The effect of trinitrophenyl nucleotide derivatives on the adenosine triphosphatase activities in maize tonoplast vesicles

The effects of trinitrophenyl (TNP) derivatives of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) on the coupled activities of the tonoplast ATPase in corn roots were investigated. The addition of TNP-ATP at micromolar concentrations (0.5-0.25 mM) significantly decreased both the ATP hydrolytic activity and the coupled proton pumping activity. The presence of TNP-ATP resulted in an increased Km for ATP and a reduced maximum enzyme velocity. Unlike TNP-ATP that strongly inhibited proton transport at low concentration, TNP-ADP inhibited this activity only slightly. TNP-AMP stimulated proton transport at low concentrations. ADP and AMP inhibited the initial rate of proton transport in a different manner. The concentrations required to inhibit half of the initial rate of proton transport activity for ADP and AMP were 80 micromolar and 2 mM, respectively. The results suggest that tonoplast vesicles may contain at least two kinds of nucleotide binding sites