Unsuspected ligand aggregation: a key problem in biochemical and biological assays with natural and xenobiotic amphipaths

ITC-derived binding affinity may be biased due to titrant (nano)-aggregation. Binding of halogenated benzotriazoles to the catalytic domain of human protein kinase CK2

The binding of four bromobenzotriazoles to the catalytic subunit of human protein kinase CK2 was assessed by two complementary methods: Microscale Thermophoresis (MST) and Isothermal Titration Calorimetry (ITC). New algorithm proposed for the global analysis of MST pseudo-titration data enabled reliable determination of binding affinities for two distinct sites, a relatively strong one with the K_d of the order of 100 nM and a substantially weaker one (K_d > 1 μM). The affinities for the strong binding site determined for the same protein-ligand systems using ITC were in most cases approximately 10-fold underestimated. The discrepancy was assigned directly to the kinetics of ligand nano-aggregates decay occurring upon injection of the concentrated ligand solution to the protein sample. The binding affinities determined in the reverse ITC experiment, in which ligands were titrated with a concentrated protein solution, agreed with the MST-derived data. Our analysis suggests that some ITC-derived K_d values, routinely reported together with PDB structures of protein-ligand complexes, may be biased due to the uncontrolled ligand (nano)-aggregation, which may occur even substantially below the solubility limit.

Neural route of pyrogen signaling to the brain

In the pathogenesis of systemic inflammation and fever, peripheral inflammatory and pyrogenic signals gain access to the brain via humoral and neural routes. One of the neural routes is represented by chemosensitive afferent fibers of the abdominal vagus. We summarize our recent studies of the role of the abdominal vagus in fever. We conclude that capsaicin-sensitive fibers traveling within the hepatic vagal branch constitute a necessary component of the afferent mechanism of the febrile response to low, but not high, doses of circulating pyrogens. We speculate that this mechanism is triggered by blood-borne prostaglandins of the E series.

Blood-borne, albumin-bound prostaglandin E2 may be involved in fever

Although the involvement of blood-borne PGE2 in fever has been hypothesized by several authors and has substantial experimental support, the current literature often rejects this hypothesis because several attempts to induce fever by a peripheral PGE2 failed. However, it is usually ignored that the amphipathic molecules of PGE2 are readily self-associating and that such an aggregation could have prevented the peripherally administered PGE2 (free form) from expressing its pyrogenic activity, thus leading to false negative results. To ensure disaggregation of PGE2, we prepared its complex within a carrier protein, human serum albumin (HSA). HSA was purified with activated charcoal and polymixin B-polyacrylamide gel and incubated with PGE2 for 1 h at 37 degrees C. Adult Chinchilla rabbits were injected intravenously with PGE2-HSA complex in either the higher (75 micrograms/kg PGE2:30 mg/kg HSA) or the lower (15 micrograms/kg:6 mg/kg) dose, and the rectal temperature (Tr) was measured. In the controls, either the ligand alone or the carrier alone was administered. At the higher dose, neither free PGE2 nor albumin alone was pyrogenic, whereas the PGE2-HSA complex produced a fever characterized by a short latency (<10 min) and a maximal Tr rise of 0.7 +/- 0.2 degrees C. At the lower dose, none of the substances affected the Tr. This study demonstrates a marked pyrogenic activity of the intravenous PGE2-HSA, but not of the free PGE2. Administration of a preformed complex may be more physiologically relevant than administration of the free ligand because of the ligand's disaggregation, protection from enzymatic degradation, and facilitated delivery to targets. Our study supports the hypothesis that peripheral PGE2 is involved in fever genesis.

Multiphasic modelling of ligand/acceptor interactions. The hydrophobicity-dependent binding of relatively small amphiphilic substances to acceptor proteins and the nature and facedness of acceptor sites

The modelling of multiphasic ligand/acceptor equilibrium binding systems proceeds at three logically distinct levels: (1) A suitable response quantity, e.g. the amount of acceptor-bound ligand nEL, is expressed as a function of the ligand concentrations [Li] (L = A,B,...) in the compartment i that contains the acceptor sites. One thus obtains a response function nEL = f1([Li]). In general, the equilibrium constants KL contained in such mathematical models are physically ill-defined. (2) Each local concentration [Li] is further expressed as a function of [Laq], the corresponding concentration in the aqueous phase, leading to nEL = f2([Laq]). In this way, the constants KL are transformed into effective constants K'L which (i) can be assessed experimentally and (ii) depend on ligand hydrophobicity in a way that is characteristic of the binding site. Formulation of the functions f1 and F2 only requires knowledge of the reactions in which the acceptor sites participate directly. (3) For each ligand, the experimentally accessible total ligand concentration Lt is expressed as a function of [Laq], leading to concentration balance equations Lt = Lt([Laq]). The latter transformation takes account of any reactions, distinct from ligand/acceptor interaction, in which the ligands are involved, e.g. binding to additional protein sites. As a result of steps 2 and 3, each binding system is described by a set of simultaneous equations dependent on the auxiliary variable [Laq]: (i) the response function f2([Laq]) and (ii) a concentration balance for each ligand Lt = Lt([Laq]). The formulae are rendered more conscise and their discussion and application to data fitting are simplified by introducing, for each ligand L, a function FL characterising the distribution of unbound monomeric ligand over the various partition compartments. When the acceptor acts on unbound ligand, the formulae are further expressed in terms of a new auxiliary variable i.e. the total concentration of unbound monomeric ligand microL. In contrast to data analysis as a function of local concentrations, analysis in terms of total ligand concentrations avoids losing sight of alternate hypotheses about the nature of the binding sites. The present formulation has also permitted clarification of several consequences of the multiphasic nature of the binding systems that, as yet, have been poorly recognised.(ABSTRACT TRUNCATED AT 400 WORDS)