IT'S ELEMENTAL!
ZINC RONALD BRESLOW, COLUMBIA UNIVERSITY
Z
INC IS AN IMPORTANT ELEMENT,
especially because it plays a key role in many enzymes that are essential to life. For this reason, we humans consume approximately 20 mg of Zn(II) in our daily diets. I have also had a series ofmore special interactions with zinc. Some years ago, my wife, Esther, was writing her master's thesis on the enzyme carbonic anhydrase and proposed the correct mechanism of the enzyme —its zinc acts both as a Lewis acid, by coordinating to a developing oxyanion, and as the coordination site for a basic hydroxide ion, the nucleophile that adds to the carbon dioxide molecule. Later, my research group synthesized zinc-based mimics ofcarbonic anhydrase and of carboxypeptidase. We also showed that carboxypeptidase itself uses the carbonic anhydrase type of mechanism. Another example of the special properties of zinc in enzyme catalyst systems was waiting for me, but I didn't know it for a while. In 1974, Paul Marks, then dean of Columbia University's medical school, came to me with a remarkable story and challenge. Charlotte Friend ofMount Sinai School of Medicine had discovered that
ZINC AT A GLANCE Name: From the German zink, tin. Atomic mass: 65.41. History: Ores were used in medieval times in China and India. The pure metal was isolated first in India in the 13th century, then in Europe by the German chemist Andreas Sigismund Marggraf in 1746. Occurrence: Comprises less than 0.007% of Earth's crust. Appearance: Bluish white, solid metal at room temperature. Behavior: Tarnishes in air and brittle when cast. The metal is a skin irritant; otherwise, it and most zinc compounds are nontoxic. Uses: Mostly used for galvanizing iron, in alloys (such as brass), and in drycell batteries. Zinc oxide is used in photocopiers and sunscreens; its sulfide is a phosphor used in cathode ray tubes. Small amounts of zinc are essential to biological function.
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certain cancer cells—pre-erythrocytes that were infected with a virus (MEL cells)— were induced to differentiate to normal red blood cells in the presence of high concentrations of dimethylsulfoxide. We set out to see ifwe could produce much more potent compounds for this remarkable "reform" of cancer cells, learn how they worked, and perhaps even find whether they were useful with other types of cancer. By a series of hypothesis-driven syntheses of novel compounds that were increasingly potent toward MEL cells, we eventually came to a compound with the acronym SAHA (suberoylanilide hydroxamic acid), with ahydrophobic group connected to ahydroxamic acid unit through apolymethylene chain. We then established that SAHA would induce other cancer cells to transform into normal cells or go into apoptosis—including the entire group of 60 human cancer cell types that are maintained for study at the National Cancer Institute. So we set out to determine the biological target ofSAHA and some related compounds.
the news that the "purified" H D A C no longer had enzymatic activity but that the addition of Zn(II) salts restored its activity "Purification" had removed the zinc, and HDAC was indeed a zinc enzyme! Our collaborator Nikola Pavletich was able to prepare crystals of SAHA bound to another H D A C and determine the structure of the enzyme/inhibitor com-
ENZYME POCKET
LONG NARROW CHAIN
BINDING END (HYDROXAMIC ACID)
F I T T I N G I N An X-ray structure of SAHA bound to the zinc of an HDAC.
plex by X-ray We saw that the hydroxamic acid group was coordinated to the Zn(II) in the enzyme. Furthermore, the structure strongly supported the idea that this enWe created a radioactive photoaffinity zyme uses zinc in the same way that carcompound based on SAHA to screen cell bonic anhydrase does, binding the carbonyl components, determining which ones bound oxygen of the acetyl group on lythe drug. At the same time, we besine while delivering a bound hycame aware of studies by M. droxide to the carbon of that %shidainjapan concerning anatgroup. Our inhibitor simply ural product, ttichostatinAOTSA), mimicked this structure. that also induced the differentiation of cancer cells. 'Ybshida had Since that time, our consorshown that TSA acted to inhibit tium of chemists and biologists the enzyme histone deacetylase has been able to determine the THIS ELEMENT (HDAC). pathway by which HDAC inhiBROUGHT TO YOU BY bition by SAHA causes important We tested our photoaffinity BIOTECHNOLOGY anticancer effects by regulating compound with H D A C and INDUSTRY gene transcription. SAHA has saw that, indeed, it was bound. ORGANIZATION been shown to be an effective anFurthermore, SAHA inhibited ticancer agent in animal trials—and for the HDAC, and the potency of our other drugs past three years, in human trials—against a as inhibitors of HDAC ran parallel to their variety ofcancers. The results are very prompotency in inducing the differentiation of ising, and there is an excellent chance that MELcells. Our biological collaborators were SAHAor some related compound willprove able to clone and express the enzyme, so I to be an important tool in the fight against suggested that they examine the purified cancer. Zinc will once again turn out to be enzyme for metals, specifically for Zn(H), uskey in the biology that is central to life, as the ing atomic absorption spectroscopy They element that permits HDAC to play its role called back with the news that HDAC conin regulating the expression of genes. tained #0 metals, but I didn't believe it. SAHA, with its hydroxamic acid group, looked Ronald Breslow is University Professor of like a metal ligand, and the catalytic action Chemistry at Columbia University He received of the enzyme HDAC looked like the kind the US. National Medal ofScience in 1991 and ofreaction that other zinc enzymes can perthe 2003 Welch Award. form. I received another telephone call with HTTP://WWW.CEN-ONLINE.ORG
