Morphine Metabolism in the Opium Poppy and Its Possible Physiological Function

Abstract

We identified a novel metabolic system of morphine in the opium poppy (Papaver somniferum L.). In response to stress, morphine is quickly metabolized to bismor-phine consisting of two morphine units, followed by accumulation in the cell wall. This bismorphine binds predominantly to pectins, which possess high galacturonic acid residue contents, through ionical bonds. Our newly developed method using artificial polysaccharides demonstrated that bismorphine bridges are formed between the two amino groups of bismorphine and the carboxyl groups of galacturonic acid residues, resulting in cross-linking of galacturonic acid-containing polysaccharides to each other. The ability of bismorphine to cross-link pectins is much higher than that of Ca 2 , which also acts as a cross-linker of these polysaccharides. Furthermore, we confirmed that cross-linking of pectins through bismorphine bridges leads to resistance against hydrol-ysis by pectinases. These results indicated that production of bismorphine is a defense response of the opium poppy. Bismorphine formation is catalyzed by anionic peroxidase that pre-exists in the capsules and leaves of opium poppies. The constitutive presence of morphine, together with bismorphine-forming peroxidase, enables the opium poppy to rapidly induce the defense system. In response to mechanical damage, the immature capsules of the opium poppy (Papaver somniferum) and related species (Papaver setigerum, etc.) immediately secrete opium consisting of various secondary constituents such as morphine, codeine, papaverine, and noscapine. Among these, morphine has attracted a great deal of attention as one of the most medicinally important analgesics and narcotics. Therefore, numerous studies on morphine have been carried out since its first isolation in 1804 by Sertü rner (1), but why the opium poppy produces this compound remains unknown. Morphine is structurally classified into alkaloid, based on the presence of nitrogen in its molecule. Many higher plants including the opium poppy synthesize a variety of alkaloids, and like urea and uric acid in animals, these alkaloids have often been suggested to be produced as nitrogenous waste products with little physiological importance for host plants. However, this hypothesis lacks precise experimental evidence (2). In addition, the significant physiological roles of some al-kaloids in host plants have been demonstrated by characterizing the biological properties of the alkaloids and their metab-olites; steroid alkaloids such as tomatine and solanidine are involved in protecting plants against herbivores and microbial pathogens (3), whereas the pyridine alkaloids, trigonelline (N-methylnicotinic acid) and N-arabinosylnicotinic acid act as precursors of the vitamin nicotinic acid (4, 5). Therefore, it is not reasonable to regard all alkaloids including morphine as waste products, although in contrast to these steroidal and pyridine alkaloids, little information is available concerning the physiological function of morphine in opium poppy. Important information on the physiological roles of several plant secondary constituents as well as the above pyridine alkaloids can be obtained by investigating the properties of their metabolites. For example, the main steroid saponins of oat (Avena sativa), avenacosides A and B, have been shown to be metabolized by endogenous-glucosidase into 26-desglu-coavenacosides A and B, respectively, which function as anti-bacterial substances against pathogens (6, 7). Furthermore, we recently investigated the metabolic pathway of flavone glucu-ronide in the skullcap plant (Scutellaria baicalensis Georgi), and the metabolite (baicalein) of baicalein 7-O-D-glucuronide has been shown to play an important role in detoxification of the large amount of H 2 O 2 produced by the oxidative burst (8-11). These results indicated that precise understanding of morphine metabolism may provide the useful evidence from which its physiological importance in the opium poppy is inferred. Recent studies have almost completely elucidated the biosynthetic mechanism of morphine (12), but metabolism of morphine in the opium poppy is largely unknown. Therefore, to determine the physiological importance of morphine , we investigated its metabolism in the opium poppy. Our results indicated that in response to mechanical damage, morphine immediately undergoes oxidation by bismorphine-forming peroxidase (BFP) 1 and is metabolized into the dimer of morphine, bismorphine. Biochemical characterization of bismorphine demonstrated that the two amino groups of this alkaloid ionically bind to the carboxyl groups of the galacturonic acid residues of cell wall polysaccharides pectins, resulting in cross-linking of pectins to each other. Furthermore, we confirmed that binding of bismorphine to pectins significantly contributes to their resistance to hydrolysis by pectinase. We report here the metabolism of morphine and its novel physiological role in the opium poppy.