Agmatine Production by Aspergillus oryzae Is Elevated by Low pH during Solid-State Cultivation

Area of Science:

• Biotechnology and Fermentation Science
• Microbial Biochemistry

Background:

• Agmatine, a bioactive compound, is generally not thought to be synthesized by fungi due to a lack of arginine decarboxylase (ADC) genes.
• Aspergillus oryzae is widely used in Asian fermented food production, but its role in agmatine synthesis is unclear.

Purpose of the Study:

• To investigate agmatine production by Aspergillus oryzae under different cultivation conditions.
• To identify the key factors influencing agmatine accumulation in rice-based fermentation.

Main Methods:

• Comparison of agmatine levels in sake produced via multiple parallel fermentation (MPF) versus enzymatic saccharification.
• Solid-state and submerged cultivation of A. oryzae to assess agmatine production.
• Optimization of cultivation parameters (temperature, pH) and addition of organic acids.
• Enzyme activity assays for l-arginine decarboxylase (ADC) and ornithine decarboxylase (ODC).

Main Results:

• Solid-state cultivation of A. oryzae produced significant amounts of agmatine (3.1 mM), unlike submerged cultivation (<0.01 mM).
• Agmatine production was enhanced at 30°C and pH 5.3, with further increases observed upon addition of organic acids (l-lactic, succinic, citric) lowering the pH.
• Solid-state culture homogenate showed high ADC activity at pH 3.0 and 30°C, while submerged culture showed very low activity.
• A. oryzae possesses ODC genes but lacks ADC orthologs, suggesting an unidentified ADC or other decarboxylases are responsible for agmatine production.

Conclusions:

• Aspergillus oryzae can produce agmatine from l-arginine, particularly under low-pH solid-state cultivation conditions.
• An unidentified, low-pH-dependent ADC activity in A. oryzae is crucial for agmatine production during solid-state fermentation.
• Optimized conditions can lead to the production of agmatine-rich ethanol-free rice syrup and fermented foods, with potential applications as nutraceuticals.