1. The turnover of protein-bound phosphorylserine in preparations of membrane fragments from ox brain cortex was studied. 2. Turnover was considered to arise from the action of intrinsic protein kinases and phosphatases on a membrane protein or proteins. 3. Properties of the kinase system were studied by measuring the rate of incorporation of32P from γ-labelled ATP into protein-bound phosphorylserine isolated from partial acid hydrolysates of membrane proteins. 4. Properties of the phosphatase system were studied by observing the rate of loss of32P from membrane preparations pre-labelled with [32P]ATP. 5. Net phosphorylation and dephosphorylation of membrane protein was observed during incubation of membrane preparations with and without ATP. 6. The rate of turnover was about 4nmol of P/h per mg of protein at 20°C; dephosphorylation was considered to be the rate-limiting step.

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