Temporal changes in phosphoamino acid phosphatase activities in murine erythroleukaemic cells.

1. The activities of phosphotyrosine, phosphothreonine and phosphoserine phosphatases were measured at various time periods in Friend murine erythroleukaemic (MEL) cells. 2. Effects of DMSO (dimethyl sulphoxide) and HMBA (hexamethylene bisacetamide), inducers of differentiation, were examined. 3. The activities of all three enzymes showed cyclic variation when measured on a daily basis over a period of several days; this was also the case when phosphothreonine phosphatase was determined in cells treated with DMSO or HMBA and when phosphoserine phosphatase was assayed after stimulation of the cells with HMBA. 4. Evidence for rhythmic variation in phosphotyrosine phosphatase activities was also obtained when the activities were determined at hourly intervals both in control cells and those treated with HMBA. 5. The time-dependent changes observed could be significant in that control of dephosphorylation may possibly be achieved by altering the rhythms.