CHARACTERIZATION OF SYNAPTOSOMAL MEMBRANE NA+, K+-ATPASE INHIBITORS

Previous work carried out in this laboratory has led to the isolation from rat brain of an aqueous soluble fraction (peak II) inhibiting synaptosomal membrane Na+, K+-ATPase and possessing other ouabain-like properties. Brain peak II was subjected to several treatments or fractionation by reversed-phase or anionic exchange HPLC and the effect of resultant fractions tested on synaptosomal membrane ATPase activity. The inhibitory components proved highly hydrophilic since they were neither extracted by hexane nor retained by a C-18 HPLC column, ruling out a lipidic nature. By anionic exchange chromatography, peak II was separated into eight fractions, two of which, named II-A and II-E, presented inhibitory activity, had low molecular weight, reacted with ninhydrin and were sensitive to acid hydrolysis. Fraction II-A was further chromatographed through a C-18 column, rendering five fractions, II-A(1) to II-A(5), inhibitory activity being confined to the most hydrophilic one (II-A(1)). Fraction II-E seems non-peptidic in nature, and its inhibitory activity was completely lost by alkalinization. II-E differs from authentic ouabain in u.v. spectrum, chromatographic behaviour and alkali sensitivity. It is suggested that two small hydrophilic compounds, probably one peptidic (II-A(1)) and another non-peptidic (II-E) in nature are involved in the regulation of Na+, K+-ATPase activity at the synaptic membranes.