyl-6-methylriboflavin; a riboflavin antimetabolite, differing from riboflavin in that the methyl groups on the isoalloxazine nucleus are in the 6,7 positions rather than the 7,8.

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iso·ri·bo·fla·vin (i″so-riґbo-fla″vin) an isomer of riboflavin that acts as an antimetabolite to riboflavin and can cause riboflavin deficiency in laboratory animals.