Identification of Protein Recognition Elements within Heparin Chains Using Enzymatic Foot-Printing in Solution and Online SEC/MS

Understanding molecular mechanisms governing interactions of glycosaminoglycans (such as heparin) with proteins remains challenging due to their enormous structural heterogeneity. Commonly accepted approaches seek to reduce the structural complexity by searching for "binding epitopes" within the limited subsets of short heparin oligomers produced either enzymatically or synthetically. A top-down approach presented in this work seeks to preserve the chemical diversity displayed by heparin by allowing the longer and structurally diverse chains to interact with the client protein. Enzymatic lysis of the protein-bound heparin chains followed by the product analysis using size exclusion chromatography with online mass spectrometry detection (SEC/MS) reveals the oligomers that are protected from lysis due to their tight association with the protein, and enables their characterization (both the oligomer length, and the number of incorporated sulfate and acetyl groups). When applied to a paradigmatic heparin/antithrombin system, the new method generates a series of oligomers with surprisingly distinct sulfation levels. The extent of sulfation of the minimal-length binder (hexamer) is relatively modest yet persistent, consistent with the notion of six sulfate groups being both essential and sufficient for antithrombin binding. However, the masses of longer surviving chains indicate complete sulfation of disaccharides beyond the hexasaccharide core. Molecular dynamics simulations confirm the existence of favorable electrostatic interactions between the high charge-density saccharide residues flanking the "canonical" antithrombin-binding hexasaccharide and the positive patch on the surface of the overall negatively charged protein. Furthermore, electrostatics may rescue the heparin/protein interaction in the absence of the canonical binding element.

Platelet Factor 4 Interactions with Short Heparin Oligomers: Implications for Folding and Assembly

Association of platelet factor 4 (PF4) with heparin is a first step in formation of aggregates implicated in the development of heparin-induced thrombocytopenia (HIT), a potentially fatal immune disorder affecting 1-5% of patients receiving heparin. Despite being a critically important element in HIT etiology, relatively little is known about the specific molecular mechanism of PF4-heparin interactions. This work uses native mass spectrometry to investigate PF4 interactions with relatively short heparin chains (up to decasaccharides). The protein is shown to be remarkably unstable at physiological ionic strength in the absence of polyanions; only monomeric species are observed, and the extent of multiple charging of corresponding ions indicates a partial loss of conformational integrity. The tetramer signal remains at or below the detection threshold in the mass spectra until the solution's ionic strength is elevated well above the physiological level, highlighting the destabilizing role played by electrostatic interactions vis-à-vis quaternary structure of this high-pI protein. The tetramer assembly is dramatically facilitated by relatively short polyanions (synthetic heparin-mimetic pentasaccharide), with the majority of the protein molecules existing in the tetrameric state even at physiological ionic strength. Each tetramer accommodates up to six pentasaccharides, with at least three such ligands required to guarantee the higher-order structure integrity. Similar results are obtained for PF4 association with longer and structurally heterogeneous heparin oligomers (decamers). These longer polyanions can also induce PF4 dimer assembly when bound to the protein in relatively low numbers, lending support to a model of PF4/heparin interaction in which the latter wraps around the protein, making contacts with multiple subunits. Taken together, these results provide a more nuanced picture of PF4-glycosaminoglycan interactions leading to complex formation. This work also advocates for a greater utilization of native mass spectrometry in elucidating molecular mechanisms underlying HIT, as well as other physiological processes driven by electrostatic interactions.

Mass spectrometry-based methods in characterization of the higher order structure of protein therapeutics

Higher order structure of protein therapeutics is an important quality attribute, which dictates both potency and safety. While modern experimental biophysics offers an impressive arsenal of state-of-the-art tools that can be used for the characterization of higher order structure, many of them are poorly suited for the characterization of biopharmaceutical products. As a result, these analyses were traditionally carried out using classical techniques that provide relatively low information content. Over the past decade, mass spectrometry made a dramatic debut in this field, enabling the characterization of higher order structure of biopharmaceuticals as complex as monoclonal antibodies at a level of detail that was previously unattainable. At present, mass spectrometry is an integral part of the analytical toolbox across the industry, which is critical not only for quality control efforts, but also for discovery and development.

Ruthenium coordination preferences in imidazole-containing systems revealed by electrospray ionization mass spectrometry and molecular modeling: Possible cues for the surprising stability of the Ru (III)/tris (hydroxymethyl)-aminomethane/imidazole complexes

Ruthenium is a platinoid that exhibits a range of unique chemical properties in solution, which are exploited in a variety of applications, including luminescent probes, anticancer therapies, and artificial photosynthesis. This paper focuses on a recently demonstrated ability of this metal in its +3 oxidation state to form highly stable complexes with tris (hydroxymethyl)aminomethane (H₂ NC(CH₂ OH)₃ , Tris-base or T) and imidazole (Im) ligands, where a single Ru^III cation is coordinated by two molecules of each T and Im. High-resolution electrospray ionization mass spectrometry (ESI MS) is used to characterize Ru^III complexes formed by placing a Ru^II complex [(NH₃ )₅ Ru^II Cl]Cl in a Tris buffer under aerobic conditions. The most abundant ionic species in ESI MS represent mononuclear complexes containing an oxidized form of the metal, ie, [X_n Ru^III T₂ - 2H]⁺ , where X could be an additional T (n = 1) or NH₃ (n = 0-2). Di- and tri-metal complexes also give rise to a series of abundant ions, with the highest mass ion representing a metal complex with an empirical formula Ru₃ C₂₄ O₂₁ N₆ H₆₆ (interpreted as cyclo(T₂ RuO)₃ , a cyclic oxo-bridged structure, where the coordination sphere of each metal is completed by two T ligands). The empirical formulae of the binuclear species are consistent with the structures representing acyclic fragments of cyclo(T₂ RuO)₃ with addition of various combinations of ammonia and dioxygen as ligands. Addition of histidine in large molar excess to this solution results in complete disassembly of poly-nuclear complexes and gives rise to a variety of ionic species in the ESI mass spectrum with a general formula [Ru^III His_k T_m (NH₃ )_n - 2H]⁺ , where k = 0 to 2, m = 0 to 3, and n = 0 to 4. Ammonia adducts are present for all observed combinations of k and m, except k = m = 2, suggesting that [His₂ Ru^III T₂ - 2H]⁺ represents a complex with a fully completed coordination sphere. The observed cornucopia of Ru^III complexes formed in the presence of histidine is in stark contrast to the previously reported selective reactivity of imidazole, which interacts with the metal by preserving the RuT₂ core and giving rise to a single abundant ruthenium complex (represented by [Im₂ Ru^III T₂ - 2H]⁺ in ESI mass spectra). Surprisingly, the behavior of a hexa-histidine peptide (HHHHHH) is similar to that of a single imidazole, rather than a single histidine amino acid: The RuT₂ core is preserved, with the following ionic species observed in ESI mass spectra: [HHHHHH·(Ru^III T₂ )_m - (3m-1)H]⁺ (m = 1-3). The remarkable selectivity of the imidazole interaction with the Ru^III T₂ core is rationalized using energetic considerations at the quantum mechanical level of theory.

Cancer metabolomic markers in urine: evidence, techniques and recommendations

Urinary tests have been used as noninvasive, cost-effective tools for screening, diagnosis and monitoring of diseases since ancient times. As we progress through the 21st century, modern analytical platforms have enabled effective measurement of metabolites, with promising results for both a deeper understanding of cancer pathophysiology and, ultimately, clinical translation. The first study to measure metabolomic urinary cancer biomarkers using NMR and mass spectrometry (MS) was published in 2006 and, since then, these techniques have been used to detect cancers of the urological system (kidney, prostate and bladder) and nonurological tumours including those of the breast, ovary, lung, liver, gastrointestinal tract, pancreas, bone and blood. This growing field warrants an assessment of the current status of research developments and recommendations to help systematize future research.

Recommendations for performing, interpreting and reporting hydrogen deuterium exchange mass spectrometry (HDX-MS) experiments

Hydrogen deuterium exchange mass spectrometry (HDX-MS) is a powerful biophysical technique being increasingly applied to a wide variety of problems. As the HDX-MS community continues to grow, adoption of best practices in data collection, analysis, presentation and interpretation will greatly enhance the accessibility of this technique to nonspecialists. Here we provide recommendations arising from community discussions emerging out of the first International Conference on Hydrogen-Exchange Mass Spectrometry (IC-HDX; 2017). It is meant to represent both a consensus viewpoint and an opportunity to stimulate further additions and refinements as the field advances.

Exploiting His-Tags for Absolute Quantitation of Exogenous Recombinant Proteins in Biological Matrices: Ruthenium as a Protein Tracer

Metal labeling and ICP MS detection offer an alternative to commonly accepted techniques that are currently used to quantitate exogenous proteins in vivo, but modifying the protein surface with metal-containing groups inevitably changes its biophysical properties and is likely to affect trafficking and biodistribution. The approach explored in this work takes advantage of the presence of hexa-histidine tags in many recombinant proteins, which have high affinity toward a range of metals. While many divalent metals bind to poly histidine sequences reversibly, oxidation of imidazole-bound Co^II or Ru^II is known to result in a dramatic increase of the binding strength. In order to evaluate the feasibility of using imidazole-bound metal oxidation as a means of attaching permanent tags to polyhistidine segments, a synthetic peptide YPDFEDYWMKHHHHHH was used as a model. Ru^II can be oxidized under ambient (aerobic) conditions, allowing any oxidation damage to the peptide beyond the metal-binding site to be avoided. The resulting peptide-Ru^III complex is very stable, with the single hexa-histidine segment capable of accommodating up to three metal ions. Localization of Ru^III within the hexa-histidine segment of the peptide was confirmed by tandem mass spectrometry. The Ru^III/peptide binding appears to be irreversible, with both low- and high-molecular weight biologically relevant scavengers failing to strip the metal from the peptide. Application of this protocol to labeling a recombinant form of an 80 kDa protein transferrin allowed Ru^III to be selectively placed within the His-tag segment. The metal label remained stable in the presence of ubiquitous scavengers and did not interfere with the receptor binding, while allowing the protein to be readily detected in serum at sub-nM concentrations. The results of this work suggest that ruthenium lends itself as an ideal metal tag for selective labeling of His-tag containing recombinant proteins to enable their sensitive detection and quantitation with ICP MS.

Mass Spectrometry Reveals a Multifaceted Role of Glycosaminoglycan Chains in Factor Xa Inactivation by Antithrombin

Factor Xa (fXa) inhibition by antithrombin (AT) enabled by heparin or heparan sulfate is critical for controlling blood coagulation. AT activation by heparin has been investigated extensively, while interaction of heparin with trapped AT/fXa intermediates has received relatively little attention. We use native electrospray ionization mass spectrometry to study the role of heparin chains of varying length [hexa-, octa-, deca-, and eicosasaccharides (dp6, dp8, dp10, and dp20, respectively)] in AT/fXa complex assembly. Despite being critical promoters of AT/Xa binding, shorter heparin chains are excluded from the final products (trapped intermediates). However, replacement of short heparin segments with dp20 gives rise to a prominent ionic signal of ternary complexes. These species are also observed when the trapped intermediate is initially prepared in the presence of a short oligoheparin (dp6), followed by addition of a longer heparin chain (dp20), indicating that binding of heparin to AT/fXa complexes takes place after the inhibition event. The importance of the heparin chain length for its ability to associate with the trapped intermediate suggests that the binding likely occurs in a bidentate fashion (where two distinct segments of oligoheparin make contacts with the protein components, while the part of the chain separating these two segments is extended into solution to minimize electrostatic repulsion). This model is corroborated by both molecular dynamics simulations with an explicit solvent and ion mobility measurements in the gas phase. The observed post-inhibition binding of heparin to the trapped AT/fXa intermediates hints at the likely role played by heparan sulfate in their catabolism.

The mechanism of reduced IgG/IgE-binding of β-lactoglobulin by pulsed electric field pretreatment combined with glycation revealed by ECD/FTICR-MS

Bovine β-lactoglobulin (β-Lg) is a major allergen existing in milk and causes about 90% of IgE-mediated cow's milk allergies. Previous studies showed that pulsed electric field (PEF) treatment could partially unfold the protein, which may contribute to the improvement of protein glycation. In this study, the effect of PEF pretreatment combined with glycation on the IgG/IgE-binding ability and the structure of β-Lg was investigated. The result showed that PEF pretreatment combined with glycation significantly reduced the IgG and IgE binding abilities, which was attributed to the changes of secondary and tertiary structure and the increase in glycation sites and degree of substitution per peptide (DSP) value determined by electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry (ECD/FTICR-MS). Unexpectedly, glycation sites (K47, K91 and K135) added by two mannose molecules were identified in glycated β-Lg with PEF pretreatment. Moreover, the results indicated that PEF pretreatment at 25 kV cm^-1 for 60 μs promoted the reduction of IgG/IgE-binding capacity by increasing the glycation degree of β-Lg, whereas single PEF treatment under the same conditions markedly enhanced the IgG/IgE-binding ability by partially unfolding the structure of β-Lg. The results suggested that ECD/FTICR-MS could help us to understand the mechanism of reduction in the IgG/IgE-binding of β-Lg by structural characterization at the molecular level. Therefore, PEF pretreatment combined with glycation may provide an alternative method for β-Lg desensitization.

Ultrasonic Pretreatment Combined with Dry-State Glycation Reduced the Immunoglobulin E/Immunoglobulin G-Binding Ability of α-Lactalbumin Revealed by High-Resolution Mass Spectrometry

Bovine α-lactalbumin (α-LA) is one of major food allergens in cow's milk. The present work sought to research the effects of ultrasonic pretreatment combined with dry heating-induced glycation between α-LA and galactose on the immunoglobulin E (IgE)/immunoglobulin G (IgG)-binding ability and glycation extent of α-LA, determined by inhibition enzyme-linked immunosorbent assay and high-resolution mass spectrometry, respectively. The IgE/IgG-binding ability of glycated α-LA was significantly decreased as a result of ultrasonic pretreatment, while the average molecular weight, incorporation ratio (IR) value, location and number of glycation sites, and degree of substitution per peptide (DSP) value were elevated. When the mixtures of α-LA and galactose were pretreated by ultrasonication at 150 W/cm², glycated α-LA possesses seven glycation sites, the highest IR and DSP values, and the lowest IgE/IgG-binding ability. Therefore, the decrease in the IgE/IgG-binding ability of α-LA depends upon not only the shielding effect of the linear epitope found to be caused by the glycation of K13, K16, K58, K93, and K98 sites but also the intensified glycation extent, which reflected in the increase of the IR value, the number of glycation sites, and the DSP value. Moreover, allergenic proteins and monosaccharides pretreated by ultrasonication and then followed by dry-state glycation were revealed as a promising way of achieving lower allergenicity of proteins in food processing.

LC/MS at the whole protein level: Studies of biomolecular structure and interactions using native LC/MS and cross-path reactive chromatography (XP-RC) MS

Interfacing liquid chromatography (LC) with electrospray ionization (ESI) to enable on-line MS detection had been initially implemented using reversed phase LC, which in the past three decades remained the default type of chromatography used for LC/MS and LC/MS/MS studies of protein structure. In contrast, the advantages of other types of LC as front-ends for ESI MS, particularly those that allow biopolymer higher order structure to be preserved throughout the separation process, enjoyed relatively little appreciation until recently. However, the past few years witnessed a dramatic surge of interest in the so-called "native" (with "non-denaturing" being perhaps a more appropriate adjective) LC/MS and LC/MS/MS analyses within the bioanalytical and biophysical communities. This review focuses on recent advances in this field, with an emphasis on size exclusion and ion exchange chromatography as front-end platforms for protein characterization by LC/MS. Also discussed are the benefits provided by the integration of chemical reactions in the native LC/MS analyses, including both ion chemistry in the gas phase (e.g., limited charge reduction for characterization of highly heterogeneous biopolymers) and solution-phase reactions (using the recently introduced technique cross-path reactive chromatography).

Multi-step conformational transitions in heat-treated protein therapeutics can be monitored in real time with temperature-controlled electrospray ionization mass spectrometry

Heat-induced conformational transitions are frequently used to probe the free energy landscapes of proteins. However, the extraction of information from thermal denaturation profiles pertaining to non-native protein conformations remains challenging due to their transient nature and significant conformational heterogeneity. Previously we developed a temperature-controlled electrospray ionization (ESI) source that allowed unfolding and association of biopolymers to be monitored by mass spectrometry (MS) in real time as a function of temperature. The scope of this technique is now extended to systems that undergo multi-step denaturation upon heat stress, as well as relatively small-scale conformational changes that are precursors to protein aggregation. The behavior of two therapeutic proteins (human antithrombin and an IgG1 monoclonal antibody) under heat-stress conditions is monitored in real time, providing evidence that relatively small-scale conformational changes in each system lead to protein oligomerization, followed by aggregation. Temperature-controlled ESI MS is particularly useful for the studies of heat-stressed multi-domain proteins such as IgG, where it allows distinct transitions to be observed. The ability of native temperature-controlled ESI MS to monitor both the conformational changes and oligomerization/degradation with high selectivity complements the classic calorimetric methods, lending itself as a powerful experimental tool for the thermostability studies of protein therapeutics.

Co(II) and Ni(II) binding of the Escherichia coli transcriptional repressor RcnR orders its N terminus, alters helix dynamics, and reduces DNA affinity

RcnR, a transcriptional regulator in Escherichia coli, derepresses the expression of the export proteins RcnAB upon binding Ni(II) or Co(II). Lack of structural information has precluded elucidation of the allosteric basis for the decreased DNA affinity in RcnR's metal-bound states. Here, using hydrogen-deuterium exchange coupled with MS (HDX-MS), we probed the RcnR structure in the presence of DNA, the cognate metal ions Ni(II) and Co(II), or the noncognate metal ion Zn(II). We found that cognate metal binding altered flexibility from the N terminus through helix 1 and modulated the RcnR-DNA interaction. Apo-RcnR and RcnR-DNA complexes and the Zn(II)-RcnR complex exhibited similar ²H uptake kinetics, with fast-exchanging segments located in the N terminus, in helix 1 (residues 14-24), and at the C terminus. The largest difference in ²H incorporation between apo- and Ni(II)- and Co(II)-bound RcnR was observed in helix 1, which contains the N terminus and His-3, and has been associated with cognate metal binding. ²H uptake in helix 1 was suppressed in the Ni(II)- and Co(II)-bound RcnR complexes, in particular in the peptide corresponding to residues 14-24, containing Arg-14 and Lys-17. Substitution of these two residues drastically affected DNA-binding affinity, resulting in rcnA expression in the absence of metal. Our results suggest that cognate metal binding to RcnR orders its N terminus, decreases helix 1 flexibility, and induces conformational changes that restrict DNA interactions with the positively charged residues Arg-14 and Lys-17. These metal-induced alterations decrease RcnR-DNA binding affinity, leading to rcnAB expression.

Mechanism of Reduction in IgG and IgE Binding of β-Lactoglobulin Induced by Ultrasound Pretreatment Combined with Dry-State Glycation: A Study Using Conventional Spectrometry and High-Resolution Mass Spectrometry

Bovine β-lactoglobulin (β-Lg) is one of major allergens in cow's milk. Previous study showed that ultrasound treatment induced the conformational changes of β-Lg and promoted the glycation in aqueous solutions, which is, however, less efficient compared with dry-state. In this work, the effect of ultrasound pretreatment combined with dry-state glycation on the IgG and IgE binding of β-Lg was studied. Dry-state glycation with mannose after ultrasound pretreatment at 0-600 W significantly reduced the IgG and IgE binding of β-Lg, with the lowest values observed at 400 W. The decrease in the IgG and IgE binding of β-Lg was attributed to the increase in glycation extent and the changes of secondary and tertiary structure, which reflected in the increase of UV absorbance, α-helix and β-sheet contents, as well as the decrease of intrinsic fluorescence intensity, surface hydrophobicity, β-turn, and random coil contents. Moreover, ultrasound pretreatment promoted the reduction of IgG and IgE binding abilities by improving glycation, reflecting in the increase of the glycation sites and the degree of substitution per peptide (DSP) value determined by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Ultrasound pretreatment at 400 W showed the most significantly enhanced glycation extent. Besides, the results suggested FTICR-MS could provide insights into the glycation at molecular level, which was conducive to the understanding of the mechanism of the reduction in the IgG and IgE binding of β-Lg. Therefore, ultrasound pretreatment combined with dry-state glycation may be a promising method for β-Lg hyposensitization.

Electrostatic Forces as Dominant Interactions Between Proteins and Polyanions: an ESI MS Study of Fibroblast Growth Factor Binding to Heparin Oligomers

The interactions between fibroblast growth factors (FGFs) and their receptors (FGFRs) are facilitated by heparan sulfate (HS) and heparin (Hp), highly sulfated biological polyelectrolytes. The molecular basis of FGF interactions with these polyelectrolytes is highly complex due to the structural heterogeneity of HS/Hp, and many details still remain elusive, especially the significance of charge density and minimal chain length of HS/Hp in growth factor recognition and multimerization. In this work, we use electrospray ionization mass spectrometry (ESI MS) to investigate the association of relatively homogeneous oligoheparins (octamer, dp8, and decamer, dp10) with acidic fibroblast growth factor (FGF-1). This growth factor forms 1:1, 2:1, and 3:1 protein/heparinoid complexes with both dp8 and dp10, and the fraction of bound protein is highly dependent on protein/heparinoid molar ratio. Multimeric complexes are preferentially formed on the highly sulfated Hp oligomers. Although a variety of oligomers appear to be binding-competent, there is a strong correlation between the affinity and the overall level of sulfation (the highest charge density polyanions binding FGF most strongly via multivalent interactions). These results show that the interactions between FGF-1 and Hp oligomers are primarily directed by electrostatics, and also demonstrate the power of ESI MS as a tool to study multiple binding equilibria between proteins and structurally heterogeneous polyanions. Graphical Abstract ᅟ.

Evaluation of Gallium as a Tracer of Exogenous Hemoglobin-Haptoglobin Complexes for Targeted Drug Delivery Applications

Haptoglobin (Hp) is a plasma glycoprotein that generates significant interest in the drug delivery community because of its potential for delivery of antiretroviral medicines with high selectivity to macrophages and monocytes, the latent reservoirs of human immunodeficiency virus. As is the case with other therapies that exploit transport networks for targeted drug delivery, the success of the design and optimization of Hp-based therapies will critically depend on the ability to accurately localize and quantitate Hp-drug conjugates on the varying and unpredictable background of endogenous proteins having identical structure. In this work, we introduce a new strategy for detecting and quantitating exogenous Hp and Hp-based drugs with high sensitivity in complex biological samples using gallium as a tracer of this protein and inductively coupled plasma mass spectrometry (ICP MS) as a method of detection. Metal label is introduced by reconstituting hemoglobin (Hb) with gallium(III)-protoporphyrin IX followed by its complexation with Hp. Formation of the Hp/Hb assembly and its stability are evaluated with native electrospray ionization mass spectrometry. Both stable isotopes of Ga give rise to an abundant signal in ICP MS of a human plasma sample spiked with the metal-labeled Hp/Hb complex. The metal label signal exceeds the spectral interferences' contributions by more than an order of magnitude even with the concentration of the exogenous protein below 10 nM, the level that is more than adequate for the planned pharmacokinetic studies of Hp-based therapeutics. Graphical Abstract ᅟ.

Assessing the iron delivery efficacy of transferrin in clinical samples by native electrospray ionization mass spectrometry

Serum transferrin is a key player in iron homeostasis, and its ability to deliver iron to cells via the endosomal pathway critically depends on the presence of carbonate that binds this protein synergistically with ferric ion. Oxalate is another ubiquitous anionic species that can act as a synergistic anion, and in fact its interaction with transferrin is notably stronger compared to carbonate, preventing the protein from releasing the metal in the endosomal environment. While this raises concerns that high oxalate levels in plasma may interfere with iron delivery to tissues, concentration of free oxalate in blood appears to be a poor predictor of impeded availability of iron, as previous studies showed that it cannot displace carbonate from ferro-transferrin on a physiologically relevant time scale under the conditions mimicing plasma. In this work we present a new method that allows different forms of ferro-transferrin (carbonate- vs. oxalate-bound) to be distinguished from each other by removing this protein from plasma without altering the composition of the protein/metal/synergistic anion complexes, and determining their accurate masses using native electrospray ionization mass spectrometry (ESI MS). The new method has been validated using a mixture of recombinant proteins, followed by its application to the analysis of clinical samples of human plasma, demonstrating that native ESI MS can be used in clinical analysis.

Evaluation of Nonferrous Metals as Potential In Vivo Tracers of Transferrin-Based Therapeutics

Transferrin (Tf) is a promising candidate for targeted drug delivery. While development of such products is impossible without the ability to monitor biodistribution of Tf-drug conjugates in tissues and reliable measurements of their levels in blood and other biological fluids, the presence of very abundant endogenous Tf presents a significant impediment to such efforts. Several noncognate metals have been evaluated in this work as possible tracers of exogenous transferrin in complex biological matrices using inductively coupled plasma mass spectrometry (ICP MS) as a detection tool. Placing Ni(II) on a His-tag of recombinant Tf resulted in formation of a marginally stable protein-metal complex, which readily transfers the metal to ubiquitous physiological scavengers, such as serum albumin. An alternative strategy targeted iron-binding pockets of Tf, where cognate Fe(III) was replaced by metal ions known to bind this protein. Both Ga(III) and In(III) were evaluated, with the latter being vastly superior as a tracer (stronger binding to Tf unaffected by the presence of metal scavengers and the retained ability to associate with Tf receptor). Spiking serum with indium-loaded Tf followed by ICP MS detection demonstrated that protein quantities as low as 0.04 nM can be readily detected in animal blood. Combining laser ablation with ICP MS detection allows distribution of exogenous Tf to be mapped within animal tissue cross-sections with spatial resolution exceeding 100 μm. The method can be readily extended to a range of other therapeutics where metalloproteins are used as either carriers or payloads. Graphical Abstract ᅟ.