NBD-Cl modification of essential residues in mitochondrial nicotinamide nucleotide transhydrogenase from beef heart

• Modification of mitochondrial nicotinamide nucleotide transhydrogenase (NADPH:NAD+ oxidoreductase, EC 1.6.1.1) with 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl), followed by measurement of the absorption or fluorescence of the transhydrogenase-NBD adducts, resulted in a biphasic labelling of approx. 4–6 sulfhydryls, presumably cysteine residues. Of these 1–2 (27%) were fast-reacting and 3–4 (73%) slow-reacting sulfhydryls. In the presence of substrates, e.g., NADPH, the labelling was monophasic and all sulfhydryls were fast-reacting, suggesting that the modified sulfhydryls are predominantly localized peripheral to the NAD(P)(H)-binding sites. The rates of modification allowed the calculation of the rate constants for each phase of the labelling. Both in the absence and in the presence of a substrate, e.g., NADPH, the extent of labelling essentially parallelled the inhibition of transhydrogenase activity. Attempts to reactivate transhydrogenase by reduction of labelled sulfhydryls were not successful. Photo-induced transfer of the NBD adduct in partially inhibited transhydrogenase, from the sulfhydryls to reactive NH2 groups of amino-acid residue(s), identified as lysine residue(s), was parallelled by an inhibition of the residual transhydrogenase activity. It is suggested that a lysine localized close to the fast-reacting NBD-Cl-reactive sulfhydryl groups is essential for activity. 

Ämnesord

NATURVETENSKAP  -- Biologi -- Biokemi och molekylärbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Biochemistry and Molecular Biology (hsv//eng)

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Biochemistry

Biokemi

Biochemistry

Biokemi

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