Surprisingly, intermediates of polyamine metabolism
showed remarkable changes during encystation. These
intermediates include putrescine, spermidine, spermine,
and N8
-acetylspermidine, all of which were present in
trophozoites. Furthermore, the levels of these metabolites
dramatically decreased and they were almost
deprived as encystation proceeded. Although there is a
possibility that polyamines are incorporated from the
culture medium, these findings led us to presume that
Entamoeba is capable of metabolizing these polyamines
during encystation, and the polyamine biosynthetic or
scavenging pathways exist in this organism. The fate of
polyamines is not well understood, but some polyamines
appear to be secreted by trophozoites (unpublished). The
presence of polyamine metabolism was totally unexpected
because several genes encoding key enzymessuch
as S-adenosylmethionine decarboxylase, spermidine
synthase, and spermine synthase, known to be involved
in polyamine biosynthesis in other organisms, have not
been detected in the Entamoeba genome [4].