Characterisation of bioactive proteins present in Actinidia species

Kiwifruit consumption is well known to have a positive impact on digestive health. Actinidin and kiwellin, two kiwifruit proteins have been characterised because of their importance in Actazin™ that is produced from kiwifruit to aid digestion. A biophysical characterisation was carried out for both proteins and the catalytic activity of actinidin was measured during the production of Actazin™ to assess if kiwifruit processing was detrimental to the catalytic rate. Actinidin, purified from A. deliciosa cultivar showed that a decrease in enzymatic activity could be from partially inactivated actinidin. Similarly actinidin from Actazin™ became inactivated when the powder was solubilised in buffer. Most of the lost activity could be recovered when a reductant was added and it is hypothesised that the catalytically important cysteine residue was becoming oxidised by a chemical species that was present in the solubilised kiwifruit powder. Freezing, thawing, and storing the processed kiwifruit pulp resulted in the largest drop of total actinidin activity of 46%. Freeze drying the frozen pulp surprisingly did not significantly affect the retention of actinidin despite incurring prolonged heat stress. Further studies that assess the effects that freezing and thawing have on the stability of actinidin is therefore suggested. Kiwellin was characterised using a number of analytical techniques and was shown to be a monomeric protein in solution that had a high melting point. It was suggested that disulfide bonds and salt bridges could be stabilising kiwellin, which was also highly dynamic in solution. Kiwellin did not show the presence of any secondary structures, which is in contrast to previous studies and small angle X-ray scattering revealed that kiwellin had an elongated shape. The dynamic nature of the protein may have implications for the interactions between kiwellin and actinidin, which has been previously shown and this could be related to the physiological function of kiwellin