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Glutamic acid: Advances in biochemistry and physiology
TIBS - March 1980
XX
The many sides of glutamate Glutamic Acid: Advances in Biochemistry and Physiology edited by L. J. Filer, S. Garattini, M. R. Kare, W. A. Reynolds and R. J. Wurtman, Raven Press, New York, 1979. $36.40 (xiv + 400pages) ISBN 0 89004 356 6 A non-essential amino acid that is present 'in large amounts in virtually all proteins seems an unlikely subject for an international symposium, with 26 papers contributed by a wide variety of authors, yet this 390 page book is devoted to glutamate. Perhaps we should not be too surprised, in view of the widespread use of monosodium glutamate (MSG) as a food additive, the central role of glutamate in ammonia metabolism, its role as an interface between amino acid and carbohydrate metabolism, and the mounting evidence that glutamate may have a neurotransmitter role of its own, as well as acting as a precursor of G A B A . Throughout the book, concern with the safety of MSG as a food additive is obvious we are told that the average per capita intake in Taiwan is 3 g per day, compared with a 'mere' 350 mg per day in the United States. There are several chapters on the unique flavour enhancing properties of MSG, and attempts to define its flavour psychometrically. The first chapter provides some interesting data on the physiology of taste in general, the effects of glutamate, and the synergism between glutamate and ribonucleotides at the taste receptors'- both glutamate and a variety of nucleotides are found in the ingredients traditionally used for flavouring in Japanese cuisine. One area of great concern over the safety of MSG has been in infant foods, and several chapters contain a thinly disguised plea for its contiiaued use, with comments on the lack of demonstrable harm, and the evidence that even relatively large amounts do not raise the blood glutamate concentration more than does a protein-rich meal. However, it is noted that adult mice and Rhesus monkeys are better able to tolerate a massive glutamate load than are neonates of these two species, and human adults have a poorer tolerance than either of these experimental species. We are also told that on a body weight basis the human baby is exposed to a greater intake of glutamate while it is being breast fed than at any other time in life. Maternal glutamate intake and blood levels seem not to affect the amount secreted in the milk. It is also comforting to be told that glutamate does not cross the -
placental barrier at concentrations greatly in excess of any that can be achieved other than by intravenous infusion. Despite this evidence of apparent safety, it is hard to justify the suggestion made in Chapter 3 that the addition of MSG to foods is desirable, because 'it replaces the glutamate that is lost or destroyed in preservation and processing.' There are excellent chapters on glutamate metabolism from Munro, with a predictably holistic approach, and from Meister, concentrating more on the cellular aspects. Predictably, Meister's chapter deals at length with the role of glutamate and the 3~-glutamyl cycle in amino acid transport, and argues convincingly for its role as one of the enzymic transport mechanisms, albeit not the sole one. Amino acid transport is also covered in Pardridge's chapter on the regulation of glutamate uptake into the brain, a chapter that gives an excellent over-view of the whole problem of understanding and interpreting blood-brain barrier transport phenomena. It is noted that there is a nett efflux of glutamate from the brain, so that the source of glutamate as a neurotransmitter, or as precursor for G A B A synthesis, must be from carbohydrate, a point that is made in the other chapters on brain glutamate. Curtis has contributed a valuable chapter on the problems of evaluating the
potential neurotransmitter role of a metabolite as ubiquitous as glutamate, where there seems to be no unique pathway for synthesis or inactivation that can be isolated from general metabolic pathways. As with any book that is the outcome of a symposium, there is a degree of overlap between chapters, and the arrangement of the chapters also seems a little capricious, with a wide separation between some chapters that would seem to belong in closer juxtaposition. The final summary chapter from Wurtman presumably provided an excellent 'take home' message from the meeting, but now it appears in print, I feel it would be more useful as an introduction (something the volume lacks) than as an envoi. Overall, this is a book that will be useful not only to those who are working on aspects of glutamate directly, but also to all those who are interested in the areas of taste, food additives, amino acid metabolism or neurotransmitter metabolism. Although it is obviously a 'state of the art' report on glutamate research, it contains much useful information collected together in a volume that is not likely to be too outdated for some time to come. D A V I D A. B E N D E R
David A. Bender is at The Courtauld Institute o f Biochemistry, The Middlesex Hospital Medical School, London W1P 7PN, U.K.
The steam and dream of virology T h e B i o c h e m i s t r y o f Viruses
by S. J. Martin, Cambridge University Press, Cambridge, London, New York and Melbourne, 1978. £10.50 (HB) £3.95 (PB) (x + 145 pages) ISBN 0 521 21678 8 (HB) 0 521 29229 8 (PB) The sharp division of virology into clinical and applied (steam) and molecular and genetical (dream) varieties in the last quarter-century has been well described by Sir Christopher Andrewes. In recent years a welcome trend towards a splicing of these strands has become evident, exemplified by such developments as the contribution of biochemistry to our understanding of serological variation among influenza viruses. Textbooks undoubtedly play an important role in determining the relative degrees of importance which students allot to the component parts of a subject, and I therefore read Dr Martin's book with interest to see how he approached the difficult task of synthesizing in a short account (138 text pages) an introduction to both
the biology of viruses and their biochemistry. Great pains have in fact been taken to provide a broad introduction to the general properties of viruses, the first 30 pages of the book being devoted to a brief history of virology and accounts of virus structure, classification, nomenclature and assay methods. The remaining chapters cover virus purification, virus architecture, and the events occurring during virus growth. The final chapter briefly addresses itself to virus evolution, vaccines and chemotherapy. The approach adopted can best be described as classical (or old-fashioned). Thirty-six pages are devoted to an account of virus structure with extensive discussion on helical and icosahedral symmetry. Although stress is placed on structure-function relationships of virion components the discussion of this important aspect is dissappointingly brief. Virion transcriptases, for example, are referred to, almost in passing, as 'minor proteins'. While this is true when we consider the
XX
The many sides of glutamate Glutamic Acid: Advances in Biochemistry and Physiology edited by L. J. Filer, S. Garattini, M. R. Kare, W. A. Reynolds and R. J. Wurtman, Raven Press, New York, 1979. $36.40 (xiv + 400pages) ISBN 0 89004 356 6 A non-essential amino acid that is present 'in large amounts in virtually all proteins seems an unlikely subject for an international symposium, with 26 papers contributed by a wide variety of authors, yet this 390 page book is devoted to glutamate. Perhaps we should not be too surprised, in view of the widespread use of monosodium glutamate (MSG) as a food additive, the central role of glutamate in ammonia metabolism, its role as an interface between amino acid and carbohydrate metabolism, and the mounting evidence that glutamate may have a neurotransmitter role of its own, as well as acting as a precursor of G A B A . Throughout the book, concern with the safety of MSG as a food additive is obvious we are told that the average per capita intake in Taiwan is 3 g per day, compared with a 'mere' 350 mg per day in the United States. There are several chapters on the unique flavour enhancing properties of MSG, and attempts to define its flavour psychometrically. The first chapter provides some interesting data on the physiology of taste in general, the effects of glutamate, and the synergism between glutamate and ribonucleotides at the taste receptors'- both glutamate and a variety of nucleotides are found in the ingredients traditionally used for flavouring in Japanese cuisine. One area of great concern over the safety of MSG has been in infant foods, and several chapters contain a thinly disguised plea for its contiiaued use, with comments on the lack of demonstrable harm, and the evidence that even relatively large amounts do not raise the blood glutamate concentration more than does a protein-rich meal. However, it is noted that adult mice and Rhesus monkeys are better able to tolerate a massive glutamate load than are neonates of these two species, and human adults have a poorer tolerance than either of these experimental species. We are also told that on a body weight basis the human baby is exposed to a greater intake of glutamate while it is being breast fed than at any other time in life. Maternal glutamate intake and blood levels seem not to affect the amount secreted in the milk. It is also comforting to be told that glutamate does not cross the -
placental barrier at concentrations greatly in excess of any that can be achieved other than by intravenous infusion. Despite this evidence of apparent safety, it is hard to justify the suggestion made in Chapter 3 that the addition of MSG to foods is desirable, because 'it replaces the glutamate that is lost or destroyed in preservation and processing.' There are excellent chapters on glutamate metabolism from Munro, with a predictably holistic approach, and from Meister, concentrating more on the cellular aspects. Predictably, Meister's chapter deals at length with the role of glutamate and the 3~-glutamyl cycle in amino acid transport, and argues convincingly for its role as one of the enzymic transport mechanisms, albeit not the sole one. Amino acid transport is also covered in Pardridge's chapter on the regulation of glutamate uptake into the brain, a chapter that gives an excellent over-view of the whole problem of understanding and interpreting blood-brain barrier transport phenomena. It is noted that there is a nett efflux of glutamate from the brain, so that the source of glutamate as a neurotransmitter, or as precursor for G A B A synthesis, must be from carbohydrate, a point that is made in the other chapters on brain glutamate. Curtis has contributed a valuable chapter on the problems of evaluating the
potential neurotransmitter role of a metabolite as ubiquitous as glutamate, where there seems to be no unique pathway for synthesis or inactivation that can be isolated from general metabolic pathways. As with any book that is the outcome of a symposium, there is a degree of overlap between chapters, and the arrangement of the chapters also seems a little capricious, with a wide separation between some chapters that would seem to belong in closer juxtaposition. The final summary chapter from Wurtman presumably provided an excellent 'take home' message from the meeting, but now it appears in print, I feel it would be more useful as an introduction (something the volume lacks) than as an envoi. Overall, this is a book that will be useful not only to those who are working on aspects of glutamate directly, but also to all those who are interested in the areas of taste, food additives, amino acid metabolism or neurotransmitter metabolism. Although it is obviously a 'state of the art' report on glutamate research, it contains much useful information collected together in a volume that is not likely to be too outdated for some time to come. D A V I D A. B E N D E R
David A. Bender is at The Courtauld Institute o f Biochemistry, The Middlesex Hospital Medical School, London W1P 7PN, U.K.
The steam and dream of virology T h e B i o c h e m i s t r y o f Viruses
by S. J. Martin, Cambridge University Press, Cambridge, London, New York and Melbourne, 1978. £10.50 (HB) £3.95 (PB) (x + 145 pages) ISBN 0 521 21678 8 (HB) 0 521 29229 8 (PB) The sharp division of virology into clinical and applied (steam) and molecular and genetical (dream) varieties in the last quarter-century has been well described by Sir Christopher Andrewes. In recent years a welcome trend towards a splicing of these strands has become evident, exemplified by such developments as the contribution of biochemistry to our understanding of serological variation among influenza viruses. Textbooks undoubtedly play an important role in determining the relative degrees of importance which students allot to the component parts of a subject, and I therefore read Dr Martin's book with interest to see how he approached the difficult task of synthesizing in a short account (138 text pages) an introduction to both
the biology of viruses and their biochemistry. Great pains have in fact been taken to provide a broad introduction to the general properties of viruses, the first 30 pages of the book being devoted to a brief history of virology and accounts of virus structure, classification, nomenclature and assay methods. The remaining chapters cover virus purification, virus architecture, and the events occurring during virus growth. The final chapter briefly addresses itself to virus evolution, vaccines and chemotherapy. The approach adopted can best be described as classical (or old-fashioned). Thirty-six pages are devoted to an account of virus structure with extensive discussion on helical and icosahedral symmetry. Although stress is placed on structure-function relationships of virion components the discussion of this important aspect is dissappointingly brief. Virion transcriptases, for example, are referred to, almost in passing, as 'minor proteins'. While this is true when we consider the