The number and type of isoforms of superoxide dismutase (SOD) and
their activities were compared in
mitochondria and peroxisomes isolated from cotyledons of three
different oilseed seedlings. Mitochondrial and
peroxisomal isoforms of SOD could be distinguished in nondenaturing
polyacrylamide gels by their differential
sensitivities to KCN and/or H2O2. The type
of SOD was not the same for each organelle in each of the three
oilseed species. For example, a single Mn–SOD was found in
cotton and cucumber mitochondria, whereas four
CuZn–SODs were present in mitochondria from sunflower. At least
one CuZn–SOD isoform was found in the
peroxisomes of all three species. Cucumber peroxisomes contained both a
CuZn–SOD and a Mn–SOD, cotton
peroxisomes contained a single CuZn–SOD, whilst four separate
CuZn–SODs, but no Mn–SOD were found in
sunflower peroxisomes. Using antibodies against CuZn–SOD from
watermelon peroxisomes or from chloroplasts
of Equisetum, a single polypeptide of c.
16·5 kDa was detected on immunoblots of peroxisomal
fractions from the three species. Post-embedment, electron-microscopic
double immunogold-labelling showed that CuZn–SOD,
with malate synthase used as marker enzyme of peroxisomes, was localized
in the matrix of these organelles of all
three species. These results suggest that CuZn–SOD is a characteristic
matrix enzyme of peroxisomes in oilseed cotyledons.