AbstractThe conformation of porcine serum ferric transferrin (Tf) and its stability against denaturation were studied by circular dichroism. Tf was estimated to have 19–24% α‐helix and 50–55% β‐sheet based on the methods of Chang et al. (Chang, C.T., Wu, C.‐S.C., & Yang, J.T., 1978, Anal. Biochem. 91, 13–31) and Provencher and Glöckner (Provencher, S.W. & Glöckner, J., 1981, Biochemistry 20, 33–37). Removal of the bound ferric ions (apo‐Tf) did not alter the overall conformation, but there were subtle changes in local conformation based on its near‐UV CD spectrum. The Tfs were stable between pH 3.5 and 11. Denaturation by guanidine hydrochloride (Gu‐HCl) showed two transitions at 1.6 and 3.4 M denaturant. The process of denaturation by acid and base was reversible, whereas that by Gu‐HCl was partially reversible. The irreversible thermal unfolding of Tfs began at temperatures above 60 °C and was not complete even at 80 °C. The bound irons (based on absorbance at 460 nm) were completely released at pH <4 or in Gu‐HCl solution above 1.7 M, when the protein began to unfold, but they remained intact in neutral solution even at 85 °C. The NH2‐ and COOH‐terminal halves of the Tf molecule obtained by limited trypsin digestion had CD spectra similar to the spectrum of native Tf, and the COOH‐terminal fragment had more stable secondary structure than the NH2‐terminal fragment.