D-Tagatose 6-Phosphate Pathways1

Lactose is usually metabolized by A-galactosidase-catalyzed cleavage to D-glucose and D-galactose, followed by the conversion of Dgalactose to D-glucose 6-phosphate via the well known Leloir pathway (10): D-galactose *D galactose 1-phosphate D-glucose 1-phosphate D-glucose 6-phosphate. In Staphylococcus aureus, however, the disaccharide is utilized exclusively via lactose phosphate, which is cleaved by a phospho-a-galactosidase to yield D-glucose and D-galactose 6-phosphate (5-7). The metabolism of exogenous D-galactose is also initiated by phosphorylation at C6 (13). DGalactose 6-phosphate is metabolized further through tagatose derivatives by a pathway recently elucidated in our laboratory (1): D-galactose 6-phosphate -* D-tagatose 6-phosphate D-tagatose 1, 6-diphosphate D-glyceraldehyde 3-phosphate + dihydroxyacetone phosphate. Other organisms besides S. aureus reported to possess phospho-,B-galactosidase activity are the group N streptococci, and these bacteria have also been shown to possess f-galactosidase (9). We have now examined representatives of this group to determine whether the occurrence of the tagatose 6-phosphate pathway extends to these organisms. In contrast to other organisms, we show here that the group N streptococci possess the enzymes of both pathways of Dgalactose metabolism. Escherichia coli B/r and Aerobacter aerogenes PRL-R3 were grown aerobically at 37 and 32 C, respectively, in a mineral medium (4) supplemented with 0.5% of the indicated sugar. Cell extracts were prepared by sonic disruption of cellular suspensions (4). S. aureus NCTC 8511 was grown and extracts were prepared as described previously (1). Streptococcus lactis ATCC 7962, S. lactis ATCC 11454, and S.