Involvement of a single thiol group in the conversion of the NAD+-dependent activity of rat liver xanthine oxidoreductase to the O2-dependent activity

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Abstract
The effects of 2-iodosobenzoic acid, 4-chloromercuribenzoate, 5,5''-dithiobis(2-nitrobenzoic acid) and tetraethylthioperoxydicarbonic diamide (disulfiram) on the NAD-dependent activity of xanthine oxidoreductase from rat liver were investigated. Only disulfiram converted the NAD-dependent activity into the O2-dependent activity quantitatively, without changing the xanthine hydroxylation rate. The modification process was a first-order reaction with respect to time (min) and disulfiram concentration (.mu.M). The kinetic data showed that modification of single SH group is sufficient for loss of the enzymic activity towards NAD as electron acceptor. The complete protection afforded by NAD against the action of disulfiram suggests that the essential SH group may be involved in binding of NAD to the xanthine oxidoreductase molecule.

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