PERSPECTIVES rosis by avoiding this toxicity as well as by lowering LDL levels. Despite these unanswered questions, the data on PCSK9 are consistent with the extensive genetic, epidemiologic, experimental, and therapeutic data that justify an aggressive public health program aimed at lowering LDL levels before the atherosclerotic process has become advanced. Early intervention may well put an end to the epidemic of coronary heart disease that ravaged Western populations in the 20th century. References and Notes 1. A. Keys, Seven Countries: A Multivariate Analysis of Death and Coronary Heart Disease (Harvard Univ. Press, Cambridge, MA, 1980). 2. J. L. Goldstein, H. H. Hobbs, M. S. Brown, in The Metabolic and Molecular Bases of Inherited Disease, C. R. Scriver, A. L. Beaudet, W. S. Sly, D. Valle, Eds. (McGraw-Hill, New York, 2001), chap. 120, pp. 2863–2913.
3. Cholesterol Treatment Trialists’ (CTT) Collaborators, Lancet 366, 1267 (2005). 4. J. C. Cohen, E. Boerwinkle Jr., T. H. Moseley, H. H. Hobbs, N. Engl. J. Med. 354, 1264 (2006). 5. K. N. Maxwell, J. L. Breslow, Proc. Natl. Acad. Sci. U.S.A. 101, 7100 (2004). 6. S. W. Park, Y.-A. Moon, J. D. Horton, J. Biol. Chem. 279, 50630 (2004). 7. S. Rashid et al., Proc. Natl. Acad. Sci. U.S.A. 102, 5374 (2005). 8. M. Abifadel et al., Nat. Genet. 34, 154 (2003). 9. K. M. Timms et al., Hum. Genet. 114, 349 (2004). 10. T. Lerens, Clin. Genet. 65, 419 (2004). 11. J. Cohen, A. Pertsemlidis, I. K. Kotowski, R. G. C. K. Graham, H. H. Hobbs, Nat. Genet. 37, 161 (2005). 12. ARIC Investigators., Am. J. Epidemiol. 129, 687 (1989). 13. Z. Chen et al., Brit. Med. J. 303, 276 (1991). 14. W. F. Enos, R. H. Holmes, J. Beyer, J. Am. Med. Assoc. 152, 1090 (1953). 15. M. R. Law, N. J. Wald, A. R. Rudnicka, Brit. Med. J. 326, 1423 (2003). 16. M. S. Brown, J. L. Goldstein, Atherosclerosis Suppl. 5, 13 (2004). 17. J. D. Horton, J. L. Goldstein, M. S. Brown, J. Clin. Invest. 109, 1125 (2002).
18. C. J. Vaughan, A. M. Gotto, Circulation 110, 886 (2004). 19. S. M. Grundy et al., Circulation 110, 227 (2004). 20. N. Bansal, J. K. Cruickshank, P. McElduff, P. N. Durrington, Curr. Opin. Lipidol. 16, 400 (2005). 21. J. D. Horton et al., Proc. Natl. Acad. Sci. U.S.A. 100, 12027 (2003). 22. K. N. Maxwell et al., J. Lipid Res. 44, 2109 (2003) 23. G. Dubuc et al., Arterioscler. Thromb. Vasc. Biol. 24, 1454 (2004). 24. M.S.B. holds shares of stock in Merck and Pfizer, two companies that discovered and market cholesterol-lowering drugs. He is on the board of directors of Pfizer. J.L.G. holds shares in five companies that discovered and market cholesterol-lowering drugs (Astra Zeneca, Bristol-MyersSquibb, Merck, Pfizer, and Schering-Plough). Since 2003, he has received consulting fees from two companies that develop drugs and related strategies to lower cholesterol (Schering-Plough and Amgen). The authors do not have planned, pending, or awarded patents related to the work discussed here, nor do they have relationships with companies that market generic drugs. Authors receive research support from the NIH and the Perot Family Foundation. 10.1126/science.1125884
CHEMISTRY
Dissolved Natural Organic Matter as a Microreactor
Organic matter dissolved in rivers and oceans undergoes poorly understood complex reactions, including the light-induced formation of a highly reactive form of oxygen that can subsequently react with pollutants and water contaminants.
John P. Hassett lmost all organic molecules dissolved in fresh and marine waters come from natural sources, ultimately derived from decay of algae and higher plants. These molecules form a very complex mixture whose underlying molecular structures are poorly characterized (1). Consequently, they are often referred to in aggregate as dissolved organic matter (DOM) and quantified as dissolved organic Enhanced online at carbon (DOC). www.sciencemag.org/cgi/ They are also recontent/full/311/5768/1723 ferred to as aquatic humic substances, in recognition of a similarity between freshwater DOM and soil organic matter. Although chemical characterization of DOM is elusive, properties of DOM that influence the physical and chemical behavior of natural and pollutant ions and molecules in water have often been investigated. On page 1743 of this issue, Latch and McNeill (2) present a striking study of how DOM can enhance the reactivity of a hydrophobic molecule by bringing it into close association with a photochemically produced, short-lived reactant, singlet oxygen. Hydrophobic contaminants in water are a major concern. Those that are chemically and bio-
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The author is in the Chemistry Department, State University of New York, College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210–2726, USA. E-mail
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logically stable, such as the insecticide DDT, accumulate in aquatic food chains and can reach levels that threaten fish-eating organisms such as humans and predatory birds. Some, such as 2,3,7,8-tetrachlorodibenzodioxin (TCDD), are very toxic. Others, such as natural and synthetic steroid hormones, can disrupt endocrine functions in aquatic organisms. DOM has both hydrophobic and hydrophilic groups, giving it micelle-like properties. Therefore, it is able to
bind other molecules by hydrophobic interactions. Hydrophobes in apparent solution in natural waters may thus exist in two states: one that is truly dissolved and one that is bound to DOM (see the figure). This binding decreases a hydrophobe’s apparent volatility, bioavailability, and attachment to particles and increases its apparent solubility. Binding also brings a hydrophobe into a chemical environment that is distinct from water, and so can affect its reactivity. DOM contains groups (chromophores) that absorb sunlight. Absorbance is strongest in the ultraviolet but often extends into the visible Hair AIR portion of the spectrum, giving the brown color often associated with water from wetlands (bogs, peatlands, and swamps). Absorption of light by WATER 1O – this chromophoric DOM (CDOM) e–hyd 2 Hd can lead to formation of short-lived Hp Particles reactive products such as singlet oxyR˙ gen (1O2). Although many of these Hb products can react with hydrophobic compounds in water, their concentraDissolved organic matter Hbio tions as measured with hydrophilic probes (furfuryl alcohol in the case Hydrophobe-DOM interactions. Truly dissolved hydrophobic of singlet oxygen) are too low molecules (Hd) can be bound (Hb) to dissolved organic matter, to be important even at diffusiondecreasing their ability to be captured by particles (Hp), taken up controlled rates. However, Blough by organisms (Hbio), or escape into the atmosphere (Hair). When (3) and Burns et al. (4) have demonbound to DOM, they are more likely to encounter photochemically strated that ions or molecules associproduced, short-lived reactants such as singlet oxygen (1O2), ated with DOM encounter higher superoxide (O2–) organic radicals (R˙), or hydrated electron (e– hyd). concentrations of short-lived reac-
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PERSPECTIVES tants and react faster than expected on the basis of bulk solution concentrations of the reactants. Thus, CDOM is a microreactor that enhances the reactivity of a molecule by bringing it into close association with a very reactive intermediate. Latch and McNeill now demonstrate that hydrophobic molecules bound to CDOM encounter much higher concentrations of singlet oxygen produced photochemically from CDOM by light in the solar spectrum. A probe that is essentially completely bound to CDOM reports a 1O2 activity that is more than 100 times the average concentration encountered by hydrophilic furfuryl alcohol. The environmental significance of this finding is that a hydrophobic molecule that is only 10% bound to CDOM will still encounter a 1O2 activity that is 10 times the average concentration in the whole solution. Therefore, the potential for degradation of compounds susceptible to reaction with 1O2 is much greater than expected on the basis of experiments with furfuryl alcohol. Latch and McNeill introduce a model for distribution of 1O2 and find that it is limited to the CDOM matrix and to a “corona” that extends out a few nanometers from each CDOM molecule. Thus, it appears that most of the solution contains essentially no 1O2. The concept of a corona predicts enhanced reactivity not only of hydrophobic compounds but also of cations such as protonated amines attracted to the net negative charge of DOM. Blough (3) in fact
observed this to be the case for cationic nitroxides reacting with photochemically produced radicals. The localized nature of these reactions leads to quantitative differences from results expected for a homogeneous solution. Hydrophilic scavengers of 1O2 or other reactive intermediates have little affinity for the DOM matrix and therefore have a low probability of being present where 1O2 is formed. Thus, they are much less effective at interfering with the reaction of a hydrophobic molecule than would be expected in a homogeneous water solution. Latch and McNeill demonstrate this for azide ion, which has little quenching effect on the intra-DOM reaction, and also for D2O, which promotes reaction in homogeneous solution by stabilizing 1O2 but has no promotional effect on the intra-DOM reaction. Conversely, a hydrophobic scavenger (β-carotene in this case) is much more effective than expected. However, molar concentrations of hydrophobic compounds in natural waters tend to be much lower than the concentrations of DOM, leaving most of the DOM molecules unoccupied. Consequently, substantial quenching of intra-DOM reactions by hydrophilic or hydrophobic scavengers is unlikely under normal environmental conditions. Latch and McNeill use custom-made probe molecules that are ideal for these types of studies. Hydrophobic molecules are difficult to work with in water solution because of their low solu-
bility and tendency to attach to surfaces. By measuring the chemiluminescence of the reaction product, the authors are able to achieve very low detection limits on small subsamples. This helps to minimize spurious results from molecules attached to the container wall that might be included if the entire solution was extracted. They are also able to observe production of a product rather than disappearance of a reactant, which greatly aids monitoring slow reactions that can still be important in the environment. Intra-DOM reactions have received little attention, but as the present work demonstrates, they may be important photochemical mechanisms for transformation of organic chemicals in the environment. This has recently been shown to be true for intra-DOM reaction of the insecticide mirex in Lake Ontario (5), and is likely to be so for other compounds and other systems as well. References 1. R. Sutton, G. Sposito, Environ. Sci. Technol. 39, 9009 (2005). 2. D. E. Latch, K. McNeill, Science 311, 1743 (2006); published online 23 February 2006 (10.1126/science.1121636). 3. N. V. Blough, Environ. Sci. Technol. 22, 77 (1988). 4. S. E. Burns, J. P. Hassett, M. V. Rossi, Environ. Sci. Technol. 31, 1365 (1997). 5. K. L. Lambrych, J. P. Hassett, Environ. Sci. Technol. 40, 858 (2006). 10.1126/science.1123389
EVOLUTION
As oxygen built up in Earth’s atmosphere during the Precambrian, organisms evolved more complex biochemical networks. This expansion made feasible oxygen-dependent metabolisms that coopted parts of preexisting anaerobic ones.
Tracing Oxygen’s Imprint on Earth’s Metabolic Evolution Paul G. Falkowski ife on Earth created, and is dependent on, nonequilibrium cycles of electron transfers involving primarily five elements: hydrogen, carbon, nitrogen, oxygen, and sulfur (1). Although biophysical and biochemical reactions catalyze specific electron transfers at a local, molecular level, the metabolic consequences are global. Through opportunity and selection, metabolic pathways evolved to form an interdependent, planetary “electron market” where reductants and oxidants are traded across the globe. The exchanges are made on a planetary scale because gases, produced by all organisms, can be transported around Earth’s surface by the ocean and atmosphere. Exactly how these five elements came to form an electron market
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The author is with the Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences and Department of Geological Sciences, Rutgers University, New Brunswick, NJ 08901, USA. E-mail:
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place remains largely unresolved, however. On page 1764 of this issue, Raymond and Segrè (2) use an ingenious bioinformatics approach to reveal the evolution of metabolic pathways. Their analysis elegantly reveals not only the profound role that molecular oxygen (O2) has played in shaping the electron market place, but also the evolutionary constraints on, and trajectories of, the ensemble of electron traders. Before the evolution of free O2 ~2.3 billion years ago (3, 4), there was a glut of reducing equivalents on Earth’s surface. The first traders consumed electrons from the large populations of potential donors, including H2, H2S, and CH4 (5). These electrons, extracted either with the release of energy or with the aid of low-energy (infrared) solar photons, were sold at relatively low energy prices to acceptors such as CO2 and, to a lesser extent, SO4. Although there was a very large pool of an alternative electron acceptor, N2, considerable metabolic energy is required to reduce the gas to NH3 at physiological tempera-
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tures. Over the first 2 billion years of Earth’s history, the electron market evolved to produce a well-structured set of metabolic pathways distributed among groups of interconnected anaerobic microbes, each selected to conduct one, or at most a small subset, of redox reactions. Because of the relatively large investment in energy to oxidize water, the biggest electron-donor pool, H2O, remained biologically inaccessible. When and how the first photosynthetic organisms evolved that were capable of oxidizing water to oxygen remains one of the great mysteries in the evolution of life on Earth (6). However, between ~3.2 and 2.4 billion years ago, either through progressive gene duplication events and selection, or by massive lateral transfer of genes, or both (7, 8), an organism emerged that was capable of extracting four electrons from two molecules of water to form free O2 as a metabolic waste product. This waste product proved not only highly useful as an electron acceptor, but also potentially damaging to the intricate metabolic
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