Permeability of ammonia methylamine and ethylamine in the cyanobacterium synechococcus r 2 anacystis nidulans pcc 7942
Ritchie, R.J.; Gibson, J.
Journal of Membrane Biology 95(2): 131-142
Permeabilities of ammonia (NH3), methylamne (CH3NH2) and ethylamine (CH3CH2NH2) in the cyanobacterium (cyanophyte) Synechococcus R-2 (Anacystis nidulans) have been measured. Based on net uptake rates of DCMU(dichlorophenyldimethylurea) treated cells, the permeability of ammonia was 6.44 .+-. 1.22 .mu.m sec-1 (n = 13). The permeabilities of methylamine and ethylamine, based on steady-state 14C labeling were more than ten times that of ammonia (Pmethylamine = 84.6 .+-. 9.47 .mu.m sec-1 (76), Pethylamine = 109 .+-. 11 .mu. sec-1 (55)). The apparent permeabilities based on net uptake rates of methylamine and ethylamine uptake were significantly lower, but this effect was partially reversible by ammonia, suggesting that net amine fluxes are rate limited by proton fluxes to an upper limit of about 700 nmol m-2 sec-1. Increasing concentrations of amines in alkaline conditions partially dissipated the pH gradient across the cell membrane, and this property could be used to calculate the relative permeabilities of different amines. The ratio of ethylamine to methylamine permeabilities was not significantly different from that calculated from the direct measurements of permeabilities; ammonia was much less effective in dissipating the pH gradient across the cell membrane than methylamine or ethylamine. An apparent permeability of ammonia of 5.7 .+-. 0.9 .mu.m sec-1 could be calculated from the permeability ratio of ammonia to methylamine and the experimentally measured permeability of methylamine. The permeability properties of ammonia and methylamine are very different; this poses problems in the interpretation of experiments where 14C-methylamine is used as an ammonia analogue.