Natural intramolecular isotope measurements in physiology: elements of thecase for an effort toward high-precision position-specific isotope analysis

Chemical information available in organisms can be categorized into three m ajor domains, macromolecular, small molecules, and isotope ratios. Informat ion about physiological state is commonly obtained by qualitative and quant itative analysis in the macromolecular and small molecule domains. Genomics and proteomics are emerging approaches to analysis of macromolecules, and both areas yield definitive information on present physiological state. The re is relatively little record of past physiological states of the individu al available in these domains. Natural isotopic variability, particularly o n an intramolecular level, is likely to retain more physiological history. Because of ubiquitous isotopic fractionation, every stereochemically unique position in every molecule has an isotope ratio that reflects the processe s of synthesis and degradation. This fact highlights a vast amount of organ ismal chemical information that is essentially unstudied. Isotope measureme nts can be classified according to the chemical complexity of the analyte i nto bulk, compound-specific, and position-specific or intramolecular levels . Recent advances in analysis of isotope ratios are transforming natural sc ience, and particularly answering questions about ecosystems using bulk met hods; however, they have had relatively little impact on physiology. This m ay be because the vast complexities of physiological questions demand very selective information available in position-specific isotope analysis (PSIA ). The relatively few high-precision PSIA studies, based on isotope ratio m ass spectrometry (IRMS), have revealed intramolecular isotope ratio differe nces in pivotal physiological compounds including amino acids, glucose, gly cerol, acetate, fatty acids, and purines. The majority of these analyses ha ve been accomplished by laborious offline methods; however, recent advances in instrumentation presage rapid PSIA that will be necessary to attack rea l physiological problems. Gas-phase pyrolysis has been shown to be an effec tive method to determine C-13/C-12 at high precision for molecular fragment s, and technologies to extend C-based PSIA to N and other organic elements are emerging. Two related efforts are warranted, (a) development of rapid, convenient, and sensitive methods for high-precision PSIA, a necessary prec ursor to (b) a concerted investigation into the relationship of metabolic s tate to intramolecular isotope ratio. Inherent in this latter goal is the n eed to identify long-lived molecules in long-lived cells that retain a reco rd of early isotopic conditions, as has been shown for postmortem human neu ronal DNA. Using known metabolic precursor-pro duct relationships between i ntramolecular positions, future studies of physiological isotope fractionat ion should reveal the relationship of diet and environment to observed isot ope ratio. This science of isotope physiology, or simply isotopics, should add an important tool for elucidation of early factors that effect later he alth, probably the most difficult class of biomedical issues. Copyright (C) 2001 John Wiley & Sons, Ltd.