Proteases are widely distributed in the natural world, found in nearly all plants, animals and microorganisms. As a group, they are well known as protein degrading enzymes and were one of the first enzymes to be commercialised. They are commercially used in a vast range of industrial applications, including detergents, pharmaceuticals and food industries. Proteases are classified into 4 main types: serine proteases, cysteine proteases, Aspartyl proteases and Metalloproteases. All having differing action mechanisms and biological processes, hence a range of applications.

Actinidin is the principal proteolytic enzyme found in varieties of kiwifruit. Classified as a cysteine protease, also known as thiol protease, it is part of the family of plant proteases that includes Papain (papaya), Bromelain (pineapple), Ficin (figs) and Zingibain (ginger).

While the cysteine proteases share similarities in their activities, differences in their protein structures and molecular weights result in differing activities towards specific protein substrates with varying pH and temperature optima. These structural differences also influence the thermal stability of the enzyme. 

As a plant-derived protease, extracted from a commercially grown fruit under a regulated environment, Actinidin offers a safe, reliable source of proteolytic activity for a range of food and therapeutic applications. With the narrow substrate specificity exhibited by Actinidin for many proteins, it is ideally suited to several specialist areas.

Commercial Protein Hydrolysates
In the production of commercial protein hydrolysates from meat, dairy and plant sources the specificity of Actinidin can be directed to achieve products with varying degrees of hydrolysis suitable for applications ranging from flavour ingredients to functional peptides. Of particular interest is the creation of flavour profiles with minimal or no bitterness.

Meat Tenderising
Consumer meat products benefit from enhanced tenderness by the treatment of the meat cut with injectable brines containing low levels of Actinidin. The pH optima and a deactivation temperature below the optimum cook temperature for meats make Actinidin, with its high enzymatic activity for collagen and fibrinogen substrates, the enzyme of choice for tenderising red meat, poultry and some seafood, while minimising the risk of over-tenderising and the loss of textural quality.

Food and Nutrition
Nutritional supplements and digestive aids based on kiwifruit preparations containing Actinidin have demonstrated benefits for a significant percentage of the population with compromised digestive systems, particularly the elderly, overcoming problems with malnutrition through poor uptake of essential (protein) nutrients. 

In vitro studies demonstrate that Actinidin will enhance the digestion of whey, casein, soy and gluten proteins over that achieved by pepsin and pancreatin alone. The Actinidin protein can tolerate passage through the acidic environment of the stomach. In many elderly people, the stomach is not producing enough acid to facilitate initial digestion; supplementation with Actinidin helps overcome this deficiency.

Actinidin demonstrates a more specific activity toward milk proteins compared to the other cysteine proteases, notably Papain. Actinidin activity on the major milk protein, casein, is pH-dependent and produces a pattern of peptides that differs under different buffering conditions.

While the cysteine proteases in general exhibit minimal activity on whey proteins, recent research has reported that Actinidin can hydrolyse the two whey proteins (α-lactalbumin and β-lactoglobulin) under optimum conditions. 

The specificity of Actinidin and its close parallel with the activity profile of Chymosin offers potential as a coagulant of dairy proteins to produce cheese varieties, replacing calf rennet. Of special importance in the vegan and vegetarian food sectors.

Actinidin has demonstrated benefits in baking for achieving improved crumb softness through its actions on gluten. 

As a clarifying agent, Actinidin can be applied to eliminate protein haze in beverages.

Biological Research
The collagenase activity exhibited by Actinidin offers a novel method for the isolation of cell populations from various tissue sources.  The temperature and pH profile of Actinidin provides conditions more suited to this application than many other proteases. 

Enzymic Wound Debridement
The development of enzymic treatments, based on Actinidin, for the removal of necrotic tissue from ulcerated skin and burn wounds, offers a potential non-surgical treatment to enhance healing.