Page 26 - Summer2011

SUlfUR aS a pRotEctivE agEnt againSt Radiation daMagE

Sulfur-containing biological molecules like glutathione and the amino acids cysteine and methionine play an important
role in redox (oxidation/reduction) reactions by tempering the damaging effects of reactive oxygen species (RoS); that is,
by acting as potent antioxidants. closely related to this role in protecting from oxidation damage associated with aerobic
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metabolism is the potential role of sulfur in protection from radiation damage due to sun exposure, radiation treatments
for cancer, or radiation exposure following a nuclear reactor meltdown.
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an awareness that sulfur protects against ionizing radiation dates back to at least 1949. an enlightening article from
1983 showed, via experiments conducted at very low temperatures, that sulfur’s reaction to radiation is a secondary effect.
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the associated primary effect is ionization of oxygen, producing the highly reactive species, o ¯, Sulfur then responds by
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binding to the o ¯ and thus preventing other molecules from reacting adversely with it.
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through an extensive review of the research literature on the response of human skin to the radiation in sunlight, i
have come up with a theory for how sulfur could be intimately involved not just in preventing harm from sunlight, but
rather by contrast in harnessing the sun’s energy and putting it to good use. i propose that sulfur, readily available from the
active cysteines in an enzyme called (inappropriately) endothelial nitric oxide synthase (enoS), reacts with two o ¯ ions
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produced by sunlight exposure to produce the highly stable and useful anion, sulfate. this reaction would take place in a
cavity formed by two abutting molecules of enoS (that is, an enoS dimer). a positively charged zinc atom centered in
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the cavity draws in the two o ¯ ions to combine them with a nearby sulfur atom attached to a cysteine residue, to form
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a sulfate anion So 4 −2 . the sulfate, then, in a subsequent reaction, combines with cholesterol to form cholesterol sulfate, a
prominent component of the outer layers of the skin (and also of hair, feathers, fur and fingernails).
an article that appeared in 2002 on the effects of irradiation treatment on aortic endothelial cells revealed that ir-
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radiation induces expression of another “inducible” nitric oxide synthase, inoS. My belief is that the purpose of the inoS
in this case is identical to the purpose of enoS in the skin: to mop up anticipated o ¯ radicals produced by the radiation,
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and to convert them to sulfate. the authors showed that if the cells are supplied with the substrate to produce nitric oxide,
l-arginine, then this causes them to initiate a programmed cell death reaction called apoptosis. What happens is that the
l-arginine binds to the inoS (and the enoS as well) and deflects these enzymes towards producing nitric oxide rather
than sulfur dioxide. Unfortunately, under the right circumstances, nitric oxide can turn into the highly reactive species
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onoo (known in the vernacular as “oh, no!”) and this can make the cell non-viable.
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a highly significant fact that supports a primary role for the noS’s in producing sulfate is that red blood cells have an
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abundance of enoS, but they are very careful to keep out its substrate l-arginine. this act has puzzled researchers, but
the answer becomes clear when you realize that red blood cells are strong producers of cholesterol sulfate, as well as
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major carriers of oxygen. this makes them a prime candidate for using enoS to convert oxygen to sulfate (taking advantage
of sunlight as a catalyst), and then shipping it to the tissues via the carrier molecule cholesterol sulfate. this action would
both protect the red blood cell from oxidative damage and reduce the risk of damage due to oxygen exposure in other
cells, as the oxygen supply contained in the sulfate constitutes safe transport of oxygen to these cells. i have little doubt
that this is a productive (but overlooked) mode of oxygen transport in the body.
the sulfur in cysteine plays a crucial role in protecting proteins from radiation damage. in experiments conducted in
the late 1950s , it was shown that proteins needed to contain only half a percent of cysteine by weight to be immune to
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any damage to the other amino acids in the protein. proteins containing no cysteine produced complex irradiation spectra
indicating that diverse chemical reactions had taken place.
an article from Nature in 1962 showed that sulfur has a remarkable ability to protect macromolecules in colloidal
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suspensions against cross-linking upon exposure to radiation. the effect was much larger than what the authors would
have expected, given their understanding of possible mechanisms, so there is still something mysterious about sulfur’s
protective role. Since molecules in the blood serum are in some sense a colloidal suspension, this behavior has relevance
to protection from ionizing radiation of proteins like serum albumin, which contains significant amounts of cysteine.
the best source of sulfur is the protein from animal products such as meat, fish and eggs. Sulfur is becoming depleted
from the soil, so vegetables contain even less sulfur than they used to. it is therefore highly likely that vegetarians suffer
from sulfur deficiency, which could affect their susceptibility to damage from radiation exposure.

1. g. atmaca, “antioxidant Effects of Sulfur-containing amino acids,” Yonsei Medical Journal, vol. 45, #5, 776-788, 2004.
2. a. charlesby, et al., “Radiation protection with Sulfur and Some Sulfur-containing compounds,” Nature, vol. 194, 782, May 26, 1962.
3. W.gordy and i. Miyagawa, “Electron spin resonance studies of mechanisms for chemical protection from ionizing radiation,” Radiation Research, vol. 12, 211-229, 1960.
4. M. Hirakawa, M. oike, K. Masuda, and Y. ito, “tumor cell apoptosis by irradiation induced nitric oxide production in vascular Endothelium,” Cancer Research, vol. 62,
1450 1457, 2002.
5. H-S Kim and c. alexander, Jr, “ESR Study of the Requirements for Sulfur Radical formation in irradiated 6-MeMpR,” J. Korean Physical Society, vol 16, no 2, 186-189,
1983.
6. p. Kleinbongard, et al., Red blood cells express afunctional endothelial nitric oxide synthase. Blood, vol. 107, no. 7, 2943-2951.
7. l.a. Kormarnisky, et al., “Sulfur: its clinical and toxicologic aspects,” Nutrition, vol. 19, 54-61, 2003.
8. H. li, et al., “crystal structures of zinc-free and -bound heme domain of human inducible nitric-oxide synthase. ” J. Biol. Chem. vol. 274, 21276-21284, July, 1999.
9. M.l. pall, nitric oxide synthase partial uncoupling as a key switching mechanism for the no/ onoo cycle. Medical Hypotheses vol. 69, no. 4, 821-5, 2007.
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10. H.M. patt, et al., “cysteine protection against x-radiation,” Science vol. 110, 213, 1949.
11. c.a. Strott, cholesterol Sulfate in Human physiology: What’s it all about? (2003) 44 J Lipid Res 1268-1278.
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