Self-aggregation, a new property of cardiac fatty acid-binding protein. Predictable influence on energy production in the heart

The binding isotherm of 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxyloxazolidine to the 12,000 molecular weight cytoplasmic fatty acid-binding protein purified from pig's heart and as analyzed by electron spin resonance is shown to be sigmoidal and dependent on protein concentration. Its maximum binding capacity, Vmax, is 9.81 microM X g-1 when the protein concentration is 1.98 g . liter-1 and 14.33 microM . g-1 when the concentration is 0.198 g . liter-1. The molar ellipticity, theta, measured by circular dichroism at 225 nm in the protein range of 0.2-3 g . liter-1 is also shown to depend on concentration, with two maxima at 1.7 and 1.2 g . liter-1. The rotational correlation time, tau eff, of a spin-labeled N-(2,2,5,5-tetramethyl-3-carbonyl-pyrroline-1-oxyl)imidazole specifically bound to tyrosine residues of the protein surface, as analyzed by electron spin resonance, is decreasing as a function of protein concentration. To account for these three concentration-dependent variations, we suggest that the protein self-aggregates since these variations will not be found if the protein is monomeric. A very significant influence of this aggregation property of the protein on the activity of fatty acid-dependent membrane-bound enzymes is predicted by a model analysis and, thus, should be considered as a new parameter in the control of energy production in the heart.