Consistency of polyphenol oxidase (PPO) thermostability in ripening apricots (Prunus armeniaca L.): Evidence for the presence of thermostable PPO forming and destabilizing mechanisms in apricots

Abstract

Destabilization of thermostable polyphenol oxidase (TS-PPO) during the ripening of peaches has been previously shown (Yemenicioǧlu, A; Cemeroǧlu, B. Tr. J. Agric. For. 1998, 22, 261-265). This work studied the effect of ripening on thermal stability of apricot PPO for three different cultivars. Kabaaşi cultivar contained thermolabile PPO, whereas TS-PPO appeared in Hacihaliloǧlu and Çataloǧlu cultivars. The TS-PPO showed biphasic inactivation curves, and its D and z values between 60 and 90 °C varied in the ranges of 357-1.12 min and 11.9-12.7 °C, respectively. In Hacihaliloǧlu cultivar the TS-PPO was very consistent and existed at all stages of ripening, whereas in Çataloǧlu cultivar it appeared only at the half-ripe stage. The loss of consistent TS-PPO in Hacihaliloǧlu apricots after partial purification by acetone precipitation and DEAE-cellulose chromatography suggested the non-covalent nature of its stabilization. The main purified fractions (F1 and F2) showed monophasic inactivation curves with similar thermal inactivation parameters (ZF1 = 10.4 °C, ZF2 = 10.1 °C). However, their kinetic properties against catechol (KmF1 = 61 mM, KmF2 = 122.7 mM) and substrate specificities were considerably different. The results of this study showed the presence of TS-PPO forming and destabilizing mechanisms in apricots. Further studies are needed for the solution of these mechanisms and to develop some new strategies that may be utilized by molecular techniques for a planned production of apricot cultivars provided with heat labile but normal PPO activity.