Activators: Papain is activated by cysteine, sulphide, sulphite and more. It is enhanced when heavy metal binding agents such as EDTA are also present. N-bromosuccinimide enhances the activity. Inhibitors: Substances which react with sulphydryl groups including heavy metals, carbonyl reagents. Aldehydes are papain inhibitors. Benzoylamidoacetonitrile is an inhibitor. See Shapira and Arnon (1967a and b) on antibody inhibitors. Papain may be inactivated by H₂O₂ generated by γ-irradiation of H₂O⁻ the active SH group being oxidised to sulphenic acid. Specific inhibitors are AEBSF, antipain, cystatin, E-64, leupeptin, PMSF, TLCK and TPCK.

• Isoelectric point (pI): 9,6 (Lit.)
• Extinction coefficient (E1%): 25,0 (Lit.)
• Enzyme commission number: E.C.3.4.22.2

Papain is a sulphydryl protease from Carica papaya latex. (A second protease, chymopapain, and a lysozyme have also been isolated from this same source.) Since native crystalline papain is quite unreactive until acted upon by mild reducing agents such as cysteine or cyanide, it may exist as a zymogen.

Papain is widely used in cell isolation. With some tissues papain has proved less damaging and more effective than other proteases of the enzymes used for dissociating turtle retina, papain produced the least trauma. Intact single photoreceptor cells have also been isolated from adult salamander retina with papain. It is used to obtain high yields of viable, morphologically intact cortical neurons from postnatal rats. Used for dissociation of postnatal rat hippocampus. It is used with fetal as well as postnatal brain regions to provide maximal dissociation and viability of neurons. It is also used to produce Fab fragments of antibodies. In addition, it is used in red cell serology to modify the red cell surface to enhance or destroy the reactivity of many red cell antigens. It is useful for platelet serology studies. Papain has also been used in the enzymatic synthesis of amino acids, peptides, and other molecules.

Papain is a cysteine protease that cleaves peptide bonds of basic amino acids, leucine, or glycine. It is also hydrolyses esters and amides. It consists of a single polypeptide chain with three disulphide bridges and a sulphydryl group necessary for activity of the enzyme.