Pyruvate stimulates transamination of leucine into α-ketoisocaproic acid and supports 3-methylbutanal production by Lactococcus lactis

AIM: To investigate the effect of pyruvate and glucose on leucine transamination and 3-methylbutanal production by Lactococcus lactis, including the comparison with cells possessing glutamate dehydrogenase (GDH) activity. METHODS AND RESULTS: Lactococcus lactis cells were incubated in chemically defined medium (CDM) with the pH controlled at 5.2 to mimic cheese conditions. Pyruvate supplementation stimulated the production of the key flavour compound 3-methylbutanal by 3-4 times after 72 h of incubation. Concurrently, alanine production increased, demonstrating the involvement of pyruvate in transamination reactions. Glucose-metabolizing cells excreted α-ketoisocaproic acid and produced even 3 times more 3-methylbutanal after 24 h than pyruvate-supplemented cells. Conjugal transfer technique was used to transfer the plasmid pGdh442 carrying the gdh gene encoding for GDH to L. lactis. Introducing GDH did not stimulate the excretion of α-ketoisocaproic acid and the production of 3-methylbutanal. CONCLUSIONS: These results demonstrate that Lactococcus uses pyruvate to transaminate leucine into α-ketoisocaproic acid which supports 3-methylbutanal production. Surprisingly, GDH activity did not stimulate leucine transamination and 3-methylbutanal production.