Taura F, Sirikantaramas S, Shoyama Y, Shoyama Y, Morimoto S (2007) Chem Biodiv 4: 1649–1663 DOI: 10.1002/cbdv.200790145
Cannabinoids, which are found only in Cannabis sativa, are secondary metabolites featuring alkylresorcinol and monoterpene moieties in their molecules. More than 60 cannabinoids have been isolated from marijuana or fresh Cannabis leaves, and their pharmacological properties have been extensively investigated. Among them, D9 -tetrahydrocannabinol (THC) is the well-known psychoactive component of marijuana. In addition, this cannabinoid is shown to exert a variety of therapeutic activities such as the relief of nausea caused by cancer chemotherapy and the suppression of spasticity associated with multiple sclerosis. Furthermore, recent studies have demonstrated that THC activates two types of cannabinoid receptors (termed CB1 and CB2), which are expressed in the mammalian brain and immune cells, respectively. Thus, THC has attracted a great deal of attention. Besides THC, several cannabinoids also show various interesting activities, although they do not activate cannabinoid receptors. For example, cannabidiol (CBD), the isomer of THC, is a potent antioxidative and anti-inflammatory agent to provide neuroprotection in acute and chronic neurodegeneration. This cannabinoid is effective against the toxicity caused by 6-hydroxydopamine, which may be relevant to ParkinsonHs disease [9]. Cannabichromene (CBC) also shows various activities including anti-inflammatory, antifungal, and antibacterial activity. The co-action of CBC with THC was investigated, as marijuana contains a considerable amount of CBC. In addition, tetrahydrocannabivarin, the propyl homologue of THC in specific types of marijuana, is a cannabinoid CB1 and CB2 receptor antagonist. Since, in the 1990s, endogenous cannabinoid receptor ligands (endocannabinoids, e.g., anandamide and 2-arachidonoylglycerol) have been identified in mammalian tissues, cannabinoids from Cannabis sativa are now often called phytocannabinoids to distinguish them from endocannabinoids. Phytocannabinoids are classified into two types, i.e., neutral cannabinoids and cannabinoid acids, based on whether they contain a carboxy group or not. In the fresh Cannabis plants, cannabinoids are biosynthesized and accumulated as cannabinoid acids, and nonenzymatically decarboxylized into their neutral forms during storage and smoking. Several plausible hypotheses had been proposed for cannabinoid biosynthesis, and most of them were based on the presumption that tetrahydrocannabinolic acid (THCA) is biosynthesized by isomerization of cannabidiolic acid (CBDA), whereas CBDA and cannabichromenic acid (CBCA) are formed by oxidative cyclization of cannabigerolic acid (CBGA) [18]. To confirm the biosynthetic pathway, feeding experiments of radiolabeled precursors were attempted, but no clear results could be obtained due to low incorporation of radioactivity into cannabinoids. To overcome these difficulties, from the middle of 1990s, several research groups, including our laboratory, started to investigate the enzymes involved in cannabinoid biosynthesis, and successfully established the biosynthetic pathway of major phytocannabinoids as illustrated in Scheme 1. In the pathway, THCA, CBDA, and CBCA are biosynthesized from the common precursor CBGA by the action of unique oxidoreductases, i.e., THCA synthase, CBDA synthase, and CBCA synthase, respectively. On the other hand, CBGA is synthesized by alkylation of olivetolic acid (OLA) with geranylpyrophosphate (GPP) by a novel prenyltransferase called geranylpyrophosphate:olivatola te geranyltransferase (GOT) [24]. This article reviews individual biosynthetic enzymes and related topics.
Abstract