Abstract
Plasma membranes of plant cells are characterized by a plant hormone (auxin)-responsive oxidation of NADH. The latter proceeds under argon. Also, when NADH oxidation is stimulated 50% by auxin addition, oxygen consumption is reduced by 40%. These findings are reconciled by direct assays using 5,5'-dithiobis-(2nitrobenzoic acid) (DTNB) (Ellman's reagent) that show protein disulfides to be electron acceptors for auxin-stimulated NADH oxidation. In the presence of an external reducing agent such as NADH, cysteine, or dithiothreitol, protein disulfides of the membrane are reduced with a concomitant stoichiometric increase in free thiols. In the absence of an external reducing agent, or in the presence of oxidized glutathione, DTNB-reactive thiols of the plasma membrane are decreased in the presence of auxins. Several auxin-reductant combinations were effective, but the same reductants plus chemically related and growth-inactive auxin analogs were not. A cell surface location of the affected thiols demonstrated with detergents and impermeant thiol reagents suggests that the protein may have a different physiological role than oxidation of NADH. For example, it may carry out some other role more closely related to the function of the auxin hormones in cell enlargement such as protein disulfide-thiol interchange.
Highlights
Brightman et al (1) described an NADH oxidative activity of the plant plasma membrane that was stimulated by the active auxins 2,4-dichlorophenoxyacetic acid (2,4-D),1 indole-3-acetic acid (IAA), and ␣-naphthaleneacetic acid (␣-NAA)
The results suggest that protein disulfides serve as acceptors for NADH reduction by plasma membrane vesicles
Oxygen Consumption Decreased as NADH Oxidation Is Increased—Measurements of oxygen consumption using an oxygen electrode (11) were indicative of some acceptor other than oxygen being responsible for the activity stimulated by auxin
Summary
2,4-D, 2,4-dichlorophenoxyacetic acid; 2,3-D, 2,3-dichlorophenoxyacetic acid; ␣-NAA, ␣-naphthaleneacetic acid; -NAA, -naphthaleneacetic acid; DTNB, 5,5Ј-dithiobis-(2-nitrobenzoic acid); DTT, dithiothreitol; IAA, indole-3-acetic acid; NTSB, 2-nitro-5-thiosulfobenzoate; Mes, 4-morpholineethanesulfonic acid. Plasma membrane vesicles were subsequently found to catalyze a protein disulfide-thiol interchange activity that was auxin-responsive (6). Both the latter and the auxin-stimulated NADH oxidase were sensitive to inhibition by brefeldin A (7). Based on this and other evidence, it was suggested that the two activities (auxin-stimulated NADH oxidation and auxin-stimulated protein disulfide-thiol interchange) might be catalyzed by the same protein of the plasma membrane. An auxin (2,4-D or IAA)-induced increase in thiols and decrease in disulfides of the plasma membrane were observed in the presence of NADH, cysteine and DTT but less so with GSH. The physiologically relevant function of the activity at the cell surface, may be the more general catalysis of disulfide-thiol interchange among membrane proteins that occurs in the absence of NADH (6)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
