BIOSYNTHESIS OF THE MONOTERPENES LIMONENE AND CARVONE IN THE FRUIT OFCARAWAY - I - DEMONSTRATION OF ENZYME-ACTIVITIES AND THEIR CHANGES WITH DEVELOPMENT

The biosynthesis of the monoterpenes limonene and carvone in the fruit of caraway (Carum carvi L.) proceeds from geranyl diphosphate via a t hree-step pathway. First, geranyl diphosphate is cyclized to (+)-limon ene by a monoterpene synthase. Second, this intermediate is stored in the essential oil ducts without further metabolism or is converted by limonene-6-hydroxylase to (+)-trans-carveol. Third, (+)-trans-carveol is oxidized by a dehydrogenase to (+)-carvone. To investigate the regu lation of monoterpene formation in caraway, we measured the time cours e of limonene and carvone accumulation during fruit development and co mpared it with monoterpene biosynthesis from [U-C-14]Suc and the chang es in the activities of the three enzymes. The activities of the enzym es explain the profiles of monoterpene accumulation quite well, with l imonene-6-hydroxylase playing a pivotal role in controlling the nature of the end product. In the youngest stages, when limonene-6-hydroxyla se is undetectable, only limonene was accumulating in appreciable leve ls. The appearance of limonene-6-hydroxylase correlates closely with t he onset of carvone accumulation. At later stages of fruit development , the activities of all three enzymes declined to low levels. Although this correlates closely with a decrease in monoterpene accumulation, the latter may also be the result of competition with other pathways f or substrate.