LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 1
INTRODUCTION
[159]. Direct evidence for cytoplasmic ionization events affecting nuclear processes has
come from studies in which individual cells were irradiated with a microbeam of particles
that permitted selective irradiation of the cytoplasm or nucleus. Irradiation of the
cytoplasm was shown to be mutagenic but not cytotoxic [160]. The major class of
mutations was similar to that generated spontaneously and thought to arise from DNA
damage due to endogenous production of ROS/RNS [161] and differed from that induced
after irradiation of the nucleus.
Studies with fluorescent dyes demonstrated generation of ROS/RNS in cells
within 15 min after irradiation with less than one a particle per cell [162]. The
intracellular production of ROS/RNS was 50-fold greater than the extracellular
production. The intracellular source was inhibited by diphenyliodonium, an inhibitor of
flavoproteins, suggesting the involvement of a plasma membrane NADPH oxidase.
However, flavoproteins are also localized to the mitochondria and endoplasmic
reticulum, possible alternative cellular organelles that generate ROS/RNS. Enhanced
cellular production of ROS/RNS has also been observed after exposure to low-LET
radiation [163]. Cobalt- 60 γ-irradiation, at 1–4 Gy, stimulated production of ROS/RNS
in HepG2 cells and depletion of GSH, which radiosensitized cells due to enhanced radical
generation. These studies do not discriminate between possible signal-amplifying
mechanisms or an irreversible mitochondrial permeability transition as a late
consequence of radiation exposure.
Consequences of Radiation-Induced Generation of ROS/RNS
Some ROS/RNS, e.g. H 2 O 2 , nitric oxide and peroxynitrite, are membrane-permeant and
are sufficiently stable to diffuse significant distances within cells, facilitating their
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