Aims: To screen various filamentous fungi belonging to Aspergillus spp. producing leucine and methionine aminopeptidases.
Methods and results: Twenty-eight Aspergillus strains representing 14 species within the genus were screened for L-leucine aminopeptidase (LAP) production in two media in shake flask fermentation. Two Aspergillus sojae (NRRL 1988 and NRRL 6271) and one Aspergillus oryzae (NRRL 6270) strains were selected as the best producers for further studies. The peak LAP activities were 2.61, 2.59 and 1.30 IU ml(-1) for the three fungi on days 2, 5 and 4 respectively. In addition to LAP, L-methionine aminopeptidase (MAP) activity was also detected. Apart from submerged fermentation, the highest LAP yields by solid-state fermentation were 11.39, 17.40 and 13.02 IU g(-1) dry matter for the above fungi. The temperature and pH optimum of the enzyme was found to be in the range of 65-75 degrees C at pH 8.0-9.0 for all three fungi. Metal ions, Co(2+) and Fe(2+) in 2 mmol l(-1) concentration apparently enhanced the relative enzyme activity and heat stability.
Conclusions: Two A. sojae (NRRL 1988 and NRRL 6271) and one A. oryzae (NRRL 6270) strains were found to be the best producers of LAP and MAP. The preliminary characterization studies revealed that the enzyme is considerably thermostable and belongs to the class metalloenzymes.
Significance and impact of the study: A good number of aspergilli were screened and the ability of the fungal aminopeptidase to release a particular N-terminal amino acid along with its high thermal stability, makes them interesting for controlling the degree of hydrolysis and flavour development for a wide range of substrate.