Acid phosphatase and phosphoamino acid phosphatases in murine erythroleukaemic cells

The activities of acid and alkaline phosphatases and phosphotyrosine, phosphoserine and phosphothreonine phosphatases were measured in Friend murine erythroleukaemic (MEL) cells. The effects of treating the cells with dimethyl sulphoxide (DMSO), an inducer of differentiation, were examined. In untreated cells alkaline phosphatase activity was undetectable, though there were significant amounts of acid phosphatase (76 +/- 15 mU/mg protein) and phosphotyrosine phosphatase (16 +/- 0.9mU/mg protein); phosphoserine and phosphothreonine phosphatase activities (9 +/- 0.4 and 7 +/- 0.6mU/mg protein, respectively) were lower than for phosphotyrosine phosphatase. Addition of 1 or 2% DMSO to the culture medium resulted in the expected cell death within 2 weeks. With 0.5% DMSO, cells remained viable for at least 8 weeks, but while some appeared to have smaller nuclei and retained their rounded appearance, others became fibroblastic within several days and adhered to the culture vessel. The treated cells which had kept their morphology showed no difference in acid phosphatase activities as compared with untreated controls; phosphotyrosine phosphatase was lower (9 +/- 0.8mU/mg protein) and phosphoserine and phosphothreonine phophatases higher (11 +/- 0.5 and 10 +/- 0.4mU/mg protein, respectively) than in the controls. The Km values for p-nitrophenyl phosphate were similar in untreated and treated cells (0.069 and 0.068mM, respectively); for phosphotyrosine the Km value was lower in the treated cells (0.97mM) than in the controls (1.9mM).