Effects of pH and type of sugar in the medium on tyrosinase activity in cultured melanoma cells.

The tyrosinase (EC 1.14.18.1) activity of cultured mouse melanoma cells B16 in the stationary phase of growth, depends greatly on the pH of the medium and the kind of sugar present. The enzyme activity of a homogenate of cells grown at pH 7.2 in Eagles's MEM supplemented with 10% new born calf serum and con taining galactose in place of glucose, was about ten times that of a homogenate of cells cultured at pH 6.3 in the same medium. The tyrosinase activity changed reversibly on changing the pH of the culture medium. When cultured at a constant pH of 7.2, cells grown with 1 mM galactose had about five times higher tyrosinase activity than cells grown with 1 mM glucose. Only a small amount of lactate accumulated in cultures with glucose and it had little effect on the enzyme activity. These two findings explain the very low tyrosinase activity of cells cultured in medium with 5 mM glucose: the low activity is due to the presence of glucose and to the low pH resulting from conversion of glucose to lactic acid.