β-D-Phosphogalactoside Galactohydrolase of Lactic Streptococci1

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Abstract
β-D-Phosphogalactoside galactohydrolase (β-Pgal) was examined in a number of lactic streptococci by use of the chromogenic substrate o-nitrophenyl-β-D-galactopyranoside-6-phosphate. Specific activity of β-Pgal ranged from 0.563 units/mg of protein in Streptococcus lactis UN, to 0.120 in S. diacetilactics 18-16. Essentially no β-D-galactoside galactohydrolase (β-gal) was found in these organisms when o-nitrophenyl-β-D-galactopyranoside served as the chromogenic substrate. S. lactis 7962 was the one exception found. This organism contained rather high levels of β-gal, and very little β-Pgal could be detected. β-Pgal activity was examined in streptococci that differed widely in both their proteolytic ability and rates of lactic acid production during growth in milk. Differences in proteolytic ability did not influence β-Pgal synthesis; also, the rate of lactic acid production was independent of the level of β-Pgal present in the cell, since the rate of lactic acid production could be increased approximately fourfold without changing the amount of β-Pgal present in the cell. Various carbohydrates were tested as potential inducers of the enzyme. Although galactose, either as the free sugar or combined with glucose in lactose, was the only inducer, noninducing sugars such as mannose or glucose showed some ability to cause fluctuations in the basal level of β-Pgal. Cells growing in mannose or glucose exhibited about 30% of the maximal enzyme levels found in cells growing in lactose or galactose. No gratuitous inducers were found.

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