Complex phosphorylation activity in neurosecretosomal membranes isolated from ox neurohypophyses

Homogenates of neural lobes of bovine pituitary glands were fractionated on Ficoll gradients to yield neurosecretosomes (nerve endings). The neurosecretosomes were lysed in a hypo-osmotic buffer and the membranes were separated from the soluble components by centrifugation. On incubation with [gamma-32P]ATP this membrane preparation showed an endogenous phosphorylation activity, which was studied by means of gel electrophoresis in the presence of sodium dodecyl sulphate, and subsequent autoradiography. The major part of the [32P]Pi detected on the gel was shown to be incorporated into three protein bands, termed A, B and C, with minimal mol.wts. of 83 000, 59 000 and 47 000 respectively. The phosphorylation of these three proteins was studied under a variety of experimental conditions. The patterns obtained were partly similar. However, important individual differences were noted, particularly with respect to the effects of cyclic AMP, Mg2+ and Ca2+. On the basis of these differences, it is suggested that in this system the phosphorylation activity is heterogenous, bands A, B and C each reflecting the presence of a different site of phosphate turnover. The relationship of bands A, B and C to several of the previously described phosphoproteins in the brain is discussed.