The heat resistance and conformational plasticity of Kunitz soybean trypsin inhibitor

The heat resistance of Kunitz soybean trypsin inhibitor (KSTI) and the conf ormational plasticity of the purified protein were examined in Tris-HCl buf fer (0.05 M, pH 8). The rate of KSTI inactivation at moderate temperatures 70-100C was slow. The time for 90% inactivation (D-value) of KSTI at 120 - 150C ranged between 81s-1047s. The activation enthalpy (Delta H-#) and entr opy (Delta S-#) change for ultrahigh temperature (UHT) inactivation of KSTI was 107 kJ mol(-1) and -26 J mol(-1) K-1, respectively. Fluorescence monit oring of KSTI structure showed a conformational change at 30-70C with a mid -point temperature (T-m) of 55. 6C. The heat induced structural change was fully reversible. There was complete regain of the KSTI native structure af ter heating at 80C, as judged from intrinsic fluorescence measurements. The results are considered in relation to the Lumry-Eyring 2-stage scheme prot ein inactivation. The UHT resistance of KSTI may be ascribed to its conform ation plasticity which allows efficient regain of its native structure afte r heat unfolding. Other possible causes of heat resistance in biologically active proteins and peptides (BAPP) are briefly discussed.