The fluorescence characteristics of 3- and 7-hydroxycoumarin, and 7-hydroxy-and 7-methoxy-4-methylcoumarin, have been determined. 7-Hydroxycoumarin shows excited-state ionization from pH1 to 9. 2. A sensitive and specific fluorimetric method for the determination of 7-hydroxycoumarin (umbelliferone), and its application to liver homogenates and other tissue preparations, are described. 3. The enzymic hydroxylation of coumarin to 7-hydroxycoumarin has been studied by this method and the optimum conditions have been determined for rabbit-liver preparations. The enzymic activity was found in the microsomal fraction and required NADPH(2) and oxygen. Activity with NADH(2) was one-third of that with NADPH(2). 4. Addition of NADP was necessary for full activity of 10000g supernatant preparations of liver. Nicotinamide added during preparation preserved coenzymic activity in tissue stored at -12 degrees . Glucose 6-phosphate had no effect on the activity of stored or fresh tissue. 5. Inhibition occurred with p-chloromercuribenzoate, and with the usual inhibitors of the microsomal drug-metabolizing enzymes, SKF acid, SKF 525A, and Lilly 7132, but not with 2,2'-bipyridyl. 6. Liver homogenates from rabbit, guinea pig, coypu, cat and pigeon showed activity, but preparations of rat or mouse liver, and of locust fat bodies, did not hydroxylate coumarin to umbelliferone. The enzyme system was absent from rat-liver homogenates and microsomal preparations. Moreover, rat liver also contained inhibitors of the rabbit-liver coumarin-7-hydroxylase system and of the further metabolism of umbelliferone by guinea-pig liver. Guinea-pig-liver preparations hydroxylated coumarin to umbelliferone and then converted this product into its glucuronide. 7. The coumarin-7-hydroxylase activity of female rabbit liver was two to three times that of male rabbit liver.
