Insights into the interaction of human liver arginase with tightly and weakly bound manganese ions by chemical modification and site-directed mutagenesis studies

Abstract

Diethyl pyrocarbonate (DEPC) caused a loss in the ability of inactive subunits of wild-type and H141F mutant human liver arginase (EC 3.5.3.1) to be reactivated by Mn 2+. The effect was reversed by hydroxylamine and involved a residue with a p K a of 6.5±0.1. Half activation with Mn 2+ was sufficient for total resistance of H141F and full activation was not impeded by a previous incubation of the half-active species with DEPC. The H101N and H126N mutants expressed 60 and 82% of the wild-type activity, respectively, without changes in K m for arginine or K i for lysine inhibition. After dialysis against EDTA, H126N was inactive in the absence of added Mn 2+ and contained <0.1 Mn 2+/subunit, whereas H101N was half active and contained 1.2±0.1 Mn 2+/subunit. Results support the concept that a weakly bound metal ion is needed only for conversion of active species to a more active active state.