The inhibitory effect of alkylureas and alkylamides on the gelation of hemoglobin S.

Analysis of the inhibition of gelation of deoxyhemoglobin S by the alkylureas and alkylamides, based on multiple equilibria formulation of the interactions of the inhibiting reagent and protein, is described and applied to minimum gelling concentration and saturation-solubility measurements in the presence of these inhibitors. Our principal expression which describes the influence of a given inhibiting reagent on the hemoglobin activity at the critical gelling concentration, (a)D = (a)w exp(N'KB[D]) has been used to predict the parameters expected of a given inhibitor, and also to provide information concerning the nature and the number of amino acids at the areas of contact of the hemoglobin tetramers that form the microtubular fibers of the gel phase of deoxyhemoglobin S. In this expression N' represents the effective number of amino acid sites per hemoglobin tetramer or the pregelation unit that enters the gelation reaction, KB is the interaction constant characterizing the interaction between the average amino acid and the inhibiting reagent, and [D] is the concentration of the inhibiting reagent. The analysis of our data includes correction for non-ideality. Our calculated gelling concentrations and saturation-solubilities are in good accord with the experimental data obtained with the various alkylureas and amides of increasing hydrophobicity or hydrocarbon content, demonstrating the utility of such analysis for the rational design of pharmacologically significant new reagents. In this regard it is to be noted that the interaction parameter, KB, used to predict the effectiveness of a given reagent is evaluated by assuming additivity of the group values of the chemical constituents of the reagent. Analysis of our data obtained as a function of concentration of several ureas (ethyl-, propyl-, butyl-, and n-pentylurea) gave N' estimates in the neighborhood of 80--100, suggesting that a substantial fraction of the surface groups of hemoglobin must be removed from contact with solvent in the course of the polymerization reaction that leads to the formation hemoglobin S gels.