Protopine
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Protopine is an alkaloid occurring in opium poppy, Corydalis tubers and other plants of the family papaveraceae, like Fumaria officinalis.
It has been found to inhibit histamine H1 receptors and platelet aggregation, and acts as an analgesic.
Biosynthesis
Protopine is derived in the plants such as the opium poppy, Papaver somniferum, from the benzylisoquinoline alkaloid (S)-reticuline through a progressive series of five enzymatic transformations: 1) berberine bridge enzyme to (S)-scoulerine; 2) (S)-cheilanthifoline synthase/CYP719A25 to (S)-cheilanthifoline; 3) (S)-stylopine synthase/CYP719A20 to (S)-stylopine; 4) (S)-tetrahydroprotoberberine N-methyltransferase to (S)-cis-N-methylstylopine.
The final step is oxidation by the enzyme methyltetrahydroprotoberberine 14-monooxygenase:
Metabolism
The enzyme protopine 6-monooxygenase converts protopine into dihydrosanguinarine. NADP+ and water are the by-products.
The enzyme uses reduced nicotinamide adenine dinucleotide phosphate (NADPH), and oxygen, with the initial product being 6-hydroxyprotopine, which spontaneously forms dihydrosanguinarine, found in plants such as Sanguinaria canadensis.