Abstract

Grapefruit juice inhibits the metabolism of substrates for enzymes of the CYP3A subfamily. However, the identity of the inhibitor has not been established. Grapefruit juice was extracted into methylene chloride and chromatographed by HPLC, and the effect of the HPLC eluent on CYP3A activity was assessed by measuring 6beta-hydroxytestosterone formation in rat liver microsomes. Significant inhibition was associated with a fraction of HPLC eluent containing a single peak, with a retention time of 16 min. The substance producing this peak was isolated using TLC and identified, using NMR and MS, as 6',7'-dihydroxybergamottin (C21H2406; molecular weight, 372), a furanocoumarin (psoralen) compound. The concentration of 6',7'-dihydroxybergamottin required to inhibit 6beta-hydroxytestosterone formation by 50% was 25 microM. Grapefruit juice reduced CYP3A activity to a significantly greater extent (p < 0.05) than did orange juice, which contained no measurable 6',7'-dihydroxybergamottin (28.6% vs. 62.2% of control activity). The addition of 6',7'-dihydroxybergamottin (30 microM) to orange juice decreased CYP3A activity to values comparable to those observed with grapefruit juice. 6',7'-Dihydroxybergamottin is a potent inhibitor of CYP3A activity, accounts for the difference in inhibition between grapefruit juice and orange juice in vitro, and may be primarily responsible for the effects of grapefruit juice on cytochrome P450 activity in humans.