A necessary modification to the preparation of papain from any high-quality latex of Carica papaya and evidence for the structural integrity of the enzyme produced by traditional methods.

A method of preparation of papain (EC 3.4.22.2) from relatively soluble types of latex of Carica papaya, including spray-dried latex produced by a controlled and relatively mild process, was devised. Spray-dried latex dissolves easily in water up to 350mg/ml at 22 degrees C, which corresponds to approx. 230mg of protein/ml. When the usual method of preparation of crystalline papain contaminated only by its oxidation products, developed by Kimmel & Smith [J. Biol. Chem. (1954) 207, 515-531], is applied to spray-dried latex, the result is a preparation of papain heavily contaminated by chymopapains A and B (EC 3.4.22.6), and to a lesser extent by papaya peptidase A. This applies also to other types of papaya-latex currently commercially available, which, though less soluble than spray-dried latex, are more soluble than the types of latex available when the method of Kimmel & Smith (1954) was developed. This contamination is avoided by adjusting the concentration of the initial latex extract to 65mg of protein/ml (or less) before salt fractionation. For spray-dried latex this corresponds to 100mg of latex/ml. Papain isolated from spray-dried latex was characterized by using 2,2'-dipyridyl disulphide and 4-chloro-7-nitrobenzofurazan as thiol-specific reactivity probes and alpha-N-benzoyl-l-arginine ethyl ester as substrate. Papain isolated from this source appears to have the same catalytic-centre characteristics as papain isolated previously from latex produced by harsher methods. The catalysis of the hydrolysis of alpha-N-benzoyl-l-arginine ethyl ester by the mixture of thiol proteinases extracted from spray-dried latex by application of the method of Kimmel & Smith (1954) appears to obey Michaelis-Menten kinetics. The presence of the other enzymes results in an increase in the value of K(m) and a decrease in the catalytic-centre activity (k(cat.)) relative to the values for the catalysis by papain.