Potassium cyanide, a commonly used activator for the proteolytic enzyme papain, may be bound by the Strecker reaction to form cyanohydrins in digestion mixtures. The formation of these N‐containing complexes depends on the pH of the digest, incubation time and temperature, and the amount of reducing sugar present. They are soluble in 5% trichloroacetic acid (TCA), so the N in the complexes will be estimated together with the non‐protein N released during digestion. Their formation probably accounts for some experimental discrepancies observed when studying the action of KCN‐activated papain on extracted leaf proteins, and calculating the percentage hydrolysis from the amount of N found in the TCA‐soluble fraction. A procedure for digesting leaf proteins by papain is described that uses thioglycollic acid as activator, and the N is estimated in the undissolved substrate and the TCA‐precipitable fraction, instead of in the TCA‐soluble fraction. Copyright © 1969 John Wiley & Sons, Ltd
