Although there are several thousand publications on the proteinases of pineapple, papaya and kiwifruit dating back many decades, controversy remains over which fruit delivers the greatest proteinase activity against common dietary proteins. This is because of the variable experimental conditions used: pH, substrate, enzyme purity and enzyme quality. Therefore the proteinase activities of fresh green and gold kiwifruit, pineapple and papaya, sometimes marketed as dietary proteinase supplements, were directly compared for their proteinase activities against bovine -casein, bovine serum albumin, chicken ovalbumin and soy protein across a pH range from pH 2 to pH 9. Proteinase activity was assessed using proteins labelled with the fluorescent dye bodipy-FL and by quantification of unlabelled protein digestion by SDS-PAGE. Bodipy-casein performed well as a substrate giving signal: noise ratios of 20:1. Bodipy-ovalbumin and bodipy-soy performed poorly as substrates. SDS-PAGE showed that bovine serum albumin and ovalbumin were readily digested by kiwifruit actinidin despite published reports to the contrary. Two per cent SDS promoted bromelain digestion of BSA at pH 7 but at pH 4 prevented BSA digestion at pH4. Gold kiwifruit was considerably better at digesting ovalbumin at pH 7 than green kiwifruit. Incubation of green kiwifruit homogenate at 37°C for 20 minutes led to a 45% loss of actinidin whereas no autocatalysis occurred at room temperature. Soy protein and casein were rapidly digested by kiwifruit and pineapple extracts. Papaya extract was very low in proteinase activity. Thus the evaluation of each fruit as an effective dietary proteinase supplement is greatly influenced by factors such as substrate choice, the presence of SDS and pH.

pineapple, papaya, kiwifruit, proteinase, ovalbumin, bovine serum albumin, soy, -casein