Historical and contemporary stable isotope tracer approaches to studying mammalian protein metabolism

Abstract

Over a century ago, Frederick Soddy provided the first evidence for the existence of isotopes; elements that occupy the same position in the periodic table are essentially chemically identical but differ in mass due to a different number of neutrons within the atomic nucleus. Allied to the discovery of isotopes was the development of some of the first forms of mass spectrometers, driven forward by the Nobel laureates JJ Thomson and FW Aston, enabling the accurate separation, identification, and quantification of the relative abundance of these isotopes. As a result, within a few years, the number of known isotopes both stable and radioactive had greatly increased and there are now over 300 stable or radioisotopes presently known. Unknown at the time, however, was the potential utility of these isotopes within biological disciplines, it was soon discovered that these stable isotopes, particularly those of carbon (¹³ C), nitrogen (¹⁵ N), oxygen (¹⁸ O), and hydrogen (² H) could be chemically introduced into organic compounds, such as fatty acids, amino acids, and sugars, and used to "trace" the metabolic fate of these compounds within biological systems. From this important breakthrough, the age of the isotope tracer was born. Over the following 80 yrs, stable isotopes would become a vital tool in not only the biological sciences, but also areas as diverse as forensics, geology, and art. This progress has been almost exclusively driven through the development of new and innovative mass spectrometry equipment from IRMS to GC-MS to LC-MS, which has allowed for the accurate quantitation of isotopic abundance within samples of complex matrices. This historical review details the development of stable isotope tracers as metabolic tools, with particular reference to their use in monitoring protein metabolism, highlighting the unique array of tools that are now available for the investigation of protein metabolism in vivo at a whole body down to a single protein level. Importantly, it will detail how this development has been closely aligned to the technological development within the area of mass spectrometry. Without the dedicated development provided by these mass spectrometrists over the past century, the use of stable isotope tracers within the field of protein metabolism would not be as widely applied as it is today, this relationship will no doubt continue to flourish in the future and stable isotope tracers will maintain their importance as a tool within the biological sciences for many years to come. © 2016 The Authors. Mass Spectrometry Reviews Published by Wiley Periodicals, Inc. Mass Spec Rev.

Keywords: mass spectrometry; protein metabolism; stable isotopes.

Figure 1

Schematic of how stable isotope tracers are used to measure protein turnover within a mammalian system highlighting: (A) Arterial‐Venous balance techniques and (B) FSR techniques (Reprinted with permission from Atherton et al., 2015 (Copyright 2015, Elsevier)).

Figure 2

Evolution of the mass spectrometer over the years, (A) JJ Thomson's original mass spectrograph for analysis of positive rays (Reprinted with permission from Thomson, 1913 (copyright 1913, The Royal Society)), (B) Dempster's single magnetic sector instrument (Reprinted with permission from Dempster, 1918 (copyright 1918, American Physical Society)), (C) Present day set‐up for a commercial IRMS with magnetic sector, note the design differs very little from that of Dempster's original design (Reprinted with permission from Begley and Scrimgeour, 1996 (copyright 1996, John Wiley and Sons)).

Figure 3

Photographic plate from JJ Thomson's original experiment into positive rays highlighting the presence of the Neon 22 isotope (Reprinted with permission from Maher, Jjunju, & Taylor, 2015 (copyright 2015, American Physical Society)).

Figure 4

Harold Urey's evidence for the presence of the hydrogen isotope of mass 2, highlighted is the faint ²H line to the left side of the overexposed central ¹H line with the symmetrical ghost peaks due to the overexposure at either side (Reprinted with permission from Urey et al., 1932b (copyright 1932, American Physical Society)).

Figure 5

Diagrammatic representation of the two pool model for calculating protein turnover using the end product method as devised by Waterlow (adapted from Duggleby & Waterlow, 2005).