Characteristic behaviors of 2-(4′-hydroxyphenylazo)-benzoic acid to native and heat-denatured serum albumins

Abstract

1. 1.|Isolating Components 1, 1′, 2 and 3 from bovine serum albumin solution heat treated at 65 °C and at pH 9.0 (Aoki, K., Hiramatsu, K., Kimura, K., Kancshina, S., Nakamura, Y. and Sato, K. (1969) Bull. Inst. Chem. Res. Kyoto Univ. 47, 274–282 and Aoki, K., Sato, K., Nagaoka, S., Kamada, M. and Hiramatsu, K. (1973) Biochim. Biophys. Acta, 328, 323–333), interactions of each of them with 2-(4′-hydroxyphenylazo)-benzoic acid (HABA) were studied. Only Component 1 enhanced the absorbance of HABA near 480 nm at pH 7.0. Other components did not affect the absorption spectrum of HABA between 400 and 600 nm. 2. 2.|The increment of an absorbance at 480 nm of HABA by Component 1 was smaller than that by the same concentration of the native bovine serum albumin. This makes us assume that Component 1 consists of two molecular species, the native bovine serum albumin (N) and bovine serum albumin modified by heating (N′). The latter corresponds to the heat-resistant bovine serum albumin in the literature. Its presence has been verified by the present investigation. 3. 3.|Amounts of N and N′ were calculated as a function of periods of heat treatment of bovine serum albumin at 65 °C, using results of electrophoretic and spectrophotometric experiments, and assuming that bovine serum albumin contains in the native state only N and that Component 1 contains only N′ after 60 min heating. It was calculated that Component 1 was completely transformed to N′ by 40 min heating at 65°C and pH 7.0. 4. 4.|Binding of HABA to N and N′ was characterized spectrophotometrically. The number of binding sites of HABA was smaller for N′, but the equilibrium constant of the binding system was approximately the same for N and N′.