Actinidin, a proteolytic enzyme, is obtained from the fruit of the Chinese gooseberry, more commonly known as Kiwifruit (Actinidia chinesis). Due to a free sulphydryl group, it has been grouped with sulphydryl proteases such as papain and ficin. This free sulphydryl group is essential for enzyme activity, a trait shared with most plant isolated sulphydryle proteases. It has been suggested based upon kinetic studies that the family of plant sulphydryl proteases is quite homologous. Even with this fact the proteins do have a fair amount of structural variety. They range in molecular weight from 23.5 kDaltons (Papain) to 33.5 kDaltons (Stem Bromelain). Unique about actinidin is its Isoelectric point which is located at 3.1, compared with the pI's of papain, ficin and stem bromelain which center around nine. Actinidin, along with papain, contains the fewest Cys residues of the aformentioned proteins with seven, ficin has eight, and stem bromelain has ten.
Actinidin like Papain has several commercial uses that range from medicinal treatment to meat tenderizing. With the increasing accuracy in identifying structural and functional elements of Actinidin, researchers have used it for comparison to other sulphydryl proteases (papain, ficin) as well as in inhibition studies. Such inhibition studies, it is hoped, will give insite into the activity of cysteine proteases that are crucial to parasitic metabolism and reproductive function, and in the near future produce a blocker that will, if not kill the parasites, at least slow it down.