Induction, and production studies of a novel glucoamylase of Aspergillus niger

Abstract

The influence of carbon and nitrogen sources on the production of a raw-starch-digesting glucoamylase was investigated. The enzyme production was variable according to the carbon source. Levels of glucoamylase were minimal in the presence of even low concentrations of glucose while its production was stimulated by other carbohydrates. Wheat bran and cellulose were the most effective inducers of glucoamylase activities, followed by rice bran. Exogenously supplied glucose inhibited the synthesis of the enzyme in cultures of A. niger growing on wheat bran. In defined medium with maltose, the glucoamylase titres were 5.2- to 16.7-fold higher with cells growing on monomeric sugars and 1.5 times higher than cells growing on other disaccharides. It appeared that synthesis of glucoamylase varied under an induction mechanism, and a repression mechanism which changed the rate of synthesis of enzyme in induced over non-induced cultures. In this organism, substantial synthesis of glucoamylase could be induced by maltose, cello-dextrin, cellulose or cellulose- and hemicellulose-containing substrates which showed low volumetric substrate uptake rate. During growth of A. niger on wheat bran, maximum volumetric productivity (Qp) of glucoamylase was 274 IU l−1 h−1 and is significantly higher than the values reported for some other potent fungi. The addition of actinomycin D (a repressor of transcription) and cycloheximide, (a repressor of translation) completely repressed glucoamylase biosynthesis, suggested that the regulation of glucoamylase synthesis in this organism occurs at both transcriptional and translational level. Thermodynamic studies revealed that the culture exerted protection against thermal inactivation when exposed to different fermentation temperatures.