Removal of the nitro and phenyl groups from NPPB decreases its inhibitory effect on cytoplasmic streaming in the alga Nitella hookeri.

Structural analogues of the arylaminobenzoate 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), prepared using a simple reductive amination sequence, were tested for their effects on cytoplasmic streaming rates in the alga Nitella hookeri. Cytoplasmic streaming was sensitive to NPPB, with an IC50 value of 24 micromol/L. Removal of the nitro group from the benzoate ring decreased the IC50 to 455 micromol/L. The introduction of an extra carbon or double bond into the aliphatic chain had no effect on activity. Loss of the phenyl group decreased potency, with an IC50 of 6.4 mmol/L. These data are the first documenting the relative inhibitory effects of structural changes to arylaminobenzoates in algae. Patch-clamp data and the effects of tetrapentyl ammonium chloride on streaming suggest that the nitro and phenyl groups may act by inhibiting both K+ and Cl- channels. This is likely, through changes in the membrane potential, to affect Ca2+ fluxes and action potentials, thereby slowing cytoplasmic streaming.