Resumo

L-sorbose enhances the extracellular levels of cellulases in T. reesei cultures in the absence of other carbon sources or in the presence of glucose or cellobiose, possibly due to cell wall alterations, reduction in the uptake or intracellular degradation of cellobiose, and expression regulation at the transcriptional level. However, little is known on the effects of sorbose on the production and secretion of β-glucosidases, enzymes of industrial interest. In this study the effects of L-sorbose on growth, morphology and production of β-glucosidase by H. grisea were investigated. The fungus was cultivated in M5 medium containing 1% carbon source. β-glucosidase activity was assayed in 50 mmol/L phosphate buffer, pH 6.0, containing 3.0 mmol/L p-nitrophenyl-β-D-glucopyranoside. Endoglucanase activity was assayed in 50 mmol/L acetate buffer, pH 5.0, using 1% CMC as substrate. Alkaline and acid phosphatase activities were assayed in 200 mmol/L glycine buffer, pH 9.0, or acetate buffer, pH 5.4, respectively, using 10 mmol/L p-nitrophenylphosphate. Sorbose increased 3-fold the lag phase period and greatly altered the morphology of the fungal hyphae. Cellobiose was a good inducer of β-glucosidase, and the addition of 5% sorbose enhanced 3.3-fold the extracellular activity levels with constant cytosolic and cell-wall bound fractions, revealing stimulation of enzyme synthesis and secretion. Sorbose stimulation of enzyme production was dependent on the presence of cellobiose as inducer. Production and secretion of phosphatases and endoglucanases was not stimulated by sorbose, but it reduced the uptake rates of glucose and cellobiose. Data discarded increased non-specific enzyme secretion and/or increased release of the enzyme from the cell-wall as possible mechanisms for the effects of sorbose on the production of β-glucosidase by H. grisea and suggested a mechanism based on prolonged action of cellobiose as inducer associated to increased secretion due to cell-wall permeability alterations.