Accelerator Mass Spectrometry in Protein Analysis

Abstract

Tools for protein quantitation have advanced through several phases. Common proteins in readily studied sources, albumin and globulins in plasma, were discovered in the nineteenth century and quantified by gravimetric mass determination of chemical precipitates. Finer fractionations and scales led to quantitation of increasingly less common proteins, but the method has obvious limits. Some proteins, enzymes, are quantified by their activity, measured as the increasing chemical product or a shrinking amount of substrate. No absolute quantitation of enzyme concentration is available without having a well-known standard for each enzyme expressing a particular action. Immunochemical methods successfully quantitate proteins, as long as antibodies are selective for the protein under study. Standard curves are required for quantification, since binding affinities are sensitive to assay conditions. Non-specific interactions further confound quantitation. Electrophoretic and chromatographic separations allowed analysis of even smaller concentrations, with quantitation accomplished through various optical properties, such as absorbance, fluorescence, or chemiluminescence. Not all proteins are responsive to the latter effects, and optical density measurements often have large uncertainties and poor sensitivity.