Douglas C. Neckers' University of N e w Mexico Albuquerque, 87106
I
I I
Photochemical Reactions of Natural Macromolecules Photoreactions of proteins
The effects of ultraviolet radiation are both advantageous and deliterious to living organisms. Ultraviolet light has been used since 1877 to sterilize foods, to maintain sterile conditions during surgery, for preventing rickets, for killing pests and insects and for many other purposes. Ultraviolet light absorption is responsible for skin tanning and has been implicated in a causative way in many kinds of skin cancers (1-4). Many of the physiological effects of ultraviolet light derive from the polynucleic acids contained in the cell nucleus. Dimers form from pyrimidines, in particular thymine, and cell mutations are said to be the result (2, 4). Other cellular and extra-cellular ~hysiologicalchanges are attributable to the proteins a n d th& constituent amino acids. In this paper, we review salient features of protein destruction by ultraviolet radiation. We concentrate on absorption of ultraviolet radiation by proteins and amino acids and the chemical transformations which result. Historical lnformation Egg albumin was reported destroyed by uv irradiation through quartz as early as 1913 (5). From a general point of view, the photodestruction of proteins is accompanied by coagulation, by the evolution of ammonia and by a yellowing of the protein. If the particular protein being irradiated happens to be an enzyme, the activity of the enzyme will often be altered (6-10). If the protein is on the external skin surface, significant changes in the absorption spectrum of the skin are observed (11) and the well-known skin darkening, sun-tanning, is observed. At the molecular level, the changes which occur in protein after ultraviolet radiation are attributed to photochemical effects which derive from chemical reactions either of specific amino acid residues within the protein or of the protein itself. To a first approximation the absorption spectrum of proteins is closely allied to the absorption spectrum of specific amino acid residues. Those amino acids which contain aromatic residues absorb the most in the ultraviolet region. The three most important absorbing amino acids are
40.000 20.000
I
10,000
$
5000
Figure 1. The ultraviolet absorption spectra of the aromatic amino acids. tryptophane (try), tyrosine (tyr) and phenylalanine (phe).
most amides show absorption in the ultraviolet region a t short wavelengths only (14, 15), the absorption of the peptide bond is insignificant except below 230 nm. In proteins containing aromatic residues, the absorption spectra of the protein in the long wavelength region results almost exclusively from the s-r* absorptions of the aromatic system. In the enzymes carbonic anhydrase (171, chymotrypsin (la), carboxypeptidase (Is), lysozyme (19), pepsin (19), and ribonuclease (20), absorption above 250 nm is much like the constituent aromatic monomers. The absorption spectrum of bovine carbonic anhydrase is contrasted with that of tryptophane in Figure 2. Mi,
I R-C-C-NH4-R' I II H 0
H
I I MOH
Peptide link
In contrast to other amino acids, these amino acids absorb significantly at wavelengths above 250 nm because they are aromatic residues. Their absorption spectra are shown in Figure 1112, 13). Proteins are condensation polymers of amino acids. The essential link is that called the peptide bond, which is an amide linkage between the amino group of one amino acid and the carboxyl group of a second amino acid. Since lFellow of the Alfred P. Sloan Foundation, 1971-73. 164
/ Journal of Chemical Education
The emission spectra of natural proteins, including phosphorescence and fluorescence also parallels the emission snectra of the emittine monomeric amino acids (2126). From fluorescence quantum yield data (27) as well as lifetime measurements (28) and similar phosphorescence data, (29, 30) one can construct energy level diagrams for the three aromatic amino acids, in similar solvents. The luminescence properties of the aromatic acids are given in Table 1. For tryptopbane, though the fluorescence spec-
-
Table 1. Luminescence Prooertiea of the Aromatic Acids at 77-K ~luorosconce maximum (A)
Pow. comoovnd
Figure 2. The ultraviolet absorption spectrum of bovine carbonic anhydrase (molecular weight = 31,000) contrasted with the spectrum of tryptophane. Carbonic anhydrase = solid line: tryptophane = dotted line.
trum is simple, Figure 3, the phosphorescence spectrum is highly structured. The total luminescence pattern of the three aromatic amino acids is shown in Figure 4. The phosphorescence spectrum of tyrosine (25) is devoid of resolvable structure as is that of phenylalanine (31). Energy transfer occurs between aromatic amino acid residues even when these amino acid residues are combined in peptides. Energy transfer occurs at the singlet level over long distances, provided that a degree of overlap occurs between the absorption spectrum of the acceptor and the emission spectrum of the donor (32). Since the fluorescence band of tyrosine shows extensive overlap with the absorption spectrum of tryptophane the formal requirements for Forster type energy transfer are satisfied (32), and in the dipeptide Try-TyrZ complete energy transfer occurs and fluorescent emission from Try only is observed. The total luminescence pattern (fluoresohosnhorescence) is also characteristic of twotocence ". phane (3j). The demee of trvntonhane fluorescence denends on the distance fietween the iryptophane and tyrosine residues. Thus in the oligopeptides, Try(Gly),,Tyr, where n = 0, . . . 4, the maximum degree of tryptophane fluorescence occurs where n = 0 and the minimum where n = 4. The emission efficiency is unaffected when the protein is denatured (34). Among the three amino acids, transfer occurs from ~henvlalanineto tvrosine to trvotonhane from both .- . singlet and triplet level;. Thus tryptophane is the energy sink into which absorbed radiation eventually localizes. Among homopolypeptides, polypeptides of one amino acid, and heteropolypeptides (polypeptides containing more than one amino acid) the luminescence characteristics follow the same pattern. Luminescence measurements for representative polypeptides are shown in Table 2. In copolymers of aromatic amino acids the transfers observed (39, 40) are summarized below.3
dar
Phosphorescenee maximum U)
05%
glu. mse
Pow-
0.5% glu-
der
case
qp ratio
Powdar
0.5% pluc m
phosphoreseaneedeeay time (secl Pow der
0.5% giu=a
tures of protein stereochemistry and of proximity of donating and accepting groups are important though little data is available at this time. What is obsewed is that synthetic disulfides and the amino acid cystine quench a t least tryptophane fluorescence. This is particularly significant since much of the secondary structural features of proteins derive from the disulfide linkage and, as we shall see later, photoinactivation of enzymes probably derives from hoto ode composition processes which disrupt the disulfide linkages. Photochemical Reactions of Proteins One of the first complete analyses of protein primary structure following irradiation was carried out by Augenstein's group (8, 39, 40). Their data for trypsin, summa-
+
-
Singlet Level
tyrosine
tryptophane
-
Triplet Leuel
tyrosinate*
tryptophane
In synthetic and natural polypeptides, additional structure features complicate the situation. Presumably fea2Remember that the nomenclature pattern of peptides is such that the first-named amino acid has the free NHz group and the last amino acid the carboxyl group. Try-Tyr is thus
3Tyrosine, being phenolic, ionizes at higher pH. Transfer at the singlet level occurs from tryptophane to tyrosinate rather than the other way around.
Figure 4. The total luminescence pattern of the three aromatic amino at the shorter wavelengths. phosphoresacids. F l ~ o r e ~ c e n cOccurs e cence at the longer wavelengths.
Volume 50, Number 3. March 1973
/
165
rized in Figure 5, show that the only amino acid residues which undergo significant photochemical change are the cystine ?HZ
"r"
~ooc-d-msc~,c-CWH
I
H
I
H
and tryptophane residues. Others showed that cystine disruption is a rather general phenomenon in protein photodestruction (41, 42). When tryptophane is not present, disruption of tyrosine (43) and phenylalanine (44) residues also has been reDorted. All other amino acid residues are relativelv unchanged during protein photodecomposition (8, 40, 44, 45). ~articularlva t lone waveleneths. Bv far most struct u & i modifications which take )lace in proteins, take place by disruption of one of the above residues. The photochemical quantum yield, a, is a measure of photoefficiency (eqn. (1)). Though literature quantum yields for amino acid and protein photodecomposition are suspect on quantitative grounds (461, qualitatively proteins which contain tryptophane are destroyed a t the cystine residues more efficiently than those without tryptophane, Table 3. produced (or reactant consumed) * = molecules of product quantaof light absorbed
(1)
Recent data from our laboratories (47) demonstrates that cystine residues are destroyed while tryptophane remains~whenthe two amino acids are irradiated in solution. Energy transfer from tryptophane to cystine is probably occurring both in the protein and among the free amino acids in solution. Chemically, cystine, like most disulfides, is easily reduced. Thus cleavage of the sulfur-sulfur bond of cystine produces easily detected sulfhydryl residues. The reactions leading to tryptophane sensitized cystine disruption can be summarized as
Tryptophane-containing proteins are therefore destroyed at the cystine residues hut not at the tryptophane centers concomitantly. Finally, recent data demonstrates that bovine carbonic anhydrase, an enzyme containing tryptophane but not cystine, is photoinactivated. These results demonstrate that photochemical reactions a t tryptophane centers are responsible for protein degradation as well. Though some tryptophane is probably located a t the active site of carbonic anhydrase, not all the tryptophane residues are there suggesting, as we observe, that the rate of photoinactivation of carbonic anhydrase should differ somewhat from the rate of enzyme inactivation. The photoproducts of tryptophane are oxidation products. Some Natural Photo-Processes Involving Proteins Vision Through the efforts of Professor and Nobel Laureate George Wald of Harvard University we know more about how man sees objects and colors than we do about many other physiological photo-processes. In the eye, proteins are altered by a photochemical act, the end result of which is a nerve impulse. The impulse is transmitted via the optic nerve to the brain. The particular kinds of proteins which participate in the vision cycle are called opsins. In the rods, the cells in the eye responsible for general visual processes, the protein is called rhodopsin. Isolated from frog retina by Boll in 1877, rhodopsin or visual purple is a rose-colored protein of molecular weight 40,000. It absorbs in the uv at 275 nm, hut has a large and broad absorption in the visible region around 500 nm. The visual sensitivity curve, Table 2. Emission Characteristics of Aromatic Amino Acids and Peptides (35-38) Fluoreseenee A,..yield 280 300 120
.>-.,a 0.06
-
Tryptophane 3 [Tryptophanel* [Tryptophanel* + Cystine Tryptophane + [Cystinel* 2 Cys [Cystinel*Cys + Hydrogen donor CySH + free radical 2 CyS. CySSCy 2 Free Radicals Non-Radical Products
- -
282 292
Phosphorercence h yield
,,.,
366
390 450 347 ,004 3611 367 :,80
34" 408
Table 3. Ouantum Yields01 Amino Acids or Protein Destruction Proteinor Aminr,Acid
Figure 5 . T h e a m i n o a c i d c o n t e n l of irradiated trypsin after irradiating with increasing a m o u n t s of 253.7 nrn light.
166
/ Journal of Chemical Education
a
,i.
Twmuphane
U0.i
Tyrosine
.WB
cysxine Trypsi"
0.n
a,#
0.45
Lymzyme
O.5R
Insulin
0.18
Quantum yield, cystin. clrsvage
-
Condition-
Retrieme
p H = 7.0
1'61
air added p H = 7.C air added water pH = 7.0 4 trypfophane residues Gtryptuphano residues notryptnphane
I471 14ii
iwni
IIP!
(471 (471
that is the eye response to specific wavelengths of light, closely parallels the absorption spectrum of rhodopsin (49).
Rhodopsin undergoes a photochemical cleavage reaction in which the major fraction of the protein is broken away. The cleavage reaction results from a cis-trans isomerization of the 11,12 double bond in the retinal portion of rhodopsin. Retinal is a highly unsaturated aldehyde
opsin Rhodospin
opsin Prelurnirhodopsin
opsin
opsin
The isomerization of the A-ll-cis-retinal-opsin protein to all tram-retinal is the sole known photochemical act in vision. Protein is bound to retinal via a Schiff's base linkage, H
I
R-C=N-protein a t the aldehyde function. In the visual cycle, the A-ll-cis-all-trans-retinalisomerization apparently changes the stereochemistry of rhodopsin sufficiently well so that enzymatic hydrolysis of the Schiff's base linkage can take place; the continued generation of sulfhydryl groups indicates a cystine disulfide is being cleaved in the protein rearrangement process as well (Fig. 6). It is truly amazing that just one photon per receptor cell per 6 minutes is sufficient to cause a visual response. In the eye, rhodopsin is continually being regenerated and retinal is continually heing refurnished. The visual cycle is such that all trans-retinal is enzymatically isomerized and protein retinene recombination occurs to reform rhodopsin. Vitamin A
feeds retinal into the cycle to replenish consumed retinal. Rhodopsin
f cis-retinal
Y
+ opsin + trans-retinal + opsin
11 eis-vitamin A,
11
."..me
trans-vitamin A,
Sun-Tanning
The skin is comprised mainly of keratin, a cystine-containing protein of characteristic insolubility. In man, the skin and its constituent proteins function mainly as a protective layer against the harmful effects of uv light. The major protective material, a highly conjugated protein called melanin is synthesized in the skin as an end result of tyrosine photooxidation. conThe Drocess of skin tannina- occurs when a copper .. taining enzyme, tyrosinase, oxidizes tyrosine. The eventual product leadina to melanin formation is indole 5,6-quinone
This reactive quinone polymerizes to highly conjugated, colored products bound in the skin. It is the skin hound polymer which is called melanin. It is the darkening agent in the outer layer of the skin.
Lurnirhodopsin
+ Metarhodopsin
Figure 6. The visual cycle.
Photochemistry and Skin Cancers. Exposure to sunlight is the primary natural cause of skin cancer in man. Men are three times more susceptible to skin cancer than women, outdoorsmen more than office workers, whites more susceptible than blacks and Texans more than Eskimos. All the accumulated evidence links the dosage of ultraviolet light, the darkness of skin pigments, and incidence of skin cancer. As in light ahsorption in skin tanning, most radiation is absorbed in the tyrosine and tryptophane on the skin surface. Cancer is not one disease and there are a number of different kinds of cancers. All cancers have one thing in common-they are diseases typified by abnormal, uncontrolled, generally too-fast cell division. Despite massive research efforts, the genesis of cancers of most kinds just is not yet understood. Nevertheless, i t is thought, at least in many instances, that tumor growth eventually begins by changes of one kind or another in the genetic material of cells-the nucleic acids. Alterations in nucleic acid composition may occur photochemically in several different ways. Direct irradiation of DNA, for example, produces thymine dimers which cause structural alternations of DNA secondary structure. A second way in which changes in nucleic acid structure might result is from chemical reactions caused by nucleic acid-photoproduct interactions. A third way is by energy transfer, that is, by absorption in one molecule and transfer to another. Any and all of the above processes may be occurring photochemically. In addition, enzymatic inactivation photochemically may well he affecting the regulatory roles of certain enzymes crucial to control of cell division. Therefore, at least one possible hypothesis relating to the induction of skin cancers is that photodecomposition of tyrosine or tryptophane in the skin outer layers eventually produces sufficient quantities of carcinogenic photoproducts so that skin cancers result. Having established that skin cancers are probably caused by ultraviolet radiation, attempts are continuing to establish the origin of the tumors. Assuming for the moment that amino acids and proteins are the radiation receptors in the skin, then the production of products after irradiation which cause tumor development chemically could be the genesis of the skin cancers. Many photometabolites of tryptophane and tyrosine are chemical carcinogens; that is to say, they induce malignant tumors of one kind or another in test animals. The three Volume 50.Number3. March 1973
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167
most important tryptophane metabolites which have been shown t o be carcinogenic are 3-hydroxykynurenine, 2amino-3-hydroxyacetophenone and 3-hydroxyanthranilic acid. The photooxidation of tryptophane produces Nformylkynurenine
a s well a s kynurenine either of which could be converted t o the carcinogenic substances by enzymes.
Literature Cited (1) Ellia. C., and Wells, A. A,, "The Chemical Action of Ultraviolet Rays: Rainhold, NswYark. 1941. (2) McLaren. A,. and Shugar, D., "Phatoehmiatry of Nucleic Acids and Proteins? PergamonPIess, Oxford. 1964. (3) Steiner. R. F.. and Wd"ryb. I.. "Excited states of Nucleie Add. and Prntei".." Plenum press. New York 1971. (4) Wong, S. Y., "The Basic Prineiplos of Nueleic Acid Chemistry." (Editor: p. Tn'o, PO..) AcademicPre3s. New York. (5) Bouie. W.T.. Science, 24.374 (19131. Bioehi. & Biophya.Acto., 24, n (19571. (61 Setlow, H., and Doyle, 9.. (71 MeLaren. A. D.. Ewmologic. 18.81 (19571. (81 Riai. S., D m , K., Rathinssamy, T. K., and Augcnsteio. L., Phefoehemistry and Phofobiology. 6.423(19671. (9) Augenstein.L., PhotochemistryondPholobiology. 7,613119681. (10) ~ . ~ k D.C.,Turner, ~ n . L. R.. Daub, E..andvanderJagt.D.L., unpublishednaulfa. (11) ~ i t ~ ~ ~ T. tB., ~ Seiji, i ~ hM.,, and MeGugan, A. D., New EnglondJ. Med, 265, 328 (19611. (12) Weinryb. I., end Steiner. R. F., "Luminescenee of Nuclei= Acids and Pmtoins" Plenum Ress. NowYork. 1'371, p. 279. (13) Konev. S. V., "Fluorescence and Phospboroscenco of Nucleie Aeida and Pr0teinn.l. Plenvm Ren%1967, p. 1. (14) Neckem, D. C.. "Mechanistic Organic Phofoehemistry: Reinhold, New York, 1997. (151 Gillsm, A. R.. and Stem. R. S., "Absorption Speefrs of Organic Molecules,.l k nold, 1964. (16) Jsffp. H., and Orehin. M.. "Ultraviolet Speefroseopy oforganic Molecu1os.l. John Wiley and Sons, Inc. New York. 1965. (171 Neeken. D. C.. Daub.E., a n d v a n d d a g t , D.L.. unpublishdresulfa. (IS1 MeClaren, A. D.. and Hidalgo-Saluatierra. 0 . . Photochemistry and Phofobiolo.gy, 3.349 (1964) (191 Longworth, J . W..Photoch~misfryendPhafabiology.7.587(19681. (201 Longworth J. W., inreferenm(12J. (21) Bowman.R.L., Canfre1d.P.A.. andUdedriend, S.,Seienrr. 122.32119551. 1221 Duggan.E.D., andUdenftisnd, S . . J B i d C h m . , 223,313~19591. 1211 Dugcan. D. E.. Bowman. R. L... Brodie.. 9.9.. . and Udenfriend. S.. Awh. Biochem. b%phys.. 68; 1119571. (241 Teal.. F. W. J.. andWeber, G.. Biochem. J.,65,476(19571.
. .
Similar chemical photooxidations occur with steroidal systems. Cholesterol, for example, is oxidized t o cbolesterol-a-epoxide a known chemical carcinogen. Though studies to date have related only t o direct radiation studies, experiments in progress probably will show t h a t photosensitizers also can be used t o cause cholesterol oxidation. In this way the aromatic amino acids can also participate a s photocarcinogens.
HO
The photogenesis of skin cancers is very complicated and many different kinds of photochemical and chemical reactions probably trigger tumor growth. We have concentrated primarily on the proteins in this paper because there seems t o be ample evidence t h a t they may be involved in the initial photochemical act. A grant from the Research Corporation supporting this work is gratefully acknowledged.
. .
.
.
.
A k d ~ & h ~, e l k s kSSR, , 9,647 (19651. I291 Weinrvb. I.. and Stciner. R. F..Biochemistrv, 7,2488 (19681 (29) fin. W . E . andsong, P. S . , J Am. them Soc., 91.1882(19681. (301 GrosswinerL. I., and Mslse. W. A,. Rodiotion Research. 10.515 (19691. (311 Nae~Chaudkuri.J..and Auwnstein. L.. Blocham. Bioohvs. Acta.. 140.381 119671.
(371 Aver. H.E.. andDoty,P.,Blochemisfry,8.170S118661. 1381 Longuorth. J. W.,Biopolym#rs. 4. 1131(19661. (39) Aueenstein.L. G..snd Ghiron. C.A..Ploc. Not. Acod Sei. U.S.. 47.1530(19611. (401 *ugonstcin. L. G.. andRiley, P..PhoforhamirfryodPhotobiology. 3,353119611. (4ll Dme, K.,Biophyaik, 1.316(19MI. (A21 Dme, K., Photoehemirtryond Pholobiolog: 6,437 (19671. I431 Fujimari,E..Biocham~sfry.5. 1034(19661. (Mi Cooper.0. R., and Dsvidron, R.J.,Biorh~m.J, '37.139(19651. (451 Perrsse, N. I.. Kondakova, N. V.. Kalabukhova. T. N., Ylsdimirov. Yu. A . and Eidus.L.Kh.Biop12ibo. 13.24119681. (461 Neekcri, D. C., and Turner, L. R.. unpuhlishedresults. 1471 Vlsdiminov, Yu. A,. Roahchupkio, 0. I.. and Rranko. E. E., Photochemistry and Photoblology, 11,227119701. K.,andRajcu&~,B..Phofochemis~ryodPhotobiolog..1.18111962~. (481 DCBO. (49) Hecht, S.,Schlaer. S., andpirenue. M. H., J , Gen. Phyaiol. Z5,819(L9121.
Chem Ed 73 Chem Ed 73, a conference to stimulate an exchange of ideas among high school teachers in Canada and the United States on the teaching of chemistry, will he held at the University of Waterloo, Waterloo, Ontario, August 21-24, 1973. The conference is under the joint auspices of the Chemical Education Divisions of the Chemical Institute of Canada and the American Chemical Society. Sessions are planned for the following topics: Individualized Chemistry Learning, What Students Think of High School Chemistry, Chemistry for Students not Bound for Universities, Environmental Chemistry, Dynamic Chemistry for the Twelve-Fifteen Age Group, Is High School Chemistry Adequate for Industry's Needs?. Low Budeet Chemistry. and many others. There will also be discussion periods and informal exchanges of inImnslwn.
, morels are a w h l d c . Enquirk5 There will he arrivrties planned for families. Student residences. ~ n m p i r e sand and reqorsrq lor reglsrratwn forms and fwrher pn,gram infurmin:on snodld he addressed to: Mr. L. H. Sibley. Chem Ed 73. 51 Cstherrnes Collegiate In,t~tureand Voratlmal School, 31 Cntlwrinr Sr..S t . Csrhermes. Ontario, Canada
168 / Journal of Chemical Education
most important tryptophane metabolites which have been shown to be carcinogenic are 3-hydroxykynurenine, 2amino-3-hydroxyacetophenone and 3-hydroxyanthranilic acid. The photooxidation of tryptophane produces Nformylkynurenine
as well as kynurenine either of which could be converted to the carcinogenic substances by enzymes.
Literature Cited ,I) Eiiia. C., and Wells, A. A,, "The Chemical Action of Ultraviolet Rays: Rainhold, NswYark. 1941. 121 McLaren. A,. and Shuaar. D.. "Phatoehmiatry of Nucleic Acids and Proteins? PergamonPIess, oxford. 1964. (3) Steiner. R. F.. and Weinryb. I.. "Excited States of Nucleie Adds and Proteins." Plenum press. New York 1971. (1) Wong, S. Y., "The Basic Prineiplos of Nueleic Acid Chemistry." (Editor: p. Tn'o, PO..) AcademicPre3s. New York. (5) Bouie. W.T.. Science, 21.374 (19131. (61 Setlow, H., and Doyle, B., Bioehi. & Biophya.Acto., 24, n 119571. I71 MeLaren. A. D.. Ewmologic. 18.81 (19571. (81 Riai. S., D m , K., Rathinssamy, T. K., and Augcnsteio. L., Photochemistry and Phofobiology. 6.423(19671. (9) Augemtein. L., Photochemistry ondPholobiology. 7,613119681. (10) ~ . ~ k D.C.,Turner, ~ n . L. R.. Daub, E..andvanderJag.D.L., unpublishednaulfa. (11) ~ i t ~ ~ ~ T. tB., ~ Seiji, i ~ hM.,, and MeGugan, A. D., New EnglondJ. Med, 265, 328 (19611. (12) Weinryb. I., end Steiner. R. F., "Luminescenee of Nuclei= Acids and Pmtoins" Plenum Ress. NowYork. 1'371, p. 279. (131 Konev. S. V., "Fluorescence and Phosphoroscenco of Nucleie Aeida and Pr0teinn.l. Plenvm Ren%1967, p. 1. (14) Neckem, D. C.. "Mechanistic Organic Phofoehemistry: Reinhold, New York, 1987. (151 Gillsm, A. R.. and Stem. R. S., "Absorption Speefrs of Organic Molecules,.l k nold, 1964. 1161 Jsffp. H., and Orehin. M.. "Ultraviolet Speefroseopy oforganic Molecu1os.l. John Wiley and Sons, Inc. New York. 1965. (171 Neeken. D. C.. Daub.E., a n d v a n d d a g , D.L.. unpublishdresulfa. (IS1 MeClaren, A. D.. and Hidalgo-Saluatierra. 0 . . Photochemistry and Phofobio1o.g~. 3.349 11964) (191 Longworth, J . W..Photoch~misfryendPhafabiology. 7.587(19681. (201 Longworth J. W., inreferenm(12J. (21) Bowman.R.L., Canfre1d.P.A.. andUdedriend, S.,Seienrr. 122.32119551. 1221 Duggan.E.D., andUdenfrisnd, S . . J B i d C h m . , 223,313~19591. 1211 Dugcan. D. E.. Bowman. R. L... Brodie.. B. B... and Udenfriend. S.. Awh. Biochem. b%phys.. 68; 1119571. I241 Teal.. F. W. J.. andWeber, G.. Biochem. J.,65,476(19571.
. .
Similar chemical photooxidations occur with steroidal systems. Cholesterol, for example, is oxidized to cbolesterol-a-epoxide a known chemical carcinogen. Though studies to date have related only to direct radiation studies, experiments in progress probably will show that photosensitizers also can be used to cause cholesterol oxidation. In this way the aromatic amino acids can also participate as photocarcinogens.
.
. .
.
.
A k d ~ & h ~, e l k s kSSR, , 9,647 (19651. I291 Weinrvb. I.. and Stciner. R. F..Biochemistrv, 7,2488 119681 (29) K U ~ ~ ;W " . E . andsong, P. S . , J Am. them Soc., 91.1882(19681. (301 GrosswinerL. I., and Mslse. W. A,. Rodiotion Research. 10.515 (19691. (311 Nae~Chaudkuri.J..and Auwnstein. L.. Blocham. Bioohvs. Acta.. 140.381 119671. Gottingen. 1951. (331 Langworth. J. W., Phororhornistryand Pheloblology, 7,587 119681. (34) Edeiho~h,H.,Brand,L., and Wi1ehek.M.. Bioehemistly, 6,547119671. (351 TenBmch. J. J.. Longworth. J. W., and Rak", R. 0.. Biorham. Biophys. Arto., 175, LO(19691. (361 Coianl. A,. Peg~ion.E.. Verdini. A. S.. and Torhoievieh. M.. Bimo!ymelr. 6.963
,.*-,. ,I-,
HO
The photogenesis of skin cancers is very complicated and many different kinds of photochemical and chemical reactions probably trigger tumor growth. We have concentrated primarily on the proteins in this paper because there seems to be ample evidence that they may be involved in the initial photochemical act. A grant from the Research Corporation supporting this work is gratefully acknowledged.
168 / Journal
of Chemical Education
(371 Aver. H.E.. andDoty,P.,Blochemisfry,8.1708118661. I381 Languorth. J. W.,Biopolym#rs. 4. 1131119661. (39) Aueenstein.L. G..snd Ghiron. C.A..Ploc. Not. Acod Sei. U.S.. 17.1530119611. (401 Augonstcin. L. G.. andRiley, P..PhoforhamirflyodPhotobiology. 3.353119611. (4ll Dme, K.,Biophyaik, 1.318(19Ml. (A21 D a e , K., Photoehemirtryond Pholobiolog: 6,137 (19671. I431 Fujimari,E..Biocham~sfry.5. 1034(19661. (MI Cooper.0. R., and Dsvidron, R.J.,Biorh~m.J, 91.139(19651. (451 perrsse, N. I.. Kondakova, N. V.. Kaiabukhova. T. N., Ylsdimi.ov. Yu. A . and Eidus.L.Kh.Biop12ibo. i3.21119681. (461 Neekcri, D. C., and Turner, L. R.. unpuhlishedresults. 1471 Vlsdiminov, Yu. A,. Roahchupkio, 0. I.. and Rranko. E. E., Photochemistry and Photoblology, 11,227119701. K.,andRajcu&~,B..Phofochemis~lyodPhotobiolog..1,181119621. (481 DCBO. (49) Hecht, S.,Schiaer. S., andpirenue. M. H., J , Gen. Phyaiol. 25,8191L9121.
