Science Update
Biochemistry By far the most striking advances in biochemistry during the past 18 months have emanated from studies of DNA. Other specialty areas, although some what less glamorous than DNA, also have seen important advances. They include developments in cancer re search, protein chemistry, neurochemistry, immunology, and lipid bio chemistry. Interest in recombinant DNA tech nology continues to dorninate a large segment of biochemistry. In some ways this technology itself is old hat by now. As a technology, it has spawned a new industry dedicated to exploiting methods for the manufacture of various mole cules, including peptide hormones, antiviral proteins, and other potentially valuable items. Already, for instance, two peptide hormones—growth hormone and insu lin—made by recombinant DNA tech nology are undergoing preliminary clinical trials. A steadily increasing number of other molecules, though not yet ready for full use outside an experi mental setting, have been made this way. Thus progress has been notewor thy across a broad front. Interferon, for example, is being made and studied in tensively by a half-dozen or more dif ferent research and development groups. Proteins that might serve for making vaccines against hoof-andmouth disease and hepatitis are being made with the help of gene-splicing technologies. Other successful projects include syntheses of amino acids po tentially useful in animal feed supple ments and of enzymes potentially useful in medicine. These advances are being helped along by the quick transfer of insights gained at the bench level. For example, several research groups are trying to mechanize certain key synthetic steps in the gene-splicing procedure. Thus, improvements in chemical methods for linking nucleotides to make long DNA molecules have prompted development of machines to do the job. Here too, some of the fundamental science is moving ahead hand-in-hand with the advancing technology because of the great demand to apply each new ad vance at great speed. The commercial pressures on the gene splicers can create a false im
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C&ENMarch2, 1981
pression that the rapid pace is being set by the lure of the marketplace. Actually, much of the speed in DNA research comes from the efficiency and efficacy of methods at the disposal of a tradi tionally highly competitive cadre of scientists. And, even when those sci entists fall back on what they term more classical techniques, their speed and daring in divulging new proposals about DNA can stem also from the conviction that whatever new might be happening with that molecule is bound to be im portant. Alternate DNA structure Two recent proposals provide ex amples. One involves the structure of DNA, the other how DNA molecules are made in living cells. The recently pro posed structure, called the Ζ structure, has parts of the DNA molecule rear ranging, with some base pairs forced closer to the edge of the long molecule instead of being straddled across its center. Once on the edge, these base pairs might be more susceptible to harmful agents, such as chemical mu tagens. This alternate structure of DNA, based on x-ray crystallographic analysis of short " m o d e l " compounds, is not in tended to replace—but rather to sup plement—the familiar Watson-Crick double helix structure. A recent proposal for how DNA is replicated in cells of higher (that is, complex) organisms tries to take into account evidence for structural scaf folding within cells. Cell biologists have gradually wooed biochemists into be lieving what they can see by electron microscopy. In turn, biochemists are convincing biologists that those visible matrices not only are made up of spe cialized and distinct molecules but also serve specialized purposes. One such purpose in the nucleus of higher cells might be to organize the lanky mole cules of DNA, preventing them from getting hopelessly tangled, particularly during replication. Intense interest is focused on how genes are controlled in higher organ isms. Here simple—one might say boldly simple—approaches have dom inated an otherwise complicated re search challenge. DNA molecules from one kind of organism have been jammed
by various means into the cells of other organisms—with varying degrees of success. The ultimate goal, from a medical standpoint, is to move genes between cells to correct genetic defi ciencies. For instance, a defective he moglobin molecule (the oxygen-carrying moiety in red blood cells) might be re placed by introducing a working (or workable) hemoglobin gene into de fective cells. So far, the basic manipulations leading in this direction have been un dertaken successfully: Skillful experi menters can move genes from cell to cell. But, except for genes that make simple enzymes that carry out useful but unbeguiling "housekeeping" chores, most transplanted genes won't work in their new settings. Perhaps that should come as no sur prise. Many of the genes already present in higher cells will work only under cer tain circumstances. Moreover, some stretches of DNA, known as pseudogenes, don't seem to function at all. These pseudogenes are segments of DNA whose sequence (that is, nucleo tide base code) bears a remarkable re semblance to that of a working gene. For example, whereas one authentic gene makes a polypeptide that's part of the protein hemoglobin, its pseudogene makes no such polypeptide. Usually, control signals present in pseudogenes prevent their synthesis and function. Though their purpose is unknown, re searchers speculate that pseudogenes are formerly active genes either on their way to refinement, to radical change (to undertake altogether different bio chemical duties), or to extinction. Changing views about cancer Several lines of evidence may be converging to provide a significantly changed view about how cells become malignant. Much of this research is new, and some of it still has not been proved universal. Cancer research, of neces sity, must be treated tentatively because the field has been strewn with so many false leads. Nonetheless, the picture is changing. For example, the suggestion was made recently that sizable rearrange ments in DNA may be the key prelude to a cell's turning malignant. Previously,
Recombinant DNA technology remains center of attention; other new advances include developments in cancer research, protein chemistry, neurochemistry, immunology, and lipid biochemistry most researchers concentrated their efforts on explaining how relatively minor changes in DNA could lead to cancer. Now, the notion that grander changes are necessary is being considered seriously. The notion deserves such consideration for several reasons. Researchers studying viruses that can cause tumors in animals, such as rodents and birds, now find DNA sequences in the genes of those viruses that are remarkably like sequences called transposons found in certain bacteria. Transposons can permit pieces of DNA to hop from one place along the molecule to another. That hopping in turn can affect whether and how certain genes are used and controlled. Such control very well may be crucial to the process whereby a cell becomes malignant. From other investigations come hints that the miscreant genes responsible for cancer are none other than normal cellular genes out of normal control. Here again, the idea that such genes or their control regions could somehow be disarranged—and hence functionally deranged—makes sense. Evidence is mounting that vast biochemical networks are involved in this rupture from normal controls. And, much to the delight of the scientists involved, the genetic and biochemical approaches seem to be converging on a coherent, though still incomplete, explanation of what's happening. Proteins operate controls Despite the attention, deserved though it may be, devoted to DNA, genetic events often are dependent on humble proteins. Some of the most widely accepted findings on how particular enzymes (catalytically active proteins) are controlled in cells are being extended into new and important domains of cellular metabolism. One prime example is known as covalent modification of enzymes. For well over a decade, biochemists have realized that certain enzymes can have their catalytic activities drastically changed when they are joined to (or have removed) one of several kinds of small residues, such as phosphate groups. This manner of control now seems more pervasive than ever. Moreover, the va-
riety of enzymes and factors responsible for exerting such control seems ever more specialized and diverse. Occasionally, a refreshing and seemingly simple new theme emerges from the throng of important control proteins found in cells. Calmodulin is a recent example. It is a small, nearly ubiquitous protein with the task of binding calcium ions. By doing so, this protein can play a pivotal role in controlling an impressive variety of cell activities, depending on cell type and the principal metabolic contributions made therein. For instance, calmodulin is believed to control prostaglandin synthesis in certain cells by acting at the peak of an enzyme cascade liberating arachidonic acid. Even simpler molecules—peptides, which are small proteins—also are implicated in ever widening control schemes. This is particularly true for nerve cells where a large variety of peptides now are believed to act as neurotransmitters. Neurotransmitters are the chemicals, usually relatively small molecules, that deliver signals between individual nerve cells. More and more different kinds of peptide neurotransmitters have been identified recently. And, to the consternation of some neurochemists, more than one type of neurotransmitter has been found in single cells. What this means is not yet possible to say. It suggests that the molecular coding among nerve cell circuits is extremely complex. Two relatively new methods for probing the intricacies of nerve cells are exciting the scientific and medical communities. One involves the use of positron emission tomography (or in animals, the combined use of a nonmetabolized glucose analog and a precise radiographic mapping technique to trace that analog's whereabouts), and the other the use of monoclonal antibodies. Both strategies call for locating precisely where in tissues certain biochemical activity is under way or where specific molecules are found. Also, positron emission tomography is being used more frequently on patients—for instance, to direct brain surgeons to avoid vital nerve centers while removing diseased centers. The technique also is providing new clues about sensory areas of the brain that are active when, say, a
person is listening to music or watching a flickering light. Immunology and other defenses Monoclonal antibodies continue to receive ever wider use. The method recently has been demonstrated with human cells, extending the technology from the rodent system, where it was developed. By far the greatest excitement in immunology comes from the emerging story of how the many kinds of antibody protein molecules are formed. Diversity is achieved by means of a multistep process that systematically introduces variety without sacrificing much in the way of biochemical economy. Crucial to the process is the need to use limited gene space to its fullest extent. Thus diversification is tied into the flowering of the immune system's cells, which differentiate and become specialized in such a way that the innumerable genetic possibilities for antibody molecules are sorted and sifted along the way. The juggling of antibody genes during these steps provides for the more than 10 8 antibody proteins that form part of one of the body's most important protective arrays. The body has other defenses at its disposal, one of which eluded chemical description for several decades. The search recently turned up an unusual family of molecules, called leukotrienes, that contain fatty acids linked to short peptides. Some of the leukotrienes are involved in anaphylaxis, a defense mechanism of the body gone awry that is sometimes life-threatening. The fatty acid portion of leukotriene molecules is related to the physiologically potent family of fatty acids called prostaglandins. Attachment of a peptide at once makes the resulting leukotriene molecule potentially more polar, and therefore able to travel into different cellular domains, and also contributes another means for establishing molecular diversity. Diversity continues to be an important, if still poorly understood or fully appreciated, theme in the biochemical world. The leukotrienes represent a whole new clan, whose full physiologic role will make a story rich in the telling. Jeffrey Fox, Washington
March 2, 1981 C&EN
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