Formation and excretion of acetylmaltose after accumulation of maltose in Escherichia coli

Abstract

malB(+)malQ strains accumulate maltose via the maltose-binding-protein-dependent transport system but are unable to metabolize it. Nevertheless, some of the maltose is modified after entering the cell. This newly formed compound exhibited a higher R(f) value than did maltose upon thin-layer and paper chromatography with the usual sugar-separating solvents. Treatment of this compound with acid and alkali reformed maltose. The identity of this compound with acetylmaltose was derived from mass spectrometry. Nuclear magnetic resonance spectra of the compound confirmed the presence of the acetyl group but did not allow its precise location on the maltose moiety. However, linkage to the 1-position of maltose could be excluded. Analysis of the mass spectra indicated that the nonreducing end of maltose was acetylated. Other substrates of the maltose transport system, such as maltotetraose, maltopentaose, and maltohexaose, were also modified after accumulation into the cell. Several products were formed; the heterogeneity of these products was probably caused by different degrees of acetylation. The enzymatic activity responsible for maltose and maltodextrin acetylation is unknown. However, it is clear that the lacA-dependent thiogalactoside transacetylase was not necessary for the acetylation of maltose. Strains that accumulate maltose via a bypass of the normal malB-dependent transport system also acetylated maltose even in the absence of any malB gene products. Thus, the acetylating activity was not connected to the malB system. Acetylmaltose as well as acetylated maltodextrins was excreted into the medium. Acetylmaltose is not a substrate of the maltose transport system. Thus, maltose acetylation may be an effective detoxification mechanism.