A common enzyme known as papain is obtained from the green papaya (pawpaw) fruit. Enzymes are proteins that can increase the rate of biological changes such as the ripening of fruit. At the end of an enzyme catalysed reaction the enzyme itself is unchanged and is able to react again.
Papain belongs to a family of related proteins with a wide variety of activities, including endopeptidases, aminopeptidases, dipeptidyl peptidases and enzymes with both exo- and endo-peptidase activity. Members of the papain family are widespread, found in baculovirus, eubacteria, yeast, and practically all protozoa, plants and mammals. The proteins are typically lysosomal or secreted, and proteolytic cleavage of the propeptide is required for enzyme activation, although bleomycin hydrolase is cytosolic in fungi and mammals. Papain-like cysteine proteinases are essentially synthesised as inactive proenzymes (zymogens) with N-terminal propeptide regions. The activation process of these enzymes includes the removal of propeptide regions, which serve a variety of functions in vivo and in vitro. The pro-region is required for the proper folding of the newly synthesised enzyme, the inactivation of the peptidase domain and stabilisation of the enzyme against denaturing at neutral to alkaline pH conditions. Amino acid residues within the pro-region mediate their membrane association, and play a role in the transport of the proenzyme to lysosomes. Among the most notable features of propeptides is their ability to inhibit the activity of their cognate enzymes and that certain propeptides exhibit high selectivity for inhibition of the peptidases from which they originate.
Uses : Papain breaks down tough meat fibres, and has been used for thousands of years to tenderise meat eaten in its native South America. Meat tenderisers in powder form with papain as an active component are widely sold.
Papain can be used to dissociate cells in the first step of cell culture preparations. A ten-minute treatment of small tissue pieces (less than 1 mm cubed) will allow papain to begin cleaving the extracellular matrix molecules holding the cells together. After ten minutes, the tissue should be treated with a protease inhibitor solution to stop the protease action. Left untreated, papain activity will lead to complete lysis of the cells. The tissue must then be triturated (passed quickly up and down through a Pasteur pipette) to break up the pieces of tissue into a single cell suspension.
Papain is added to some toothpastes and mint sweets as a tooth whitener. Its whitening effect is, however, minimal because the papain is present in low concentrations and is quickly diluted by saliva. It would take several months of use, to have a noticeable effect.
Papain is the main ingredient of Papacarie, a gel used for chemomechanical dental caries removal. It does not require drilling and does not interfere in the bond strength of restorative materials to dentin.