Sulfur Respiration by Eukaryotic Cytoplasm:
Accidental, Vestigial, or Functional?
Searcy, D.G.
Biology Department, University of Massachusetts, Amherst, MA 01003, U.S.A.
The "Universal Tree of Life" suggests that eukaryotic
nucleocytoplasm is related to Archaea. Thermophilic, sulfur-
metabolizing Archaea may most nearly resemble the ancestral
eukaryotic phenotype, in some aspects. Nonetheless, modern
eukaryotic cells are not thermophilic, and elemental sulfur
(S8) reduction has attracted little study. We
examined the question of whether modern eukaryotic cytoplasm
might be able to respire upon S8, using human
erythrocytes as a simple model of the cytoplasm. Washed
erythrocytes incubated with glucose plus S8 and
purged with N2 produced 170 micro-mol
H2S (L cells)-1 min-1, which
continued at a nearly constant rate for several hours. In sealed
vials, 25 mM HS- accumulated. Reduction of
S8 was glucose-dependent, where 3 mol H2S
were produced per mol glucose consumed. In oxygenated samples
H2S production was 80% of the anoxic rate. Cells
incubated 24 h with no added S8 produced 0.5 mmol
H2S (L cells)-1, suggesting endogenous
reserves of reducible sulfur. In cell lysates, H2S
production occurred after addition of either NADH, NADPH, or
reduced glutathione. All eukaryotes tested so far, including
representatives of each of the 4 Kingdoms, reduced S8
to H2S. In contrast, mitochondria apparently
originated from H2S-oxidizing alpha-
Proteobacteria. Thus, the ancient mitochondrial symbiosis may
have been based upon sulfur exchange between a sulfur-reducing
host and a sulfur-oxidizing symbiont. In modern eukaryotic cells
sulfur might yet function as an electron carrier, cycling
between the cytoplasm and mitochondria.
Searcy, D. G. (1992) Origins of mitochondria and chloroplasts
from sulfur-based symbioses. In: The Origin and Evolution of
the Cell. H. Hartman and K. Matsuno, Eds. World Scientific,
Singapore, pp. 47-78.
| LOCATION |
DATE |
TIME |
| Lecture Hall I |
Tuesday, April 7 |
02:00 pm |