Isolation of three species of soluble sucrase from larval midgut of the silkworm, Bombyx mori and some kinetic properties

1. 1. Small but significant differences were observed in the total and specific activity of larval midgut sucrase among nine strains of the silkworm, Bombyx mori. The hybrid strain that is commercially available for use of cocoon production showed a rather high activity, which might indicate the significant role of this enzyme in the silkworm physiology. 2. 2. Three species of soluble midgut sucrase were isolated on column chromatography of DEAE-cellulose from the fifth instar larvae in nine silkworm strains. They were named Peaks III, II and I according to the order of elution from the column. Peak II was a predominant soluble sucrase. 3. 3. The kinetic parameters of three soluble sucrases in the midgut tissue from a hybrid silkworm strain were investigated. pH optima were pH 5.5–7.5, pH 6.4 and pH 6.3–7.4 in Peaks I, II and III, respectively (hereafter we refer to the data in this order). K m values toward sucrose at pH 6.5 were 7.5 mM, 14.8 mM and 5.1 mM. Substrate inhibition was observed in three soluble sucrases above 200 mM. Metal ions such as Cu2+, Pb2+, Zn2+ and Fe3+ activated three soluble sucrases markedly, as much as 17-fold against the control in Peak III with Cu2+. Tris and Tricine showed inhibition against three soluble sucrases. Tris inhibited three soluble sucrases similarly, in a mixed type of inhibition. K i values of Tris were 3.5 mM. 3.2 mM and 0.5 mM. 4. 4. These results indicate that three soluble sucrase in the larval midgut of the silkworm, Bombyx mori are the distinct entities with similar kinetic properties but with distinct kinetic parameters and suggest that they play a role slightly different from each other in the midgut cell.