β-アミラーゼのアスコルビン酸に依る阻害に關する研究 : (第2報)システインに依る阻害の除去

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(1) The complex nature of the inhibition of sweet potato β-amylase caused by ascorbic acid plus Cu++ was demonstrated by testing the degree of reactivation at different stages with cysteine. (Fig. 3 and Fig. 4). (2) As MAPSON suggested in the case of urease, -SH group of enzyme reacts rapidly with Cu+ formed by ascorbic acid and results in inhibition which is completely reversed by excessive cysteine. The effect of cysteine concentration on the degree of reactivation shows that -SH group of enzyme competes with cysteine for Cu+. (Fig. 2). (3) While inhibition by Cu+ diminishes as ascorbic acid decreases, another type of inhibition proceeds, which is not reversed by cysteine. Thus the observed inhibition is the sum of these two components. (Fig. 3 and Fig. 4) (4) The mechanism of the latter (not reversed by cysteine) is not yet determined, but there may be a change occurring in -SH group of enzyme intramolecularly. (5) This inhibition may be of no importance as the regulatory mechanism of β-amylase action in vivo. But, it is rather interesting that the enzyme acts as a kind of tracer indicating the mechanism of autoxidation of ascorbic acid in the presence of Cu++ as a catalyst.
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