Balicalin, the predominant flavone glucuronide of scutellariae radix, is absorbed from the rat gastrointestinal tract as the aglycone and restored toits original form

When baicalin was orally administered to conventional rats, it was detected in their plasma for 24 h after administration, but baicalein, the aglycone of baicalin, was not detected. However, when baicalin was given to germ-fr ee rats, only a small amount of baicalin was detected in their plasma withi n 2 h after the administration, its AUC(O-lim) (the area under the concentr ation-time curve from 0 to last determination time) being 12.0% of that in conventional rats. Subsequently, a considerable amount (55 .1+/-6 .2%) of b aicalin was recovered from the gastrointestinal tract even 4 h after admini stration. When baicalein was orally administered to conventional rats, howe ver, baicalin appeared rapidly in their plasma at an AUC(O-lim) value simil ar to that obtained after oral administration of baicalin, despite the abse nce of baicalein in plasma. When intestinal absorption was evaluated by the rat jejunal loop method, baicalein was absorbed readily, but only traces o f baicalin were absorbed. Moreover, in conventional rats a small amount (13 . 41+/-3 .1%) of baicalin and an appreciable amount (21 .9+/-3 .4%) of bai calein were recovered from the gastrointestinal tract even 4 h after oral a dministration of baicalin, but only a small amount (3 . 93+/-1 . 43%) of ba icalein was detected in the intestinal tract Ih after administration of bai calein. Baicalin was transformed to baicalein readily by the rat gastric and caecal contents. When baicalin was administered orally to conventional rats, an a ppreciable amount of baicalein was recovered in their gastrointestinal trac ts. Moreover, baicalein was efficiently conjugated to baicalin in rat intes tinal and hepatic microsomes. These results indicate that baicalin itself is poorly absorbed from the rat gut, but is hydrolysed to baicalein by intestinal bacteria and then restor ed to its original form from the absorbed baicalein in the body.