chung-shih Tang University o f Hawaii Honolulu, Hawaii 96822
A Simple Method for Demonstrating an Enzymatic Reaction
I t would be desirable to impress students with a simple but dramatic experience on the effect of enzymes before the subject is introduced. The glucosinolates (thioglucosides) are a group of compounds distributed in members of certain plant families, such as Crucifel-ae (e.g., cabbage, cauliflower). A characteristic reaction of glucosinolates is their enzymatic hydrolysis thioglucosidase
----+
&-SO,-
glucosinolate
R-N=C=S + C6H12O6+ SO,%-+ H + isothiocyanate . glucose
The enzyme which catalyzes the hydrolysis of thioglucoside is thioglucosidase. A rearrangement on the part of the aglucon occurs during the reaction and an isothiocyanate may be formed as the final product. More detailed information can be found in a recent review on sulfur compounds in plants by Ettlinger and Kjaer. The preceding enzymatic reaction also occurs when mature papaya seeds are macerated. To demonstrate this, papaya seeds obtsined from fresh papaya are utilized after removing the outer gelatinous coating (sarcotesta), after which the seed is washed thoroughly under running water for approximately 1 min. This seed is then put into one's mouth and crushed with the front teeth. The inner contents of the seed are carefully sensed with the front part of the tongue which gives a sweet taste with a slight degree of bitterness. If the tasting period is prolonged, a pungent flavor close to that of mustard oil gradually appears. However, this flavor is usually not distinctive. After this
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experience, the chewed seed is discarded and the mouth rinsed thoroughly with tap water. A whole papaya seed is now tasted; the gelatinous membrane is first gently broken with the front teeth. The juicy content with a rather flat taste should be sensed. Now, crush the seed as before and a strong pungent flavor similar to that of mustard oil is immediatelv released. The sharp contrast in flavor between these two papaya seeds with and without the outside gelatinous coating is attributed to enzvmatic reaction. Macerated ripe papaya seeds contain approximately 3000 ppm of benzyl isothiocyanate (R = C6H&H2-). Apparently, the glucosinolate is confined to the embryo and endosperm of the seed while the enzyme, thioglucosidase, is essentially contained in the gelatinous coating material (sarcotesta). Benzyl isothiocyanate, a mustard oil, is formed as soon as the enzyme and the substrate are mixed in the mouth, thus producing the pungent flavor noted. The slow evolution of a faint mustard flavor from the crushed seed without, the gelatinous coating suggests that enzymatic activity may also be present in the embryo and endosperm in small amounts but that the enzyme activity is much higher in the gelatinous material. If the papaya seed is boiled in water for two minutes no mustard-like flavor will be detected when chewing the whole seed. This is a simple demonstration of the heat labile characteristics of enzymes. Journal Series No. 1185 of the Hawaii Agricultural Experiment Station. ETTLINGER, M. G., and KJAER,A., "Itecent Advances in Phytochemistry" (Editor: MABRY,T. J.), Appleton-CenturyCrofts, Inc., New York, 1968, Vol. 1, p. 59.
