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Rate of electron transfer between cytochrome b561 and extravesicular ascorbic acid
P M Kelley1, V Jalukar, D Njus
1Department of Biological Sciences, Wayne State University, Detroit, Michigan 48202.
The Journal of Biological Chemistry
|November 15, 1990
Summary
Cytosolic ascorbic acid (vitamin C) efficiently donates electrons to cytochrome b561, regenerating intravesicular vitamin C. This process ensures a continuous supply of reducing equivalents for vesicle enzymes.
Area of Science:
- Biochemistry
- Cell Biology
- Membrane Transport
Background:
- Cytochrome b561 is crucial for electron transfer across secretory vesicle membranes.
- It regenerates intravesicular ascorbic acid, vital for cellular processes.
Purpose of the Study:
- To determine if cytosolic ascorbic acid is kinetically capable of being the external electron donor for cytochrome b561.
- To quantify electron transfer rates between cytochrome b561 and external ascorbate/semidehydroascorbate.
Main Methods:
- Utilized stopped-flow spectrophotometry to measure electron transfer rates.
- Assessed reduction of cytochrome b561 by ascorbate.
- Deduced oxidation rates of cytochrome b561 by semidehydroascorbate from steady-state electron flow.
Main Results:
- The rate constant for cytochrome b561 reduction by ascorbate was 450 M-1 s-1 at pH 7.0.
- The rate constant for cytochrome b561 oxidation by semidehydroascorbate was 1.2 x 10(6) M-1 s-1 at pH 7.0.
- Rate constants align with the reduction potentials of cytochrome b561 and ascorbate/semidehydroascorbate.
Conclusions:
- Cytosolic ascorbate rapidly reduces cytochrome b561, maintaining its reduced state.
- Cytochrome b561 effectively shuttles electrons from cytosolic ascorbate to intravesicular semidehydroascorbate.
- This supports the role of cytochrome b561 in providing reducing equivalents for intravesicular monooxygenases.