RESEARCH
Carcinogens Have Common Features Electron donating and stereochemical properties of hydrocarbons are proposed as key properties in cancer induction Carcinogenicity of organic compounds appears to be related directly to their electron donating ability and to their geometry, Dr. Charles B. Huggins of the University of Chicago's Ben May Laboratory for Cancer Research told the 99th Annual Meeting of the Na tional Academy of Sciences, held in Washington, D.C. (C&EN, April 30, page 43). Dr. Huggins and his co-worker Dr. Nien-Chu Yang find that some substituents at critical locations on the parent molecule can give carcinogenic properties to otherwise inactive hy
drocarbons; among these are methyl, amino, dimethylamino, and acetylamino groups, as well as alicyclic and aromatic rings. For ex-ample, 7,12dimethylbenz ( a ) anthracene is one of the most potent cancer producers known, Dr. Huggins says. It induces cancer in rats after one feeding, thereby matching the effect of a single exposure to a sublethal dose (400 roentgens) of gamma rays. The par ent compound, benz ( a ) anthracene, has no carcinogenic activity; one methyl group in either meso position is weakly carcinogenic while two
methyl groups endow the molecule with very great ability to induce cancer. Similarly, 2-aminophenanthrene is a strong carcinogen, while phenanthrene is not carcinogenic. And cholanthrene, formed by addition of a cyclopenteno ring across the C-7 and C-8 positions of benz ( a ) anthra cene, and benzo(a)pyrene (formed by addition of an extra aromatic ring to benz (a) anthracene) are both cancerproducing compounds. Cancer-promoting substituents of aromatic compounds, such as methyl
Carcinogens Fit Base Pair Frame
Benzo(a)pyrene
Cytosine/guanine
Progesterone
7,12-Dimethylbenz(a)anthracene GEOMETRY. Models help explain the theory proposed by University* of Chicago's Dr? Charles B. Huggins and Dr. NienChu Yang as to the ability of certain molecules to induce cancer. The carcinogenic hydrbcarbons, 7,12-dimethylbenz(a)anthracene and benzo(a)pyrene, fit perfectly within the three-dimensional frame which encases nucleic acid base 40
C&EN
MAY
14,
1962
pairs such as cytosine/guanine. Steroids, which are bulkier molecules (5 to 6 A. against 3.6 Α.), cannot fit entirely within the frame. This suggests that cancer-inducing hydrocarbons can be accommodated perfectly within the two helical poly nucleotide strands of a nucleic acid molecule; steroids can not fit-within the strands, the Chicago scientists say
and amino, share the common ability to donate electrons to appropriate acceptors. This is in accordance with the observation made by Dr. A. SzentGyorgyi and co-workers at the Institute for Muscle Research, The Marine Biological Laboratory, Woods Hole, Mass., in 1960, that carcinogenicity of aromatic hydrocarbons is correlated with their ability to form charge -transfer complexes with local acceptors. But charge transfer is not the only criterion for cancer-inducing properties of compounds, Dr. Huggins says. For example, a number of hydrocarbons that can donate electrons— indole and phenanthroline, for example—are noncarcinogenic. Geometry. The molecular similarity of carcinogens and steroids was pointed out previously by Harvard's Dr. Louis F. Fieser, Dr. Huggins notes. However, the Chicago scientists have extended a study of this geometric similarity to include the purine and pyrimidine base pairs—for example, guanine/cytosine and adenine/thymine—of nucleic acids. They made a molecular model of the base pairs guanine/cytosine' and constructed a frame around it. Models of all the known aromatic carcinogens as well as models of steroids such as testosterone and progesterone can be inserted within this frame. They thus show the similarity in geometric pattern among the three classes of compounds—carcinogenic aromatics, steroids, and base pairs of nucleic acids. This geometric similarity underlies Dr. Huggins' and Dr. Yang's theory. It is already known that nucleic acids consist of two helical, intertwined polynucleotide strands. Hydrogen bonding occurs between pyrimidine and purine bases on opposite strands. If the helix were unraveled, it would resemble a ladder, the linked base pairs resembling the rungs. Carcinogenic aromatics—like the base p a i r s are planar and have identical thickness (3.6 A.); they could fit perfectly within the ladderlike frame. Steroids, being thicker (5 to 6 A.), could not. This may explain why steroids are not carcinogenic, the Chicago scientists believe. To induce cancer, therefore, it seems that hydrocarbons must not only be electron donors, but must also closely resemble the base pairs of nucleic acids in solid geometry, Dr. Huggins says. The recipient of the charge transfer isn't known; but very probably it's a nucleic acid, he adds.
Trypsin Can Function in Anhydrous Medium Evidence shows trypsin acts as a catalyst in pure dimethyl sulfoxide, indicates reaction path Trypsin shows catalytic activity in pure dimethyl sulfoxide, according to chemists at Adelphi College, Garden City, N.Y. The Adelphi workers find that crystalline trypsin reacts with p-nitrophenyl acetate in pure dimethyl sulfoxide, and have followed the reaction spectrophotometrically. They have also found that trypsin exhibits proteolytic activity in mixtures containing 95% dimethyl sulfoxide and 5% water. The reaction between trypsin and p-nitrophenyl acetate in dimethyl sulfoxide may throw some light on the reactive site of the protein, say Adelphi's Dr. Frederick A. Bettelheim, Kenneth Smith, and Dr. Aaron Lukton (now at Brooklyn College). Their spectrophotometric data give evidence of a two-step reaction between trypsin and the nitrophenyl acetate in dimethyl sulfoxide. First, there is a shift of the enzyme peak from 280 m/x to longer wave lengths. This shift is very rapid, indicating a fast first step in the reaction. Then peaks at 320 and 432 m^ gradually appear; these indicate that
a much slower second step takes place. The peaks at 320 and 432 m^ correspond to the liberation of p-nitrophenol. In experiments using equimolar concentrations of enzyme and substrate, the amount of p-nitrophenol liberated shows that the reaction is complete. The shift in the 280 m{x peak indicates that a rapid intermediate formation may occur between the enzyme and the substrate, p-nitrophehyl acetate. This probably involves the tyrosine group of trypsin, Dr. Bettelheim says. Not only does the enzyme peak shift toward longer wave lengths, but its intensity also decreases. Since the absorption coefficient of the intermediate is not known, it's not certain that the absorption decrease is caused by a decrease in concentration of the intermediate complex, he adds. The Adelphi chemists have tried to isolate the intermediate by precipitating it from dimethyl sulfoxide with an excess of ether, but were not successful as both the precipitate and filtrate
ENZYME. Dr. F. A. Bettelheim uses a spectrophotometer to study the reaction between trypsin and p-nitrophenyl acetate in dimethyl sulfoxide. The data obtained from the Adelphi College work give evidence of a two-step reaction path, and aid in determining the reactive site of the enzyme MAY
14, 1 9 6 2 C & E N
41
