Evaluating Acetaldehyde Synthesis from L-[14C(U)] Threonine by Streptococcus thermophilus and Lactobacillus bulgaricus

Abstract

To evaluate the synthesis of acetaldehyde from threonine during growth of yogurt cultures, Streptococcus tbermopbilus MS1 and Lactobacillus bulgaricus MR1 were grown in defined medium in which 10% of the total threonine was composed of L-[carbon-14(U)] threonine. Acetaldehyde production was monitored by formation of 2,4-dinitrophenylhydrazone followed by separation and analysis using high performance liquid chromatography. After growth for 8 h at 42C, approximately 2.0% of the total acetaldehyde (780.4 nmol) produced was from L-[carbon-14]threonine. Threonine aldolase activity was determined in cell-free extracts from S. tbermopbilus and L. bulgaricus grown in Elliker broth. Increasing incubation temperature from 30 to 42C decreased threonine aldolase activity in cells of the streptococcus harvested after 8 h of incubation Effect of incubation temperature was more dramatic in cells harvested after 18 h where the activity of cells grown at 42C was 89% lower than that of cells grown at 30C. Cell extracts from S. tbermoploilus MS1 possessed higher threonine aldolase activity than did those from L. bulgaricus MR1. Increased assay temperature from 30 to 42C increased threonine aldolase activity in S. tberrnopbilus MS1.