(11) TRANSITION TO CUBIC MORPHOLOGY OF MITOCHONDRIAL INNER MEMBRANES IN FASTING AMOEBA MAY BE A RESPONSE TO INCREASED OXYGEN RADICAL GENERATION

Y. Deng and C.A. Mannella
Wadsworth Center, Box 509, Empire State Plaza, Albany, NY 12201-0509

The amoeba Chaos carolinensis can survive without intake of food organisms for several weeks.  During early stages of fasting, the mitochondrial cristae undergo a dramatic change from random tubular to a highly ordered cubic morphology [1] that has been reconstructed by EM tomography [2]. The mechanisms triggering this change in mitochondrial morphology are unknown.  Whole-cell polarographic measurements indicate that the respiration of fed cells is cyanide-sensitive, and becomes cyanide-resistant one day after food depletion. Also, starved (but not fed) cells display spontaneous cyanide-sensitive oxygen evolution a few minutes following respiratory inhibition, suggesting enhanced generation and subsequent catalase-mediated reduction of H2O2.  There are two likely sources of peroxide production in these cells:

1) Peroxisome-like beta-oxidation of fatty acids, which generates H2O2 directly.  There are early observations that Chaos changes its catabolism from carbohydrate to lipid-based substrates during fasting [3].
2) Increased production of reactive oxygen species (ROS) by the mitochondrial respiratory chain, possibly due to degradation of inner membrane lipids.  ROS would be reduced to H2O2 by mitochondrial superoxide dismutase (SOD) activity and the H2O2 subsequently reacts with catalase in the cytosol.

Elevated production of H2O2 in starved amoeba suggests two interesting, inter-related hypotheses.  First, might the peroxidation of mitochondrial membrane lipids trigger the observed transition of mitochondrial membranes to cubic morphology?  Second, there are reports of non-enzymatic SOD activity in mitochondria associated, presumably, with direct reduction of ROS by protons at the inner membrane [4].  Might the cubic transition of the inner membrane enhance this non-enzymatic SOD activity, by creation of an elaborate, interconnected network of catalytic membrane surfaces?  Experiments to test these two novel hypotheses are underway, e.g., to assay levels of ROS generation and lipid peroxidation in fed and starved amoeba.  It is worth noting that many other examples of naturally occurring cubic membranes are associated with systems with elevated ROS, e.g. chloroplasts, spermatozoa, oocytes, viral-infected cells, aged cells, and photocytes [5].

Supported by CAMURUS Lipid Research Foundation, Sweden and NIH/NCRR grant RR01219.

[1]  Deng, Y., Mieczkowski, M. (1998) Protoplasma 203:16-25
[2]  Deng, Y. et al. (1999) J. Struct. Biol. 127:231-239
[3]  Daniels, E.W., Breyer, E.P. (1968) Zeitsechrift für Zellforschung 91:159-169
[4]  Guidot, D.M. et al. (1995) J. Clin. Invest. 96:1131-1136 
[5]  Landh, T. (1996) PhD thesis. Lund, Sweden.

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