A sulphate metabolizing centre in Euglena mitochondria

Area of Science:

• Mitochondrial biochemistry
• Sulphur metabolism in algae

Background:

• Euglena gracilis mitochondria possess an external sulphate activating system.
• Previous attempts to link sulphate activation to oxidative phosphorylation were unsuccessful.

Purpose of the Study:

• To investigate the relationship between oxidative phosphorylation and sulphate activation in Euglena mitochondria.
• To identify the cellular location and regulatory factors of sulphate activation and reduction.

Main Methods:

• Mitochondrial assays with varying phosphate (Pi) concentrations.
• Inhibition studies using oxidative phosphorylation uncouplers (DNP, CCCP) and specific inhibitors (chloramphenicol, NaN3, KCN).
• Digitonin fractionation to isolate mitochondrial components (mitoplasts, outer membranes, intermembrane fraction).

Main Results:

• Phosphate concentration (10 mM Pi) inhibits sulphate activation; a 10-fold reduction allows normal function.
• Sulphate activation is inhibited by uncouplers (DNP, CCCP).
• Sulphate reduction to cysteine occurs on mitoplasts and is inhibited by various agents.
• Sulphate activation and reduction are localized to a mitochondrial inner membrane-associated centre.

Conclusions:

• Sulphate activation and reduction in Euglena are coupled processes occurring at a specific mitochondrial site.
• Phosphate concentration is a critical regulator of sulphate activation.
• This mitochondrial centre supplies the cell with activated sulphate and cysteine.