Thermodynamics of native protein/foreign surface interactions. IV. Calorimetric and microelectrophoretic study of human fibrinogen sorption onto glass and LTI-carbon.

1. According to a working hypothesis put forward in the previous papers of this series2-4, the initial phases of native blood/foreign surface interactions have been considered within the framework of a physicochemical model of contact activation at the molecular level. The salient features of this hypothesis are: a) the arrival and adsorption of native plasma proteins on a contact surface overwhelmingly precedes that of the cellular blood components; b) the interaction energy that arises between a particular foreign surface and native plasma proteins settling on it, is a characteristic quantity depending upon the effective surface molecular structure as well as the nature of the proteins; c) the "intensity" of native protein/foreign surface interactions can be treated in terms of thermodynamic quantities since these energy terms are independent of the type of forces acting between protein and surface; and d) depending upon the degree to which the adsorption of a native protein is thermodynamically favored by enthalpy and/or entropy factors, the interaction energy can be utilized to induce conformational changes of varying degree in the sorbed protein. 2. Using glass and low temperature isotropic (LTI) carbon adsorbents, i.e., a known procoagulant and a relevant biomaterial, respectively, the adsorption properties and the potential surface-induced conformational changes of high-purity native human fibrinogen (clottability greater than or equal to 92%) were studied, at 25 degrees C, by 3 independent methods. In all of the experiments performed, a) both adsorbents were employed in the form of particles less than or equal to 1.0 mu representing specific surface areas of 9.85 M2/Gm and 27.7 (nominal) M2/Gm for the glass and LTI-carbon powders, respectively, and b) the fibrinogen was absorbed from a standaridized buffer (pH = 7.2, ionic strength 0.05) using the same fixed surface area/protein solution volume ratio with a given adsorbent. 3. The 25 degrees C adsorption isotherms of fibrinogen obtained for both the glass and LTI-carbon powder adsorbents are not amenable to Langmuir type analysis but indicate multilayer sorption with a possible change in binding mechanism after the completion of the first sorbed monolayer. The adsorptivities attained at first monolayer coverage were slightly greater on glass (approximately 0.76 mug/cm2) than on LTI-carbon (approximately 0.52 mug/cm2). 4. Using a custom-built, thermistorized, isothermal-jacketed microcalorimeter routinely capable of resolving temperature changes of 0.00001 degrees C in 100 ml of aqueous sample volume, the "intensity" of interaction between fibrinogen and each of the microparticulate adsorbents was studied by the direct measurement of the net overall enthalpy changes, hI(SLP)25 arising as a result of protein adsorption. From these values, the net heat of protein sorption has been determined...