Quantitative measurement by microdensitometry of tyrosinase (dopa oxidase) development in whole small ascidian embryos.

Embryos of the ascidian, Ciona intestinalis, were fixed in either cold (5 degree C) 70% ethanol or cold absolute methanol during their tyrosinase development phase and incubated in buffered (pH 7.2) solutions of the enzyme substrate L-dihydroxyphenylalanine. Optical density of the reaction product (melanin) was measured in the whole small embryos at 450 nm with a Vickers M85 scanning and integrating microdensitometer. The frequency distribution of the reaction density in embryos of a population was Gaussian, and the mean optical density in embryos samples (N = 25) increased linearly with incubation time when a saturation level of substrate was used. Absolute optical density units of dopa oxidase activity in embryos increased linearly in proportion to the development time preceding melanin granulogenesis thereby suggesting that the enzyme activity measured by this procedure is proportional to the amount of tyrosinase present. Since this developmental increase in activity was blocked by treatment of the embryos with puromycin, an inhibitor of protein synthesis, the change is apparently caused by new enzyme synthesis. The microdensitometry assay also confirmed results obtained previously with a radiometric assay: embryos cleavage-inhibited at 7 h development time with cytochalasin B to produce giant melanocytes developed only the same amount of enzyme activity as control embryos.