D-tagatose 1,6-diphosphate aldolase from lactic streptococci: purification, properties, and use in measuring intracellular tagatose 1,6-diphosphate.

Two D-ketohexose 1,6-diphosphate aldolases are present in Streptococcus cremoris E8 and S. lactis C10. One aldolase, which was induced by growth on either lactose or galactose, was active with both tagatose 1,6-diphosphate (TDP) and fructose 1,6-diphosphate (FDP), having a lower Km and a higher Vmax with TDP as the substrate. This enzyme, named TDP aldolase, had properties typical of a class I aldolase, being insensitive to EDTA and showing substrate-dependent inactivation by sodium borohydride. Sodium dodecyl sulfate-gel electrophoresis indicated a subunit molecular weight of 34,500. The amino acid composition of TDP aldolase is reported. When the enzyme was incubated with either triose phosphates or FDP, the equilibrium mixture contained an FDP/TDP ratio of 6.9:1. The other aldolase, which had properties typical of a class II aldolase, showed activity with FDP but not with TDP. The intracellular TDP concentration, measured with the purified TDP aldolase, was 0.4 to 4.0 mM in cells growing on lactose or galactose and was lower (0 to 1.0 mM) in cells growing on glucose. The intracellular concentration of FDP was always higher than that of TDP. The role of ketohexose diphosphates in the regulation of end product fermentation by lactic streptococci is discussed.