Proteomic study for the prediction of μCT imaging with iodine

Iodine-based staining techniques are commonly used in histological imaging and micro-computed tomography ({micro}CT) due to iodines affinity for binding to specific molecules. However, the basis for tissue-specific contrast has not yet been sufficiently explored. In this study, we analyse the human proteome at four different levels: individual proteins, protein families, tissues with additional expression values for selected proteins, and organs as a distinct combination of different tissues. At each level, we try to identify proteins/groups with high potential for iodine binding, especially those rich in aromatic heterocyclic amino acids. Using bioinformatic methods, we evaluate the occurrence of aromatic/non-aromatic heterocyclic, carbocyclic, and the remaining 15 amino acids in 20,650 proteins, 1,487 families, 57 tissues, and 16 organs. At the protein level, structural proteins such as titin, nebulin, obscurin, mucin, filaggrin and hornerin have a high absolute number of aromatic heterocyclic amino acids, which could explain the high {micro}CT contrast in muscle, skin and mucosal tissues. At the next level, however, structural families (such as the Laminin-family) rank significantly lower in comparison. These results are reflected in tissues and organs for which protein expression is available. Here, no significant correlations between the enrichment of heterocycles and the intensity of iodine staining can be observed. Furthermore, the enrichment of amino acids in each tissue/organ is relatively similar and shows no significant difference. Our results provide a general basis for iodine-based tissue imaging and serve as a potential starting point for future research, e.g. for cross-species applications and for the structural and functional effects of iodination.