The inhibitory activity of a brain extract on synaptosomal Na+, K+-ATPase is sensitive to carboxypeptidase A and to chelating agents.

In the present study some properties of an inhibitory extract of synaptosomal membrane Na+, K+-ATPase were investigated. This extract (peak II) was prepared by gel filtration in Sephadex G-50 of a soluble fraction of the rat cerebral cortex. Ultrafiltration of peak II through Amicon membranes indicated that the inhibitor has a low MW (less than 1000). The inhibitory activity was not modified by heating in neutral pH at 95 degrees C for 20 min but it was destroyed by charring in acid pH at 200 degrees C for 120 min. The inhibitory activity decreased by incubation of peak II with carboxypeptidase A. These findings suggest that the factor responsible for the inhibition of Na+, K+-ATPase activity is probably a polypeptide. On the other hand, the inhibition was reverted by the chelators EDTA and EGTA, indicating the participation of an ionic compound as well. The increase of Mg2+ concentration during the enzyme assay did not increase the inhibition, indicating that the ion involved might not be vanadate. It is suggested that both a polypeptide and an ionic compound coparticipate in the inhibitory effect of peak II on Na+, K+-ATPase activity.