THERMAL-STABILITY OF HORSE SPLEEN APOFERRITIN AND HUMAN RECOMBINANT-HAPOFERRITIN

The thermal stability of horse spleen apoferritin, a heteropolymer com posed of 90% L and 10% H chains, has been studied by differential scan ning calorimetry and compared with that of the human recombinant H hom opolymer. The denaturation temperatures (T-m) are significantly higher for the horse spleen polymer than for the recombinant protein under a ll experimental conditions (e.g., at pH 7, T-m values are greater than or equal to 93 and 77 degrees C, respectively). The thermal denaturat ion process displays substantial reversibility for both polymers up to a few degrees below T-m, as indicated by CD measurements in the far a nd near uv regions. At temperatures higher than T-m the thermograms ar e influenced by the exothermic contribution of aggregation and/or prec ipitation. The H homopolymer thermogram, which is not distorted by the exotherm, is consistent with a multistate denaturation process. Acid dissociation of apoferritin produces stable dimeric subunits. The ther mal unfolding of both dimeric subunits is reversible at least up to T- m and is characterized by an inversion of stability relative to the po lymers (at pH 3.5, T-m is 42 degrees C for the horse spleen and 50 deg rees C for the H subunit). These results indicate that the stabilizati on of the polymeric structure arises mainly from interactions between dimers, in accordance with the crystallographic evidence that the dime rs are the building blocks of the polymeric molecule. (C) 1996 Academi c Press, Inc.