Inhibitory Effect of Ascorbic Acid on β-Amylase

Part 5. Effect of Temperature and pH and a Comparison of Inhibition by Mercaptide-forming Heavy Metals

Abstract

1) Effect of temperature and pH on the inhibition of sweet potato β-amylase by -ascorbic acid plus Cu⁺⁺ has been studied. Moreover, these properties of this inhibition have been compared with those of inhibition caused by other mercaptide-forming heavy metals viz. Ag⁺. HgCl₂ and PCMB.
2) As it was reported previously, this inhibition contains two reactions in series, inhibition by cuprous mercaptide formation (I) and subsequent oxidation of mercaptide by atmospheric O₂ (II). From the data presented in Fig. 1, it is presumed that reaction (I) occurs instantaneously, and has no temperature coefficient, while reaction (II) is a slow process and requires activation energy.
3) As pH becomes lower, the extent of reaction (I) decreases presumably as the result of lower affinity of Cu⁺ to -SH groups of enzyme in such a condition. Consequently reaction (II) proceeds at a slower rate. (Fig. 2_??_Fig. 5)
4) Above-mentioned features of this inhibition including reactivation by cysteine, which was reported previously, are quite the same as those in the case of inhibition by Ag⁺ and differ in all respects from those by HgCl₂ and PCMB. These results are summarised in Table 1.