Abstract: Lactate dehydrogenase (LDH) is a key enzyme that catalyzes the transformation of L-lactic acid from pyruvate in Rhizopus oryzae. Investigations of its activity during the fermentation have important significances on metabolic engineering and strains selection for increasing lactate production. In this study, the relationship between LDH activity and L-lactic acid production rate of Rhizopus oryzae AS3.1208 in shaking flask was investigated. The results indicated that glucose as carbon source had a higher L-lactic acid production rate and LDH activity than the sweet potato starch source. (NH4)2SO4 was the optimum nitrogen source compared with beef extract and peptone. It had the highest L-lactic acid production rate and LDH activity at a concentration of 0.40%. The addition of CaCO3 to fermentation medium for the neutralization of lactic acid produced had an absolute effect on lactate production as well as LDH activity. A higher L-lactic acid production and LDH activity could be achieved at 36～40h fermentation when the 250ml flask was filled by 100ml fermentation liquid with sweet potato starch as carbon source.

Key words: Rhizopus oryzae, lactate dehydrogenase, L-lactic acid, metabolic control