GLUTAMATE BIOSYNTHESIS IN LACTOCOCCUS-LACTIS SUBSP LACTIS NCDO-2118

Unlike other lactic acid bacteria, Lactococcus lactis subsp. lactis NC DO 2118 was able to grow in a medium lacking glutamate and the amino a cids of the glutamate family. Growth in such a medium proceeded after a lag phase of about 2 days and with a reduced growth rate (0.11 h(-1) ) compared to that in the reference medium containing glutamate (0.16 h(-1)). The enzymatic studies showed that a phosphoenolpyruvate carbox ylase activity was present, while the malic enzyme and the enzymes of the glyoxylic shunt were not detected. As in most anaerobic bacteria, no alpha-ketoglutarate dehydrogenase activity could be detected, and t he citric acid cycle was restricted to a reductive pathway leading to succinate formation and an oxidative branch enabling the synthesis of alpha-ketoglutarate. The metabolic bottleneck responsible for the limi ted growth rate was located in this latter pathway. As regards the syn thesis of glutamate from alpha-ketoglutarate, no glutamate dehydrogena se was detected. While the glutamate synthase-glutamine synthetase sys tem was detected at a low level, high transaminase activity was measur ed. The conversion of alpha-ketoglutarate to glutamate by the transami nase, the reverse of the normal physiological direction, operated with different amino acids as nitrogen donor, All of the enzymes assayed w ere shown to be constitutive.