FAILURE OF MERCURIC-CHLORIDE TO SELECTIVELY INHIBIT BETA-AMYLASE IN SORGHUM MALT

Mercuric chloride has been reported to be a suitable reagent for the determination of alpha-amylase activity in sorghum malt, based on its ability to selectively inhibit beta-amylase. In this re-investigation, the alpha- and beta-amylase activities of eight sorghum malts were determined after treatment of malt extracts with various concentrations of mercuric chloride. At a malt: mercuric chloride ratio of 8.3 x 10(5):1, incomplete inhibition of beta-amylase activity, as measured by the Betamyl assay, occurred in all extracts. However, this concentration resulted in significant inhibition of alpha-amylase activity in all extracts, as measured by both the Ceralpha assay and the Phadebas assay. In addition, alpha-amylase activity was found to be significantly inhibited at malt: mercuric chloride ratios as low as 1.0 x 10(5):1, when measured by the AmyloZyme assay. These findings do not support the original report that a malt:mercuric chloride ratio of 4.0 x 10(3):1 will selectively inhibit beta-amylase in sorghum malt. Furthermore, in this context it should be emphasised that the original report was based upon inhibition studies conducted on beta-amylase derived from barley, not sorghum malt.