Obscure compounds, dark glands, healing balms
Polyketides and their derivatives hardly feature in our
mainstream biochemistry teaching, but these compounds, derived essentially via
condensation of acetate units, constitute in many plant species a significant
proportion of the ‘secondary metabolites’. Despite being relatively little
known, polyketides have had their place in history: the lethal compound in
hemlock extract, used to execute Socrates, is the polyketide coniine. However,
these compounds have more benign uses. Extracts from different plants have been
used in traditional medicine and preparations from Hypericum perforatum (St John’s wort) are used
extensively as a complementary remedy for depression. Active components are
hypericin and pseudohypericin, in the naphthodianthrone group of ketide
derivatives. The medicinal use of these compounds has led Zobayed
et al. (Chiba, Japan, pp. 793–804) to investigate their
distribution in the plant, their likely site of synthesis, and the relationship
between accumulation and photosynthetic rates. The results show clearly that
the distribution within the plant of hypericin/pseudohypericin is closely
correlated with the presence of dark glands, one of several types of secretory
structure possessed by H. perforatum. The denser the dark glands, the higher is the concentration of
hypericin/pseudohypericin. Conversely, organs with very few dark glands contain
very little of these compounds. Dark glands themselves differ little in
content; it is the number of dark glands that matters. The densest array of
dark glands is on the stamens, consistent with earlier findings that flowers
are a rich source of hypericin/pseudohypericin. Further, in addition to
indicating that dark glands are the likely organs of secretion of hypericin/pseudohypericin,
the occurrence of the immediate precursor of hypericin suggests that at least
the final stages of biosynthesis take place in the dark gland cells. Finally,
the density of dark glands and hence the concentration of hypericin/pseudohypericin
in leaves increases when the net photosynthesis rate increases, indicating a
relationship between C fixation and hypericin synthesis.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
