Two separate transhydrogenase activities are present in plant mitochondria

Area of Science:

• Mitochondrial biochemistry
• Plant physiology
• Enzymology

Background:

• Plant mitochondria contain enzymes involved in redox reactions.
• Transhydrogenase (TH) activity, the transfer of hydride equivalents, is crucial for cellular energy metabolism.
• The specific enzymes and mechanisms of TH in plants are not fully elucidated.

Purpose of the Study:

• To investigate the enzymes responsible for hydride transfer from NADPH to NAD(+) in plant mitochondria.
• To characterize the properties and identify the novel transhydrogenase (TH) activity in potato and pea mitochondria.
• To understand the implications of non-energy-linked TH for NADP-linked processes in plant mitochondria.

Main Methods:

• Utilized inside-out submitochondrial particles from potato tubers and pea leaves.
• Employed a substrate-regenerating system to monitor hydride transfer.
• Used diphenyleneiodonium (DPI) as a Complex I inhibitor.
• Performed gel-filtration chromatography on solubilized mitochondrial membrane complexes.

Main Results:

• Identified two distinct enzymatic activities catalyzing NADPH to NAD(+) hydride transfer.
• DPI-sensitive activity was attributed to NADH-ubiquinone oxidoreductase (Complex I).
• A DPI-insensitive TH activity was characterized, eluting at ~220 kDa, specific for the 4B proton of NADH, and potentially similar to soluble bacterial TH.

Conclusions:

• Plant mitochondria possess both Complex I and a distinct, non-energy-linked transhydrogenase (TH).
• The novel TH directly couples matrix NAD(H) and NADP(H) pools.
• This non-energy-linked TH activity has significant implications for regulating NADP-dependent metabolic pathways in plant mitochondria.