Papain

Papain, also known as papaya proteinase I, is a cysteine protease enzyme present in papaya (Carica papaya) and mountain papaya (Vasconcellea cundinamarcensis).

Papain family
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.

Structure
The papain precursor protein contains 345 amino acid residues, and consists of a signal sequence (1-18), a propeptide (19-133) and the mature peptide (134-345). The amino acid numbers are based on the mature peptide. The protein is stabilised by three disulfide bridges. Its three-dimensional structure consists of two distinct structural domains with a cleft between them. This cleft contains the active site, which contains a catalytic diad that has been likened to the catalytic triad of chymotrypsin. The catalytic diad is made up of the amino acids - cysteine-25 (from which it gets its classification) and histidine-159. Aspartate-158 was thought to play a role analogous to the role of aspartate in the serine protease catalytic triad, but that has since then been disproved. Nonetheless, it does appear to play a role.

Function
The mechanism by which it breaks peptide bonds involves deprotonation of Cys-25 by His-159. Asparagine-175 helps to orient the imidazole ring of His-159 to allow this deprotonation to take place. Cys-25 then performs a nucleophilic attack on the carbonyl carbon of a peptide backbone. This frees the amino terminal of the peptide, and forms a covalent acyl-enzyme intermediate. The enzyme is then deacylated by a water molecule, and releases the carboxy terminal portion of the peptide. In immunology, papain is known to cleave the Fc (crystallisable) portion of immunoglobulins (antibodies) from the Fab (antigen-binding) portion.

Papain is a relatively heat resistant enzyme, with a temperature optimal range of 60-70°C.

Papain prefers to cleave at: (hydrophobic)-(Arg or Lys)- cleaves here -(not Val). Hydrophobic is Ala, Val, Leu, Ile, Phe, Trp, or Tyr.

Uses
Its utility is in breaking down tough meat fibres and has been used for thousands of years in its native South America. It is sold as a component of powdered meat tenderiser available in most supermarkets. Papain, in the form of such a powdered meat tenderiser, made into a paste with water, is also a home remedy treatment for jellyfish, bee, and yellow jacket (wasps) stings; mosquito bites; and possibly stingray wounds, breaking down the protein toxins in the venom.

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 breaking down 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 (if left untreated, papain's 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.

It is also used as an ingredient in various enzymatic debriding preparations, notably Accuzyme. These are used in the care of some chronic wounds to clean up dead tissue.

Papain can also be found as an ingredient in some toothpastes or mints as teeth-whitener. Its whitening effect in toothpastes and mints is minimal, however, because the papain is present in low concentrations, and is quickly diluted by saliva. It would take several months of using the whitening product to have noticeably whiter teeth.

It is the main ingredient of Papacarie, a gel used for chemomechanical dental caries removal. Besides the advantage of avoiding the use of rotary cutting tools, it does not interfere in the bond strength of restorative materials to dentin.

Papain has been known to interfere with urine drug test for cannabinoids. It is found in some drug detox products.

Immunoglobulins
An antibody digested by papain yields three fragments: two 50 kDa Fab fragments and one 50kDa Fc fragment. The papain-digested antibody is unable to promote agglutination, precipitation, opsonization, and lysis.

Production
Papain is usually produced as a crude, dried material by collecting the latex from the fruit of the papaya tree. The latex is collected after scoring the neck of the fruit, where it may either dry on the fruit or drip into a container. This latex is then further dried. It is now classified as a dried, crude material. A purification step is necessary to remove contaminating substances. This purification consists of the solubilization and extraction of the active papain enzyme system through a government-registered process. This purified papain may be supplied as powder or as liquid.

Action by the FDA to restrict marketing of all topical drug products containing papain

 * See also article: Proteases (medical and related uses)

On September 23, 2008, the FDA warned companies to stop marketing topical drug products containing papain by November 4, 2008. The FDA said, "Papain-containing drug products in topical form historically have been marketed without approval...". According to the FDA's statement on the subject, "These unapproved products have put consumers' health in jeopardy, from reports of permanent vision loss with unapproved balanced salt solutions to a serious drop in blood pressure and increased heart rate from the topical papain products," said Janet Woodcock, M.D., director for the Center for Drug Evaluation and Research. In the same FDA announcement, the FDA states the following:

About Unapproved Topical Papain Products: Topical drug ointments containing papain are used to remove dead or contaminated tissue in acute and chronic lesions, such as diabetic ulcers, pressure ulcers, varicose ulcers, and traumatic infected wounds. Trade names for these products include Accuzyme, Allanfil, Allanzyme, Ethezyme, Gladase, Kovia, Panafil, Pap Urea, and Ziox. Other products are marketed under the names of the active ingredients, for instance, papain-urea ointment.

The FDA is taking action today against these products because it has received reports of serious adverse events in patients using products containing papain. Reports include hypersensitivity (allergic) reactions that lead to hypotension (low blood pressure) and tachycardia (rapid heart rate). In addition, patients who are allergic to latex can also be allergic to papaya, the source of papain. Therefore, patients with latex sensitivity may be at increased risk of suffering an adverse reaction to a topical papain drug product.

FDA urges consumers who are using topical drug products containing papain, and who have questions or concerns, to contact their health care provider about discontinuing treatment with these products. There are a number of FDA-approved topical products that have been found safe and effective as wound healing agents and that do not contain papain.

"Removing unapproved topical drug products containing papain and unapproved ophthalmic balanced salt solutions is yet another step forward for patient safety," said Deborah M. Author, director, Office of Compliance for CDER, FDA.

Human cysteine proteases from papain family
CTSB;     CTSC;      CTSF;      CTSH;      CTSK;      CTSL1;     CTSL2; CTSL3;    CTSO;      CTSS;      CTSW;      CTSZ;      TINAG;     TINAGL1;