Relative versus absolute quantitation in disease glycomics

The glycome of a diagnostic biological material such as blood, urine, saliva, tissue, or cell cultures comprises of a vast array of structurally distinct glycans attached to the protein complement. Aberrant glycan structures and distributions result from changes in specific glycosyltransferase activities and have different biological significance, making proper quantitation of glycans highly important. In this review, common HPLC/CE and LC-MS/MS-based methods for glycomics, their advantages and disadvantages, will be discussed with respect to the main quantitative strategies. With the increasing interest in absolute quantitation for glycomics, we discuss absolute and relative glycome quantitation and how it affects the resulting conclusions drawn from glycomics studies. We argue that while absolute quantitation of glycomes may be attractive for some areas of clinical glycomics, relative quantitation of glycans remains the most informative and time/cost-effective method to obtain biological insight into the regulation of the cellular glycosylation machinery and the synthesis of the resultant glycan structures in most research questions due to the enzymatic relatedness of the biosynthesized glycans. Recent developments in multiplexing of glycomes by the introduction of stable isotopic labeling of glycans show promise for providing another level of information to the existing benefits of relative quantitation.