NADPH and protoporphyrin IX dependent conversion of violaxanthin to a Retro-carotenoid in Narcissus pseudonarcissus chromoplasts in vitro

In homogenates of chromoplasts isolated from petals of Narcissus pseudonarcissus, violaxanthin is efficiently converted into a novel retro-carotenoid [(3S,5R,3'S,SR, all-E)- 5,5'-dihydro-3,5,3',5'-tetrahydroxy-beta,beta-carotene]. Here we have purified and characterized this compound by NMR spectroscopy. Its formation took place in chromoplast homogenates after supplementation with NAD(P)H in the presence of protoporphyrin IX. The tetrapyrrole could either be biosynthetically formed from added delta-aminolevulinate or supplemented externally. The reaction was greatly stimulated under anaerobic conditions, probably due to inhibition of a respiratory redox pathway known to operate in chromoplast membranes. Saturating behaviour with respect to substrates, high substrate specificities and proteinase susceptibility show the enzymic nature of the reaction. Neither chromoplast membranes alone nor the soluble stroma components deprived of membranes were capable of carrying out the reaction, indicative of a differentially localized multi-component system being involved in catalysis.