Metabolism of sinapic acid and related compounds in the rat.

1. Administration of sinapic acid to the rat results in the excretion of 3-hydroxy-5-methoxyphenylpropionic acid, dihydrosinapic acid, 3-hydroxy-5-methoxycinnamic acid and unchanged sinapic acid in the urine. The sinapic acid conjugate sinalbin is also catabolized to free sinapic acid and 3-hydroxy-5-methoxyphenylpropionic acid in the rat. 2. 3,4,5-Trimethoxycinnamic acid is metabolized in part to sinapic acid and 3-hydroxy-5-methoxyphenylpropionic acid. 3. 3,5-Dimethoxycinnamic acid is metabolized to 3-hydroxy-5-methoxycinnamic acid and 3-hydroxy-5-methoxyphenylpropionic acid. 4. The metabolic interrelationships of these compounds were studied by the administration of intermediates and a metabolic pathway is proposed. 5. The metabolism of the corresponding benzoic acids was studied, but these compounds and their metabolites were shown not to be intermediates or products of the metabolism of the related cinnamic acids.