Tyrosinase activity in human ocular malignant melanoma.

We examined tyrosinase activity in pigmented specimens from three cases of ocular malignant melanoma. Tissue (cholate-trypsin-treated) extract was prepared in cholate-phosphate buffer by homogenization, centrifugation, trypsin digestion, and hydroxylapatite column chromatography. Tyrosinase activity was spectrophotometrically assayed as dopa (L-3,4-dihydroxylphenylalanine) oxidase activity. Tyrosinase activity was detected in the cholate-trypsin-treated extracts. Enzyme activity was inhibited by phenylthiourea but not by 3-iodo-tyrosine. The enzyme was inactivated when extract was preheated or digested with pronase. We believe that our findings confirm the presence of tyrosinase activity in ocular malignant melanoma.