Enzyme activities of cell-free extracts from mutant strains of lactic streptococci subjected to sublethal heating or freeze-thawing

Abstract

Two mutant lactose-negative (Lac −), proteinase-negative (Prt −) strains of lactic streptococci, Streptococcus lactis 25Sp and S. cremoris KHA2, and their parents, S. lactis C2 and S. cremoris KH Lac + Prt +, were grown in a suitable medium with the pH maintained at 6.5 by addition of NH 4OH. Cells were harvested by centrifugation, resuspended, and then heated sublethally at 54 or 69 °C for 15 sec. Cells also were frozen and stored for 1 week at −20 or −100 °C. Cell-free extracts of cells heated at 54 °C had more proteinase and aminopeptidase activities than did a similar extract of cells heated at 69 °C. The greatest enzyme activities occurred in the cell-free extracts prepared from cells frozen and stored at −100 °C. Specific activities of proteinase and dipeptidase generally decreased in extracts of freeze-shocked cells compared to those in extracts of untreated cells. Enzyme activity of extracts also decreased in the presence of 5% NaCl at pH 5.0. Cell-free extracts at pH values of 5 to 8 were heated at 69 °C for 1, 5, or 10 min. Heating them for 10 min caused a loss of dipeptidase activity which was most pronounced at pH 5.0 and least pronounced at pH 7.0.