Table 1.

Classification of alkaloids. Alkaloids biosynthesized in plants are classified based on their biochemical precursors (biosynthetic pathway), chemical structures, and according to their occurrence in different genera of the plant kingdom.

Group	Characteristic	Representative Compounds	Plant Source	Reference
	Feature
Biosynthetic Pathway—Alkaloids Biosynthesized from Common Precursor
Tetrahydroisoquinoline alkaloids	Tyrosine acts as precursor	Berberine	Argemone mexicana, Berberis aristata, B. aquifolium, B. heterophylla, B. beaniana, Coscinium fenestratum, C. chinensis, C. japonica, C. rhizome, Hydratis Canadensis, Chelidonium majus, Coptidis rhizome	[9]
Indole alkaloids	Tryptophan acts as precursor	Ajmalicin	Rauvolfia spp., Catharanthus roseus, Mitragyna speciosa	[10,11,12]
		strychnine, brucine	Strychnos nuxvomica
Pyrrolizidine alkaloids	Ornithine acts as precursor	Senecionine	Jacobaea vulgaris, Brachyglottis repanda, Emilia sp., Erechtites hieraciifolius, Petasites sp., Syneilesis sp., Crotalaria sp., Senecio sp., Cynoglossum sp., Symphytum sp., Heliotropium sp., Caltha leptosepala, Castilleja sp.	[13,14]
Tropane alkaloids		Scopolamine	Hyoscyamus niger, Datura sp., Brugmansia sp., Duboisia sp.
Piperidine alkaloids	Lysine acts as precursor	Piperine	Piper nigrum, P. longum	[15,16,17]
Quinolizidine alkaloids		Lupinine	Lupinus argenteus
		Cytisine	Laburnum anagyroides, L. alpinum, Cytisus canariensis
Indolizidine alkaloids		Swainsonine	Astragalus earlei, A. mollissimus, A. wootoni, A. pehuenches, Oxytropis lambertii, O. sericea, O. campestris, Swainsona luteola, S. greyana, S. galegifolia
Pyridine alkaloids	L-Aspartate acts as precursor	Nicotine	Nicotiana tabacum, Nicotiana rustica, Duboisia hopwoodii	[18,19]
Pyridinone alkaloids		Cerpegin	Ceropegia bulbosa, C. juncea,
Quinoline alkaloids	Anthranillic acid acts as precursor	Skimmianine	Skimmia japonica, Zanthoxylum nitidum Camptotheca acuminata	[20,21,22]
		Camptothecin
Quinazoline alkaloids		Vasicine	Adhatoda vasica, Peganum harmala
Xanthine alkaloid	Adenosine (SAM cycle) acts as precursor	Theobromine	Theobroma cacao, Camellia sinensis, Cola acuminate, Paullinia cupana, Ilex guayusa	[23]
		Caffeine	Coffea arabica, C. canephora, C. liberica, C. racemose, Theobroma cacao, Camellia sinensis, Cola acuminate, Paullinia cupana, Ilex guayusa, I. vomitoria, I. paraguariensis
Steroid alkaloids	Formed by the inclusion of one or two nitrogen atoms to a preformed steroid molecule	Veratridine, jervine and cyclopamine;	Veratrum album, V. californicum, V. viride, Schoenocaulon officinale	[24,25]
		Zygacine	Toxicoscordion venenosum, Zigadenus glaberrimus
Terpenoid alkaloids	Formed by the introduction of a nitrogen atom from methylamine, ethylamine, or β-aminoethanol to terpenoidal skeletons	Secodaphniphyllate	Daphniphyllum macropodum, D. teijsmanni, D. humile	[26]
		Aconitine	Aconitum napellus, A. variegatum, A. noveboracense, A. vulparia, A. delphinifolium
Chromone alkaloids	Formed by the linkage of a structure consisting of a nitrogen system to the “A” ring of chromone	Rohitukine	Amoora rohituka, Dysoxylum binectariferum, Schumanniophyton magnificum, S. problematicum	[27,28,29,30]
		Dysoline	Dysoxylum binectariferum
		Cassiadinine	Senna siamea
		Ficine and isoficine	Ficus pantoniana
Flavoalkaloids		capitavine	Buchenavia capitata
		Aquiledine, isoaquiledine	Aquilegia ecalcarata
Chemical Structure—Alkaloids Grouped based on Nature of Heterocyclic Ring
Heterocyclic alkaloids (typical alkaloids)	Mononuclear	Hygrine	Erythroxylum coca, Convolvulus hanadae	[31,32,33]
		Boldine	Peumus boldus, Lindera aggregata
	Polynuclear	Atropine	Atropa belladonna, Datura innoxia, D. metel, D. stramonium, Brugmansia sp., Hyoscyamus sp.
		Reserpine	Rauvolfia serpentine
		Quinine	Cinchona officinalis
Non-heterocyclic alkaloids (atypical alkaloids)	Phenylethylamine skeleton	Ephedrine	Ephedra sinica, E. viridis, E. fragilis, E. distachya, E. ciliate	[34,35,36]
		Capsaicin	Capsicum frutescens, C. annuum, C. chinense, C. baccatum
	Tropolone skeleton	Colchicine	Colchicum autumnale, Gloriosa superba
	Modified diterpenes	Paclitaxel	Taxus baccata, T. brevifolia, T. chinensis
Taxonomy—Alkaloids Grouped based on Distribution by Botanical Origin
Opium alkaloids	Present in the Papaveraceae family	Morphine, codeine, papeverine, and thebaine	Papaver somniferum	[37]
Solanum alkaloids	Present in the Solanaceae family	Solanine, tomatidine, and solasodamine	Solanum tuberosum, S. lycopersicum, S. melongena	[5]
Daphniphyllum alkaloids	Present in the Daphniphyllaceae family	Daphniphylline, daphilactone-B	Daphniphyllum macropodum	[26]
Vinca alkaloids	Present in the Apocynaceae family	Catharanthine leurosine, vincristine, and vinblastine	Catharanthus roseus	[38]
Protoberberine alkaloids	Present in Annonaceae Ranunculaceae, Berberidaceae, Menispermaceae families; shares the same protoberberine skeleton	Berberine	Berberis aristata, B. aquifolium, B. heterophylla	[9,39,40]
		Jatrorrhizine	Enantia chlorantha, Thalictrum lucidumm, Thalictrum revolutum
		Palmatine	Phellodendron amurense, Guatteria friesiana
Ephedra alkaloids	Present in Ephedra genus of the Ephedraceae family	Ephedrine	Ephedra sinica, E. viridis, E. fragilis, E. distachya, E. ciliate	[41]