Chemicals in Cacti ... 2/3

Betalains

Many angiosperms (flowering plants) contain water soluble coloured pigments like anthocyanins (reds, blues and purples) and betalains (reds, orange, purple, pink and yellows) but not both - they are mutually exclusive.
Anthocyanins are present in carnations, delphiniums and african violets, and a lot of red fruits and vegetables eg strawberies and grapes. It is also reponsible for the change in colour of leaves in Autumn.

Betalains (eg. betacyanins are red/purple, and betaxanthins are yellow) have a limited distribution being present in the Beta genus (after which they are named) like beetroot, the portulacas, Bouganvilleas, Aizoaceae/Mesembs, Amaranthaceae and cacti families - all of which are closely related to each other (Betalains are exclusive to the order of Caryophyllales).
Betalains show in the red/purple colour of cacti bodies, and Opuntia or prickly pear pads/joints, especially when stressed by dehydration etc.
Betalains have been studied for their beneficial effects for (wo)man with their anti-oxidant properties especially in preventing the oxidation of LDL (the bad cholesterol), which when oxidised sticks to the artery walls and contributing to strokes/heart attacks.

Other less important coloured pigments, for cacti anyway, are the carotenoids - found in carrots, pumpkin and squashes - giving the characteristic yellow/orange colour to these plants.

Betalains are synthesised by a long chemical process from a common amino acid called tyrosine, the first step being the manufacture of L-Dopa (found in the animal brain - responsible for movement and mood) as above in alkaloids.