In the tropical species Carica papaya, the articulated and anastomosing laticifers form a dense network of vessels displayed in all aerial parts of the plant. Damaging the papaya tree inevitably severs its laticifers, eliciting an abrupt release of latex. Besides the well-known cysteine proteinases, papain, chymopapain, caricain and glycyl endopeptidase, papaya latex is also a rich source of other enzymes. Together, these enzymes could provide an important contribution to plant defence mechanisms by sanitising and sealing the wounded areas on the tree.

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Papaya proteinase III (PPIII) was purified, as the S-methylthio derivative from the latex of Carica papaya L., by ion-exchange chromatography. Separation of reactivable PPIII from the irreversibly oxidized molecular species of this enzyme was readily achieved after a selective conversion of the reactivated proteinase into the S-monomethoxypoly(ethylene glycol)thio derivative (S-mPEG thio PPIII). From this derivative, a PPIII preparation titrating 1 mol of thiol/mol of enzyme was regenerated. From the physicochemical properties of S-mPEG thio PPIII that were investigated, it is concluded that interactions between the mPEG and the PPIII chains occur only to a limited extent. In addition to its usefulness for purifying thiol-containing enzymes, the mPEG modification resulting from mixed disulfide bond formation may find other practical applications.

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