Ammonium (NH
4+) uptake in the cyanobacterium
Nostoc muscorum ISU (
Anabaena ATCC 27893) and interaction of copper (Cu
2+) and sulfhydryl agents was studied. N
2-grown cells scavenged extracellular NH
4+ via two energy-dependent transport systems: the ‘high-’ (
Km=11μM,
Vmax=0.22nmol/min/mg protein) and ‘low-affinity’ (
Km=66μM,
Vmax=1.25nmol/min/mg protein). Both transport systems were competitively inhibited by methylamine (high-affinity
Ki=20μM; low affinity
Ki=80μM), and showed distinct pH profiles. Addition of Cu
2+ (0.1μM) stimulated NH
4+ uptake by the high-affinity system (
Km=8μM,
Vmax=0.42nmol/min/mg protein). Similar effect was not observed with other bivalent cations (Hg
2+, Ni
2+, Zn
2+, Mn
2+) applied at equimolar concentrations. The sulfhydryl reducing agents, cysteine and dithiothreitol, inhibited the high-affinity system noncompetitively and caused efflux of accumulated NH
4+. Cu
2+ eliminated the inhibitory effect of sulfhydryl reducing agents on NH
4+ uptake. Inhibition of NH
4+ uptake by sulfhydryl blocking agents (N-ethylmaleimide or
p-chloromercuribenzoate) which was not reversible by Cu
2+ suggested that oxidation of available sulfhydryl residues of membrane proteins (carriers) is an important factor in NH
4+ translocation in
Nostoc muscorum.
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