KINETICS OF K-P-NITROPHENYL PHOSPHATASE STIMULATION BY A BRAIN SOLUBLE FRACTION()

We have already described the separation of two brain soluble fraction s by Sephadex G-50, one of which stimulates (peak I) and the other inh ibits (peak II) Na+, K+-ATPase and K+-p-nitrophenylphosphatase (K+-p-N PPase) activities. Here we examine the features of synaptosomal membra ne p-NPPase activity in the presence and absence of brain peak I. It w as observed that stimulation of Mg2+, K+-p-NPPase activity by peak I w as concentration dependent. The ability of peak I to stimulate p-NPPas e activity was lost by heat treatment followed by brief centrifugation . Pure serum albumin also stimulated enzyme activity. K+-p-NPPase stim ulation by peak I proved dependent on K+ concentration but independent of Mg2+ and substrate p-nitrophenylphosphate concentrations. Since ou r determinations were performed in a non-phosphorylating condition ref lecting the Na+, K+-ATPase Na+ site, it is suggested that peak I may s timulate the Na(+)dependent enzyme phosphorylation known to take place from the internal cytoplasmic side.