K. Grunewald et al., Ketocarotenoid biosynthesis outside of plastids in the unicellular green alga Haematococcus pluvialis, J BIOL CHEM, 276(8), 2001, pp. 6023-6029
The carotenoid biosynthetic pathway in algae and plants takes place within
plastids. In these organelles, carotenoids occur either in a free form or b
ound to proteins. Under stress, the unicellular green alga Haematococcus pl
uvialis accumulates secondary carotenoids, mainly astaxanthin esters, in cy
toplasmic lipid vesicles up to 4% of its dry mass. It is therefore one of t
he favored organisms for the biotechnological production of these antioxida
tive compounds. We have studied the cellular localization and regulation of
the enzyme beta -carotene oxygenase in H. pluvialis that catalyzes the int
roduction of keto functions at position C-4 of the beta -ionone ring of bet
a -carotene and zeaxanthin, Using immunogold labeling of ultrathin sections
and Western blot analysis of cell fractions, we discovered that under indu
ctive conditions, beta -carotene oxygenase was localized both in the chloro
plast and in the cytoplasmic lipid vesicles, which are (according to their
lipid composition) derived from cytoplasmic membranes. However, p-carotene
oxygenase activity was confined to the lipid vesicle compartment, Because a
n early carotenogenic enzyme in the pathway, phytoene desaturase, was found
only in the chloroplast (Grunewald, K,, Eckert, M., Hirschberg, J,, and Ha
gen, C, (2000) Plant Physiol. 122, 1261-1268), a transport of intermediates
from the site of early biosynthetic steps in the chloroplast to the site o
f oxygenation and accumulation in cytoplasmic lipid vesicles is proposed.