Low molecular weight proteins stable to heat, acid, and organic solvents were extracted from tomato leaves and from mango, papaya, and walnut seed. These proteins were further purified by a new method utilizing triacylglycerol lipase (EC 3.1.1.3) and (diethy1amino)ethyl-Sephadex. Electrophoretic analysis and assays for redox activity and interaction with crude nonreduced papain (which contains EC 3.4.22.2. and EC 3.4.22.3) and the specific sulfhydryl reagent ammonium 4-chloro-7-sulfobenzofuranozan showed that these proteins have low molecular weights, net positive charges, and electrophoretic migration characteristics similar to thionin; abundant sulfhydryl groups; redox properties; and interactions with crude papain proteolytic activity. The possibility that these proteins may be thionins is discussed, and the pseudo-first-order kinetics of their reaction with insulin is demonstrated.

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