NADPH generation in Aspergillus nidulans: is the mannitol cycle involved?

Area of Science:

• Biochemistry
• Mycology
• Molecular Biology

Background:

• A cyclic pathway for NADPH generation via mannitol and fructose interconversion has been proposed in fungi.
• This pathway, the mannitol cycle, is hypothesized to play a role in fungal metabolism.

Purpose of the Study:

• To investigate the presence and function of the proposed mannitol cycle in Aspergillus nidulans.
• To determine the subcellular localization of key enzymes involved in the mannitol cycle.
• To assess the regulation of mannitol cycle enzyme activity under different growth conditions.

Main Methods:

• Enzyme activity assays were performed on extracts from Aspergillus nidulans mycelia grown on various carbon and nitrogen sources.
• Subcellular localization of enzymes was determined using biochemical fractionation techniques.
• Activities of other NADP-linked dehydrogenases were measured for comparative analysis.

Main Results:

• Three enzymes of the proposed mannitol cycle were localized exclusively in the cytosol.
• Two isoenzymes of mannitol-l-phosphate dehydrogenase were found in both the mitochondrion and cytosol.
• No coordinated changes in mannitol cycle enzyme activities were observed under different growth conditions, including growth on nitrate (NO3-).
• Growth on nitrate increased the activity of other NADP-linked dehydrogenases, such as NADP-isocitrate dehydrogenase and glucose-6-phosphate dehydrogenase.

Conclusions:

• The study found no evidence to support the operation of the mannitol cycle in Aspergillus nidulans.
• The proposed role of the mannitol cycle in NADPH generation in this fungus is not supported by the experimental data.
• Alternative pathways, potentially involving other NADP-linked dehydrogenases, may be responsible for NADPH generation in Aspergillus nidulans.