Tyrosine is an important precursor of alkaloids with the phenyl and phenylpropyl nuclei. There are four basic alkaloid pathways.
2.3.1. Mescaline pathway
This alkaloid pathway starts with PLP decarboxylation to tyramine, and subsequently via SAM dimethylation synthesizes hordeine (Figure 35).
The second synthesis pathway from L-tyrosine is to dopamine across hydrox-ylation patterns and PLP activity. Dopamine is a very important compound in the
synthesis of alkaloids, especially in animals. Only dopamine can be converted to an other alkaloid, for example mescaline. Anhalamine, anhalonine and anhaloni-dine can also be synthesized in this way. Like mescaline, they are typical of simple tetrahydroisoquinoline alkaloids.
From L-tyrosine, and alternatively also from L-phenylalanine, kreysigine synthesis begins with dopamine (Figure 36). S-autumnaline is derived via a Mannich-like reaction. S-autumnaline is converted into floramultine by the oxidative coupling. Subsequently, the kreysigine is synthesized through the
activities of SAM. From S-autumnaline, other alkaloids can also be derived. The destination of this pathway is colchicine (Figure 36).
From dopamine, the pathway of tetrahydroisoquinoline alkaloids, such as emetine and cephaeline (Figure 37), also begins. Dopamine and secologanin undergo a Mannich-like to produce N-deacetylisoipecoside and ipecoside, and after hydrolysis and transformation, this is converted to emetine and cephaeline.
From L-tyrosine, or alternatively from L-phenylalanine, there is one further alkaloid biosynthesis pathway. This is the galanthamine pathway (Figure 38). Galanthamine synthesizes with tyramine, norbelladine, lycorine, crinine, N-demethylnarwedine and N-demethylgalanthamine. Schiff base and reduction reaction, oxidative coupling and enzyme NADPH and SAM activity occur in this pathway. Schiff base is a reaction for the elimination of water in formation with the C—N bonds process.
From norbelladine, through the activity of the SAM, the 4'-0-methylnorbelladine synthesizes, and again is transformed to lycorine, crinine and, by oxidative coupling, to N-demethylarwedine, which is the object of enzyme NADPH activity. Galanthamine is synthesized by transformation trough the activity of the SAM from N-demethylgalanthamine.
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