ABSTRACT
When ionizing radiation interacts with a cell, protein, lipid and DNA are damaged and one of the earliest questions addressed in radiobiology was “What is the critical target for radiation within the cell?”. Early studies demonstrated that amino acids in proteins were sensitive to damage by radiation: the most prone being cystine, methionine, histidine, tyrosine, threonine and serine (Kumta and Tappel 1961). Irradiation did not result in simple hydrolysis of bonds releasing peptide fragments, but caused degradation and breakdown of the amino acids and the formation of insoluble aggregates (Kumta and Tappel 1961). Hydroxyl radicals generated by gamma radiation were found to modify as well as fragment polypeptides, resulting in unfolded proteins (Wolff and Dean 1986), but the modifi cations and the extent of fragmentation were altered when the protein was situated in a membrane (Wolff et al. 1986). The lipid in the membrane decreased protein fragmentation for a given dose of hydroxyl radicals, indicating that the lipid was reacting with the free radicals. However, further addition of iron or copper to the system resulted in greater protein fragmentation, and it was concluded that the increased fragmentation was due to radicals generated from the lipid hydroperoxides (Dean 1987). Cellular membranes were originally considered to be a critical target for radiation (Bacq and Alexander 1961). Dr. Alper (1963) proposed that ionizing radiation resulted in two forms of damage: type N, which contributed to cell death after irradiation under anoxic conditions and was the result of primary energy deposition in the DNA, and type O, which was responsible for the radiosensitization of cells in the presence of oxygen and was due to membrane damage. Since DNA was known to be closely associated with the nuclear membrane in mammalian cells and with the cell membrane in bacteria, damage to the membrane was believed to result in death by disruption of DNA structure and function (Alper 1979, 1987). The induction of lipid peroxidation at unsaturated fatty acid residues by radiation is well established (reviewed in Leyko and Bartosz 1986), and a link with radiosensitivity was demonstrated when the cytosol of a cell line (L5178Y) was found to protect against lipid peroxidation, while the cytosol of a radiosensitive mutant of this cell line (M10) did not (Nakazawa et al. 1982). Vitamin E, a membrane soluble antioxidant, also increased the survival of irradiated mice (Malik et al. 1978), whereas vitamin E-defi cient mice showed enhanced radiosensitivity (Konings and Drijver 1979). The effect of radiation on membranes is still a very relevant topic of research today (reviewed in Corre et al. 2010). Evidence indicates that radiation leads not only to lipid peroxidation or protein modifi cation within the plasma
membrane, but disruption of the lipid bilayer, loss of barrier function, an alteration in the localization and size of lipid rafts that contain receptors and secondary messenger systems for signaling, and the production of ceramide followed by induction of apoptosis. Although some biological effects from ionizing radiation can be attributed to oxidation of proteins and lipid, the dose required to generate alterations, such as membrane permeability, occurs at a higher level than required to induce DNA damage (Kankura et al. 1969). Evidence still indicates that the most critical radiation target in the cell is DNA. In fact, lipid peroxidation has been shown to induce DNA damage (Pietronigro et al. 1977; Inouye 1984), so interventions to decrease lipid peroxidation could decrease cell killing by altering the level of DNA damage.
