THE PROBLEM OF PLASTEIN FORMATION: I. THE FORMATION OF A PLASTEIN BY PAPAIN

Abstract

Papain, activated by cyanide, cysteine, or hydrogen sulphide, produces a plastein, a protein-like substance, from concentrated peptic or papain digests of egg albumin. This activity is suppressed by boiling, aeration, the addition of copper salts, hydrogen peroxide, iodoacetate, or alloxan, indicating that free SH groups are essential. The optimum pH for plastein formation is 4.8; that for hydrolysis of the plastein by papain is about pH 4.2. It is concluded that concentration alone controls the direction of the enzyme action, the optimum pH and oxidation-reduction conditions being practically identical for both formation and hydrolysis of plastein.The rate of plastein formation varies with substrate concentration, above a minimum value. With constant substrate concentration, the rate of plastein formation varies as the square-root of the enzyme concentration. Hydroxylamine reduces the activity of the papain by about one-half. A similar effect is produced by treatment with phenylhydrazine, followed by benzaldehyde. Phenylhydrazine alone has no effect, whereas benzaldehyde alone depresses the activity very strongly.