American Journal of Clinical Nutrition, Vol. 69, No. 3, 580,
March 1999
© 1999 American Society for Clinical Nutrition
Peptides in Mammalian Protein Metabolism: Tissue Utilization and Clinical Targeting
Siamak A Adibi
UPMC Health System Clinical Nutrition Research Unit 200 Lothrop Street MUH E-321 Pittsburgh, PA 15213–2582 E-mail: adibi{at}medl.dept-med.pitt.edu
Peptides in Mammalian Protein Metabolism: Tissue Utilization and Clinical Targeting, edited by GK Grimble and FRC Backwell, 1998, 180 pages, hardcover, $33.00. Portland Press, Ltd, London.
Despite substantial evidence published over the past 30 y, there is still little recognition of the discovery that dipeptides, like amino acids, could serve as substrates for protein nutrition, and problems of tonicity, solubility, and stability of amino acid solutions can be resolved with the use of amino acids in dipeptide form. One reason could be that this discovery has not been adequately covered in scientific publications. Therefore, this book is a welcome publication.
From the table of contents, it is apparent that the authors chosen are all highly experienced investigators who have made important contributions to the field. As a result, the book is a comprehensive and excellent source of wide-ranging issues on the metabolism of peptides and their use as substrates in nutritional therapies. Despite its title, the book covers the existence of oligopeptide transporters in microorganisms and their roles in intestinal absorption and renal reabsorption of peptide-like drugs. These inclusions are quite appropriate because it was thought initially that substrates for the oligopeptide transporters were only di- and tripeptides, but it has become apparent that these transporters have a much wider substrate specificity. In fact, this knowledge already has had a great effect on improving oral absorption and bioavailability of drugs used for the treatment of prevalent conditions such as infection. The only relevant subject not covered in the book is the discovery of an oligopeptide transporter in the lysosomal membrane in the liver that appears to provide an active mechanism for completion of the final stage of protein degradation in this tissue (J Biol Chem 1997;272:11786–90). This omission probably occurred because the book was written before the publication of this article.
A notable strength of the book is that in this one volume readers can find chapters on the biochemistry, physiology, molecular biology, and clinical applications of dipeptide transport. In particular, the chapter on molecular biology of dipeptide transport is of current interest. There is no doubt that the recent cloning of intestinal (Pept-1) and renal (Pept-2) oligopeptide transporters has opened a new era in the investigation of the regulation of protein absorption in the gut and nitrogen conservation in the kidney. Another notable feature of the book is its discussion of dipeptide absorption in ruminants and its practical applications in farm animals. Apparently, studies of plasma composition in cows suggested that peptides may constitute a high proportion of total amino acid absorption in ruminants. However, the use of classical techniques failed to show any evidence for the existence of an oligopeptide transporter. Finally, the use of a technique of molecular biology provided the anticipated evidence. This was done by injecting Xenopus laevis oocytes with poly(A) + RNA isolated from sheep stomach.
A major technical problem in studies of peptide metabolism has been the determination of their concentrations in biological fluids. The book contains an excellent review of this topic; however, its coverage by most of the authors in the book seems superfluous. The quality and quantity of science supporting the observations reviewed in each chapter varies. For example, the advocacy of the use of protein hydrolysates as the nitrogen source in parenteral nutrition does not appear to be well supported by data on its efficacy and safety. These shortcomings should not undermine the fact that the book is a valuable source of information on a field that is currently on the cutting edge of science. In particular, readers not familiar with the field will be rewarded with new knowledge that should be of interest to all life scientists.
